Health promoting benefits of krill oil: mechanisms, bioactive combinations, and advanced encapsulation technologies

Abstract

Krill oil, derived from Antarctic krill (Euphausia superba) species, is drawing increased attention due to its distinct composition, being rich in omega-3 fatty acids, phospholipids, and astaxanthin. Recent studies highlight the potential benefits of krill oil as a dietary supplement for enhancing various health-related factors. Research indicates that supplementing with krill oil positively affects markers of inflammation, oxidative stress, muscle function, glucose metabolism, and lipid profiles. Additionally, advancements in encapsulation technologies aim to optimize the delivery and efficacy of krill oil supplements. The review outlines the selection of emulsifiers and wall materials, along with techniques employed in creating four novel encapsulation methods for krill oil: micro/nanoemulsions, microcapsules, liposomes, and nanostructured lipid carriers. The review also provides scientific literature on the physiological impacts and underlying mechanisms of krill oil supplementation. It explores its influence on glucose homeostasis, oxidative stress responses, inflammatory pathways, lipid metabolism, and muscle physiology.

Introduction

The approach of preventive healthcare and holistic well-being is gaining momentum which gives a notable shift towards the natural supplements. People, now are more proactive about their health and are opting for natural remedies over manufactured ones. The main reason behind this transformation is the growing recognition of the pivotal role of nutrition plays in overall human health (Alneyadi et al., 2024). This ongoing awareness amongst consumers for health leads to delve more into the concepts of supplements which are rich in vitamins, minerals, and bioactive substances sourced from plants and animals. Among these, the Antarctic krill, scientifically named Euphausia superba, known to be an excellent source of various nutraceutical components is a new area of interest amongst the food and pharmaceutical industries (Costanzo et al. 2016). The biomass of Antarctic krill estimates to be around 379 million metric tons, with annual post-larval production ranging from 342 to 536 million metric tons. To ensure sustainability, the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) has set a catch limit of 620,000 tons per year. However, the actual catch remains significantly lower, at approximately 250,000 tons annually, indicating underutilization of krill resources (Xie et al., 2019). This number is increasing evidently with declining fish stocks due to overfishing and environmental pollution which makes the krill as a promising alternative marine resource. Presently, the use of krill is restricted primarily in the aquaculture industry and sport fishing market. Therefore, efforts are underway to develop krill-based products for human consumption, with krill oil emerging as an excellent source owing to its rich nutritional profile. Enriched with essential nutrients such as n-3 polyunsaturated fatty acids (n-3 PUFAs), phospholipids (PLs), astaxanthin, vitamins, flavonoids, and minerals (Joob & Wiwanitkit, 2015). Nutritional scientists researched into the properties of krill oil, hailing it as a new nutraceutical with the potential to improve quality of health in humans. The distinctive composition of lipids and rich antioxidant profile of krill oil set it apart in the market of health supplements, highlighting its potential as a superior nutritional source. Various researchers studied the nutritional significance of krill oil in comparison to the other conventional sources present in the market, for instance Ulven et al. (2011) studied the impacts of krill oil and fish oil on serum lipids, oxidative stress indicators, inflammation and to determine whether two distinct molecular forms of omega-3 polyunsaturated fatty acids: triacylglycerol and phospholipids, have distinct effects on plasma level of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The study found that both krill and fish oil significantly elevated the plasma levels of EPA, DHA, and Docosapentaenoic acid (DPA) compared to controls, with no marked differences between the two oils in terms of changes in serum lipids or markers of oxidative stress and inflammation. This means that even though the EPA + DHA dose in the Krill oil was only 62.8% of that in the fish oil, krill oil and fish oil are comparable dietary sources of n-3 PUFAs. Moreover, Rundblad et al., (2018) examined the health benefits of consuming, lean, fatty fish in accordance with dietary recommendations, krill oil with a comparable n-3 fatty acid content, and a control oil. The study concludes that lean and fatty fish when consumed through supplementation with krill oil have good benefits to health. While krill oil is better for blood glucose regulation than fish, fish is still a wonderful source of other nutrients that are essential for health, such as Vitamin D. Furthermore, Alkhedhairi et al., (2022) determined how krill oil supplementation affected muscle size and function in healthy older adults. Krill oil supplementation for 6 months results in statistically and clinically significant increases in muscle function and size in healthy older adults.

Several other articles have also pointed to the health benefits of krill oil intake such as reducing intestinal inflammation by improving intestinal barrier integrity and epithelial restitution (Xie et al., 2019); significant increases in plasma levels of EPA and DHA (Maki et al., 2009); reducing fasting serum triglycerides levels (TG) (Berge et al., 2015); effective in the management of the emotional symptoms of premenstrual syndrome; increase of high-density lipoproteins (HDL) and reduction in total cholesterol, low-density lipoproteins (LDL), and TGs (Kwantes & Grundmann, 2015); mice fed with krill oil demonstrated lower infiltration of inflammatory cells into the joint and synovial layer hyperplasia (Ierna et al., 2010); DHA levels in rats brain increased after krill oil consumption and it may protect from depression-like behaviour also it can improve learning acquisition and working memory (Wibrand et al., 2013). Despite studying the health benefits, numerous research endeavors worldwide are focusing more in comprehending the mechanisms underlying its action, conducting safety and toxicity studies, and enhancing the stability of krill oil through encapsulation and emulsification processes along with the combination of this compound with other bioactive components (Fig. 1). Therefore, the aim of this review is to provide a comprehensive examination of the potential health advantages associated with krill oil, exploring diverse encapsulation and emulsification methodologies detailed in the literature, and addressing relevant safety considerations associated with the prolonged consumption. Additionally, the authors attempt to consolidate insights into the synergistic effects of combining krill oil with other bioactive compounds, while also examining the market products. Furthermore, this review seeks to merge existing knowledge, identify research gaps, and offer valuable insights on the advantages and limitations of utilizing krill oil as a nutraceutical in various health aspects.

Fig. 1.

Network of keywords

Krill oil: nature’s nutrient powerhouse

Krill oil is commonly sold as a dietary supplement and is rich in omega-3 fatty acids, mainly EPA and DHA. These fatty acids are similar to those found in fish oil, but in krill oil, they are bound to phospholipids, mainly phosphatidylcholine (Backes & Howard, 2014). This phospholipid form of omega-3 fatty acids is known to be better absorbed by the body than the triglyceride form found in fish oil (Schuchardt et al., 2011). In addition to omega-3 fatty acids, krill oil contains phospholipid-derived fatty acids, choline, and astaxanthin. The astaxanthin content gives krill oil its red color and provides antioxidant benefits (Akanbi & Barrow, 2018). The overall phospholipid content of krill oil is high, ranging from 39.29% to 80.69%, with phosphatidylcholine being the most prominent phospholipid (Fu et al., 2023).The fatty acid profile of krill oil appears to be 26.1–30.7% saturated fatty acids, 24.2–25.9% monounsaturated fatty acids, and 34.1–48.5% omega-3 fatty acids. The omega-3 fatty acids are mostly bound as phospholipids, with some bound as triglycerides. The phosphatidylcholine content has been calculated to be 34 ± 5 g per 100 g oil (Xie et al., 2019). Thus, the increasing popularity of krill oil as a source of omega-3 fatty acid is due to its abundant nutrient composition, providing a strong substitute for conventional sources such as fish oil. Krill oil is becoming more and more popular as it can successfully reduce total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and triglyceride levels. These effects are similar to those shown with fish oil use at similar dosages. This shift in preference is significant as it addresses concerns regarding the finite nature of fish and fish oil resources, highlighting the sustainable appeal of krill oil as a viable nutritional supplement (Son et al., 2021).

Similarly, additional studies have also shown the lowering effect of krill oil against the factors which are otherwise responsible for the cardiovascular diseases (Huang et al., 2023). Furthermore, phosphatidylcholine is the primary phospholipids in krill oil, composed of nearly 60–70% omega-3 fatty acids bound to it (Ramprasath et al., 2015). The interlinkage of phospholipid and long-chain omega-3 fatty acids make the transfer of fatty acid molecules easier through the intestinal wall, enhancing bioavailability and also, improving the omega-3: omega-6 PUFA ratio. In several investigations, krill oil has lowered serum triglyceride in a manner which is dependent on dosage (Ramprasath et al., 2015; Rundblad et al., 2018). Numerous studies, both in animals and humans, had also suggested that krill oil may be better than fish oil in improving plasma and red blood cell, n-3 PUFA levels and also improving cardiovascular disease risk markers (Ramprasath et al., 2015; Schuchardt et al., 2011). These characteristics of krill oil theoretically make it a better replacement for fish oil (Ramprasath et al., 2015). Furthermore, in another study, Sun et al. (2017) reported that when krill oil is fed to mice, it resulted in lesser body weight gain, less accumulation of fat in tissues such as liver and adipose. In addition, dyslipidemia generated by high fat diet was somewhat alleviated by krill oil feeding with considerable reduction of blood low density lipoprotein-cholesterol (LDL-C) content. Further, Bunea et al. (2004) discovered that by considerably lowering levels of triglycerides, low density lipoprotein, and total cholesterol while raising levels of high-density lipoprotein at lower and equal dosages, krill oil is useful for the treatment of hyperlipidemia. In addition, mildly obese males who receive continuous krill powder treatment have lower levels of anandamide and plasma triglycerides. There is not much doubt that krill oil can help manage joint illness because of its positive impact on the biological process of inflammation (Berge et al., 2013). Krill oil has ability to inhibit the inflammation and reduces arthritic symptoms within a short treatment period. Sampalis et al., (2003) observed that krill oil is far more effective than omega-3 fish oil for the full management of premenstrual symptoms, and that it can greatly reduce dysmenorrhea and the emotional symptoms of premenstrual syndrome. Research showed that krill oil was significantly more effective than fish oil for the reduction of glucose (Maki et al., 2009). Sun et al. (2017) found that krill oil consumption enhanced the metabolism of glucose in mice, resulting in a glucose tolerance of roughly 22% compared to 32% of Area under curve. The rapid blood glucose levels for the high fibre, krill oil, and normal control groups were 8.5 mmol/L, 9.8 mmol/L, and 9.3 mmol/L, respectively, under the curve for the krill oil vs. high fibre diet. Furthermore, krill oil feeding decreased oxidative damage in the liver by increasing the amount of superoxide dismutase and decreasing the amount of malondialdehyde. It has been proposed that dietary modifications, such as increasing protein and fatty acid intake, may have therapeutic benefits for maintaining older people’s health and muscle function (Bhattacharya et al., 2021).

Mechanism of action

Krill oil is a complex mixture of vitamins, flavonoids, PUFAs, and astaxanthin that has a variety of pharmacological effects that are mediated by a number of different pathways. This important point was in detailed clarified by Colletti et al. (2021), which highlights the complex mechanism of action that underlies the health benefits of krill oil. The (n-3) PUFAs, particularly EPA and DHA, which act as natural ligands for peroxisome proliferator-activated receptors (PPARs), are essential to its therapeutic potential (Rincón-Cervera et al., 2020) PPARα, which is mostly present in hepatic cells, controls lipids accumulation (Wang et al., 2020), whereas PPARγ, which is present in inflammatory cells and adipose tissues, controls insulin sensitivity, adipocyte differentiation, and has anti-inflammatory properties (Ma et al., 2018). Furthermore, the activation of PPARγ by EPA and DHA further involves the expression of FAT/CD36, a transmembrane fatty acid transporter, facilitating the cellular uptake of these essential fatty acids (Zhang et al., 2021). Interestingly, FAT/CD36 expression is also regulated by PPARγ, which leads to a positive feedback loop that increases PUFA uptake and eventually encourages the synthesis of adiponectin (Monsalve et al., 2013). Furthermore, EPA and DHA exhibit anti-inflammatory effects beyond PPAR activation. By limiting the release of proinflammatory mediators PUFAs have the ability to reduce inflammatory responses by means of a non-covalent interaction. This reduction in inflammatory responses leads to a decrease in the release of interleukin-6 and tumour necrosis factor alpha after lipopolysaccharide stimulation (Korbecki & Bobiński Rafałand Dutka, 2019). The G protein-coupled transmembrane receptor GPR120 is a key component in the pharmacological actions of krill oil. GPR120 is activated by EPA and DHA, which starts intracellular signalling cascades that raise cAMP and Ca2 + concentrations, hence encouraging extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation (Son et al., 2021). For instance, EPA and DHA are essential for controlling inflammatory processes taking place in adipose tissue. The effect on GPR120 has been studied; DHA inhibits Jun N-terminal kinase phosphorylation and the activation of the inhibitor of nuclear factor-κB (IκB) kinase complex, which reduces the amount of tumor necrotic factor-α released by lipopolysaccharides-treated macrophages (Si et al., 2016). Its role was validated by GPR120 knockdown, and DHA promoted the GPR120-βarr2 complex’s formation to counteract pro-inflammatory stimuli from lipopolysaccharides exposure. Through their respective GPR120s, DHA and EPA dramatically reduced the expression of Monocyte chemoattractant protein-1, Interleukin-1β, and tumor necrotic factor-α genes in 3T3-L1 adipocytes (Talukdar et al., 2010). A detailed mechanism of action of krill oil in human body is given in Fig. 2. Beyond its PUFA content, astaxanthin is largely responsible for krill oil’s antioxidant properties. By triggering nuclear factor erythroid 2-related factor 2 (Nrf-2), a transcription factor that regulates the antioxidant machinery, this potent antioxidant reduces oxidative stress (Barros et al., 2014). Astaxanthin’s defence against oxidative damage is facilitated by its capacity to increase the synthesis of antioxidant molecules and activate antioxidant enzymes. Furthermore, the antioxidant properties of astaxanthin go beyond scavenging reactive oxygen species as it also inhibits insulin resistance linked to the activation of several kinases, including Jun N-terminal kinase, via reducing oxidative stress (Solinas & Becattini, 2017). This multimodal antioxidant action translates into improved glucose metabolism, insulin sensitivity, and GLUT4 translocation, with potential therapeutic implications for the management of type 2 diabetes (Feng et al., 2020).

Fig. 2.

Underlined mechanism of action of krill oil in different part of the human body

Krill oil in combination with another bioactive compounds

To optimize health and well-being, researchers have increasingly explored the synergistic effects of combining krill oil with other bioactive compounds. For instance, it was investigated that combining Krill oil combined with Bifidobacterium animalis F1-7 significantly reduced the inflammatory response and lipid metabolism imbalance associated with atherosclerosis. By influencing the TLR4/MYD88/NF-B pathway, it may be able to significantly reducing the inflammatory response. Moreover, it improves the atherosclerosis by promoting metabolism of bile acid and controlling levels of cholesterol via the FXR/FGF15/CYP7A1 pathway. Therefore, compared with Bifidobacterium animalis F1-7 and Krill oil alone, their combination depicted obvious benefits in anti-inflammatory effects. The Bifidobacterium animalis further enhanced the capability of krill oil to reduce inflammation and to protect the intestinal mucosal barrier, thereby improving atherosclerosis (Liang et al., 2021). Similarly, in another study liver index, adiposity index, incline in body weight, and levels of triglycerides, total cholesterol, and LDL-cholesterol was investigated when diet supplemented with the combination of fish and krill oil was given to mice. Interestingly, the effects were more pronounced in male mice compared to the female mice. The authors noted that the variation could have resulted from the variable gut microbiota composition of male and female organisms, which exhibits differing reactions to the oil treatment. Following the oil treatment, the researchers observed a reduced Firmicutes to Bacteroidetes ratio in male mice as compared to female mice at the phylum level. It is well recognized that this ratio is connected to both food-based energy absorption and obesity. Furthermore, specific bacterial genera displayed distinct reactions to the oil treatment in male and female mice at the genus level. For example, after the oil treatment, male mice had higher concentrations of certain potentially advantageous bacteria (like Barnesiella), while female mice showed different patterns of some bacteria (such Helicobacter and Prevotella) (Han et al., 2018). In addition, krill oil in combination with Vitamin D and Lactobacillus reuteri also resulted in the reduction of inflammation, enhancement in epithelial barrier integrity and rebalancing the gut microbiota (Costanzo et al., 2018). In addition, similar combination like krill oil with Astaxanthin and Hyaluronic acid studied by Park et al. (2020) found to reduce the serum biomarkers associated with articular cartilage degeneration, along with reduced expression of pro-inflammatory cytokines and mediators in knee joint tissue. Mechanism of action of these combinations is given in the Table 1. However, there is still a scope of understanding full mechanism behind the effects of these combinations. Continued investigation through additional research and clinical trials is essential to fully understand the therapeutic capabilities and refine the application of these synergistic combinations for enhancing overall health and well-being.

Table 1.

Krill oil in combination with another component

	Component (Species/bioactive compounds)	Effect of the combination	Mechanism	References
1	Bifidobacterium animalis subsp. lactis F1-7	Improved the lipid metabolism disorder and inflammatory response associated with atherosclerosis	It regulates the farnesoid X receptor (FXR)/cholesterol 7-alpha hydroxylase (CYP7A1) pathway to reduce lipid accumulation. It improves the inflammatory response by downregulating the Toll-like receptor 4 (TLR4)/myeloid differentiation factor 88 (MyD88) pathway	(Liang et al., 2021)
2	Fish oil	Oil treatment had a greater efficacy in obesity control in male mice compared with female mice	The level of total cholesterol, the potential effects of sex hormones and the distribution and composition of the adipose tissue between sexes are extremely different, indicating that diet oil may influence males and females differently	(Han et al., 2018)
3	Lactobacillus reuteri and vitamin D	Krill oil, Lactobacillus reuteri, and vitamin D mixture reduces the gut inflammation	Levels of proinflammatory cytokines, TNF-α, IL-1β, and IL-6, were significantly down-regulated in the inflamed tissues of KLD + DSS (dextran sodium sulphate)-treated mice as compared to DSS-treated mice, while the anti-inflammatory cytokine IL-10 and vitamin D receptor were significantly increased	(Costanzo et al., 2017)
4	Astaxanthin and Hyaluronic acid	Mixture of krill oil, astaxanthin, and hyaluronic acid in a rat model of osteoarthritis induced by monosodium iodoacetate ameliorated joint pain and decreased the severity of articular cartilage destruction in rats	Significantly reduced serum levels of the articular cartilage degeneration biomarkers cartilage oligomeric matrix protein and crosslinked C-telopeptide of type II collagen, and the pro-inflammatory cytokines tumor necrosis factor alpha, interleukin-1β, and interleukin-6, as well as mRNA expression levels of inflammatory mediators, inducible nitric oxide synthase and cyclooxygenase-2, and matrix-degrading enzymes, matrix metalloproteinase (MMP)-2 and MMP-9, in the knee joint tissue	(Park et al., 2020)
5	Lactobacillus rhamnosus	Stable viability of probiotic under different storage temperature (4 to 25℃) when co-microincapsulation with krill oil by spray drying	The content of phospholipids, MUFA, and PUFA was retained in the co-microcapsules	(Zavaleta et al., 2022)

Scope of emulsification and encapsulation of krill oil

Encapsulation

Encapsulation can be defined as the method of entrapping an active agent within another substance which acts as a protective barrier. It is a technology that involves applying a physical barrier to safeguard bioactive components from the adverse environmental conditions (Fig. 3). Functional compounds like vitamins, minerals, polyphenols, essential oils, unsaturated fatty acids, PUFAs which are susceptible to degradation processes due to heat, light and moisture can be encapsulated and prevented (Reque & Brandelli, 2021). Krill oil, offers a new abundant source of EPA and DHA on the market, with a biomass estimated between 500 and 2500 million (Martin, 2007). As compared to other marine oils, Krill oil contains a high proportion of omega-3 fatty acids bound to phospholipids and diverse naturally occurring antioxidants, mainly astaxanthin (Deutsch, 2007; Massrieh, 2008). However, its incorporation in foods is limited due to its low solubility in the hydrophilic media (S. Liu et al., 2010) and its oxidative instability (Bustos et al., 2003). Microcapsules are considered as an effective method for the oxidative stabilisation of edible oils, and is used for the protection and the delivery of functional lipids in food applications (Bustos et al., 2003). Formation of microcapsules by complex coacervation involves the electrostatic attraction between two biopolymers of opposing charges (Liu et al., 2010). Some of the studies have shown the effect of complex coacervation of krill oil on its oxidative stability and solubility. For instance, (Aziz et al., 2014) conducted a study focusing on gelatin-gum Arabic multinuclear microcapsules containing krill oil, produced by coacervation process. They employed a three-level-by-three-factor Box–Behnken design to assess the effects of the core material to wall ratio, stirring speed, and pH on encapsulation efficiency. Their experimental results indicated pH as the most significant factor affecting the encapsulation efficiency of krill oil, exhibiting both linear and quadratic effects, along with a bilinear effect with the core material to wall ratio. Stirring speed, however, did not show any significant effects. Optimal conditions yielding 92% encapsulation efficiency were determined as follows: core material to wall ratio of 1.75:1, pH of 3.8, and stirring speed of 3. The resulting multinuclear microcapsules, formed via complex coacervation without cross-linking agents, exhibited circular morphology and adequate stability. The authors highlighted that multinuclear microcapsules generally offer superior controlled release properties as compared to the mononuclear designs, releasing core material slowly even upon complete wall destruction. On the other hand, mononuclear microcapsules with reservoir-type structures tend to release their contents rapidly, even when the wall is only partially damaged. Furthermore, the multinuclear capsules showed enhanced stability, reduced susceptibility to breakage, and higher encapsulation efficiency. In a related study by, Kermasha et al., (2018), esterified krill oil, obtained from the transesterification of krill oil with 3,4-dihydroxyphenylacetic acid was encapsulated, via complex coacervation using gelatine and gum-arabic in the presence of phenolic acids which further resulted in the formation of phenolic lipids. This investigation highlighted the significant influence of pH change on microcapsule morphology, encapsulation efficiency and storage stability. The author observed that the change in pH regulates the balance of charges with in the molecules thereby modulating the intensity of electrostatic interactions which are otherwise crucial for the complex formation. Additionally, apart from the pH, different emulsification devices affect the encapsulation efficiency and the size of the microcapsules. The authors indicated that ultrasonic liquid processor produced a homogenous, stable emulsion, free of particles as compared to the one obtained in homogeniser. These findings may be attributed to the fact that the ultrasonicator applies shock waves using differential pressure. This may have prevented the interactions between the gelatine and phenolic lipids which otherwise resulted in the pH change of the system, thus providing a stable homogeneous emulsion as compared to the homogeniser. Similarly, Shi et al., (2018) also focused on the encapsulation of esterified EPA/DHA to phospholipids particularly phosphatidylcholine, however the wall material used in this study was krill protein isolate which was obtained through the method of isoelectric solubilization/precipitation followed by freeze-drying. The isoelectric solubilization/precipitation method showed potential as a wall material to microencapsulate krill oil and thus, expand application of krill oil/protein for human consumption. The feasibility of yeast cells for encapsulation of krill oil though freeze-drying and homogenization was analysed by Fu et al. (2021). Yeast cells membranes acted as a good all-natural alternative microencapsulation technology system as yeast cells allows diffusivity of lipid molecules. The oxidative stability of the capsule in terms of peroxide value was analysed, as peroxide value of Krill oil-Yeast cell complex was lower as compared to krill oil due to fact that structure of Yeast cell membrane provides double carbohydrate wall/lipid membrane provided shelf-stable product. The bio accessibility of DHA and EPA was much higher in Krill oil-Yeast cells (69.62% and 66.67% respectively), this effect can be explained by how the digested particles affect the mixed micellar phase’s ability to solubilize particles. Chitosan, a cationic polysaccharide, is also commonly utilized for lipophilic chemical encapsulation and transport because of its low toxicity, biocompatibility, non-antigenicity, and biodegradability by Haider et al. (2017) who suggested that even after two weeks of storage, Krill Oil-loaded Chitosan Nano Particles displayed a slight reduction in band shifts of ROOH and triglyceride ester groups, indicating greater krill oil stability in Chitosan Nano Particles.

Fig. 3.

Methods to enhance the oxidative stability of krill oil

Furthermore, by limiting krill oil’s exposure to oxygen, various drying methods can be utilized that can effectively encapsulate the oil and stabilize it. As previously discussed, an essential first step in creating a stable encapsulated krill-oil capsules are choosing the wall materials. The wall material should have low oxygen diffusivity and a dense coating film acting as an oxygen-transfer barrier in order to produce oxidative stable krill oil. There are some studies that analyze the efficiency of microencapsulation of krill oil using different wall materials like maltodextrin, Gum Arabic and combination of both. For instance, Takashige et al. (2018) reported the encapsulation of krill oil by spray drying using a saponin, Quillayanin, as an emulsifier and maltodextrin as a wall material. Similarly, Sultana et al. (2021a) prolonged the shelf-life and prevented the oxidation of PUFAs of krill oil by encapsulating it with maltodextrin as wall material and Quillaja saponin as emulsifiers. The retentions of EPA and DHA in spray-dried powders immediately after spray drying were 99% and 100%, respectively, for samples subjected to high-pressure homogenization. The author also observed that the retention of EPA in krill oil microencapsulated with Maltodextrin was higher than that of DHA because DHA is more readily oxidized than EPA, even when it takes the form of fatty acids or esters, respectively.

Furthermore, Freeze drying and spray drying are common dehydration techniques used to create stable nano emulsions for encapsulation of krill oil. El-Messery et al., (2020) investigated the effect of two different dehydration approaches in respect of encapsulate stability, chemical stability of encapsulated krill oil, and in vitro bioaccessibility of encapsulated krill oil (%) with in a nano-emulsion system. For this process the combination of three different biopolymers; whey protein concentrate, maltodextrin, and gum Arabic-as a stabilizers and wall materials for the encapsulate was used. The study observed a significant increase in particle size, approximately 7-folds in the spray drying process, which may be due to the high temperature of the process that might have led to the oxidation of krill oil on the outer surface of the encapsulates and induced the Maillard reaction between maltodextrin and whey protein concentrate. Furthermore, the krill oil concentration with 8% (w/v) exhibited optimum encapsulation efficiency in both freeze-dried and spray-dried samples. In terms of in-vitro bioaccessibility, the spray-dried nanoemulsion with 8% (w/v) krill oil concentration had higher value compared to freeze-dried counterparts making the latter process good in facilitating better release and absorption of the encapsulated krill oil during digestion followed by lower permeability to gases which provided extra protection to core material, thus resulting in higher encapsulation efficiency. Similar to this, microencapsulation of krill oil using spray drying was claimed to be an efficient method by Ortiz Sánchez et al. (2021) using air inlet/outlet temperatures of 160/80 °C which appears to be suitable technique for addressing the instability and low solubility of krill oil and aqueous media while also ensuring consumers access to astaxanthin and omega-3 polyunsaturated fatty acids. Additionally, the analysis of the encapsulated krill oil revealed a significant content of EPA at 3.11%, DHA at 1.57%, and a low omega-6/omega-3 ratio of 0.05, which suggest that the microencapsulated krill oil could potentially offer beneficial effects to human health. Furthermore, various authors are currently exploring a novel technology known as nanostructured lipid carriers, serving as delivery system which enhances the solubility of krill oil in water while also minimizes oxidation. Nanostructured lipid carriers derived from oil in water (O/W) nanoemulsions holds a significant potential to serve as a carrier system for bioactive compounds present in foods. Their small size, high entrapment efficiency and the potential of scalability, has made them very promising to the food industry (Takashige et al., 2018). Nanostructured lipid carriers are composed of solid lipid, liquid lipid, surfactant and water as major ingredients, thus both solid and liquid lipids exist in the lipid phase of Nanostructured lipid carriers at room temperature. Incorporating liquid oil into the core of the solid lipid in Nanostructured lipid carriers enables simultaneous entrapment of bioactives dissolved in the liquid oil within the solid lipid, resulting in enhancement in the drug loading as well as in the controlled drug release. Moreover, this system also leads to the improvement in the bioavailability and the nutritional value of bioactive compounds along with the increase in the functionality (shelf-life, consumer acceptability and safety of foods) and offering controlled release of the entrapped nutrients. Ultrasonication is a processing technique which is used to prepare nanostructured lipid carrier loaded with krill oil using solid lipid. Encapsulation efficiency of EPA and DHA in this optimized nanostructured lipid carrier was found to be high which might be due to high solubility and low diffusion coefficient of EPA and DHA in the phospholipid form. Furthermore, may be due to the more space which have been provided by the imperfectly crystalline nanostructured lipid carrier. Similar results were reported by Zhu et al. (2015) who also prepared nanostructured lipid carrier based encapsulates containing krill oil prepared from palm stearin as a solid lipid whereas lecithin was used as a surfactant. The resultant nanostructured lipid carrier exhibited a spherical or ovoid structure, characterized by small dimensions (< 150 nm), a narrow polydispersity index (< 0.2), and notably high entrapment efficiency (> 96%). The analysis on the basis of Differential scanning calorimetry revealed a less-ordered crystalline structure further an indicative of enhanced loading capacity. Furthermore, the nanostructured lipid carrier demonstrated efficiency in shielding bioactive compounds within the krill oil from photooxidation when exposed to UV light. Additionally, the feasibility of pasteurization and lyophilization was demonstrated, showcasing promising applications in functional beverages and milk powder fortification. Nanostructured lipid carrier containing krill oil was stable for up to 70 days at a temperature of 4ºC due to kinetic energy depression of nanoparticles at lower temperatures.

Emulsion

An emulsion is a heterogeneous system consisting of at least one immiscible liquid dispersed in another in the form of droplets. It is a mixture of two or more immiscible liquids under specific transforming process which will adopt a macroscopic homogeneous aspect (Perrin et al., 2023). Emulsions of krill oil, a rich source of antioxidants, have been developed to address various issues such as odor, particle size distribution, and stability research studies have explored the use of different emulsifiers and co-emulsifiers to optimize krill oil emulsions (Table 2). For example, a study by Zhao et al. (2020) found that Antarctic krill oil emulsion could be created using Tween-80 as an emulsifier, with combination of oil concentration of 1 percent (v/v) and a ratio of surfactant to oil phase of 1:5 (v/v). In this study droplet analysis revealed that particles were mostly spherical, the dispersion was uniform and the size was mostly less than 100 nm. Another study by Liu et al. (2021) developed high internal phase emulsions of Antarctic krill oil diluted by soyabean oil using casein as a co-emulsifier, which showed that increasing the krill oil level subsequently led to a more viscoelastic interfacial membrane, reducing lipid oxidation and improving stability. These studies demonstrates that the choice of emulsifiers and co-emulsifiers is crucial for the stability and functionality of krill oil emulsions. The use of high internal phase emulsions for instance, allows for higher concentrations of hydrophobic bioactives and a semi-solid viscoelastic structure that can slow lipid oxidation and improve stability. Along with surfactants, to analyze and control the microbial activity catalase could be added. The addition of catalase to krill oil emulsions can lead to increased Thiobarbituric acid reactive substances and hydroxyl radicals, which are indicators of lipid oxidation. However, this approach is not suitable for improving emulsion stability, as catalase is not an effective antioxidant for lipid oxidation in emulsions (Zheng et al., 2021). Instead, antioxidant proteins and protein hydrolysates, such as Chlorella pyrenoidosa, enhances oxidative stability of krill oil emulsions which could be used as both antioxidants and emulsifiers. Addition of Chlorella pyrenoidosa resulted in inhibition of linoleic acid oxidation and the emulsion remained stable for 1 month (Liu et al., 2022). In addition to that, commercial emulsifiers like Tween 80, Tween 20 and Span are commonly used in oil-in-water emulsions in the food industry. However, these emulsions are prone to oxidation due to the presence of lipid oxidation promoting components in the aqueous phase. Therefore, to address this issue, antioxidant and protein hydrolysates can be used and improving the oxidative stability of emulsions (Liu et al., 2022). Polar and non-polar antioxidants such as Trolox and alpha-tocopherol could potentially affect the physical as well as oxidative stability of oil emulsions prepared by altering pH with acetic acid buffer (Wu et al., 2016). The addition of Trolox was found to inhibit lipid oxidation, whereas the addition of 50 µm ferrous (II) chloride resulted in a prominent rise in both lipid hyperoxides and Thiobarbituric acid reactive substances after 1 day of storage (Wu et al., 2016). This could be attributed with high concentration of antioxidant rich phospholipids in krill oil, which can form antioxidative compounds like Stecker aldehydes and pyrroles, thus inhibiting non-enzymatic browning and oxidation and negative charge on emulsion droplet led to increased reactivity of iron in emulsion (Wu et al., 2016). Fish gelatin or Maillard reaction products have also been found to influence oxidative stability of krill oil emulsions, with emulsions containing these additives being shelf-stable for up to 25 days due to their antioxidative activity (Shen et al., 2014). Oral bioavailability of Krill oil could be increased with addition of hydroxymethyl cellulose powder along with lysolecithin and glycerin. Results indicated that addition of glycerin resulted in finer particle formation with uniform droplets. Moreover, a better hypotriglyceridemic function of Krill oil could be noticed after oral administration of formulation in rats with hypertriglyceridemia (Seto et al., 2018).

Table 2.

Encapsulation and emulsion techniques used for krill oil

S.no	Carrier material	Techniques/Processing conditions	Results	Reference
1	Krill protein	Freezing followed by Freeze drying: • Temperature: − 24 °C • Time: 24 h • Freeze-drying for 72 h	• Microcapsules contained ω-3 PUFAs at 43–60 (EPA and DHA at 28–41 and 9–11 g/100 g of total fatty acids respectively) • Emulsion Stability Index: 1.0	(Shi et al., 2018)
2	Maltodextrin	Spray drying: • Feed rate: 20 mL/min • Atomizer speed: 10,000 rpm • Air flow rate: 110 kg/h	• Surface-oil ratio: EPA: 4.2 wt% DHA: 52.2 wt% • Particle Size: 0.48– 9.29 μm • EPA and DHA stable at 25 °C	(Sultana et al., 2021a, b)
3	Yeast cells	Homogenization followed by freeze-drying/ Homogenization: • Speed:10,000 rpm • Time: 5 min Freeze drying: • Temperature: − 80 °C • Pressure: 0.001 mbar • Time: 48 h	• Bioaccessibility: DHA: 69.62% ± 7.67% EPA: 66.67% ± 4.55% • Peroxide Value: 8.78 ± 0.07 meqvO2/kg oil	(Fu et al., 2021)
4	Nanostructured Lipid Carrier	Homogenization followed by ultrasonication/ Homogenization: • Speed: 25,000 rpm • Time: 2 min Ultrasonication: • Power: 380 W • Time: 8 min	• Particle size: 112 nm • Polydispersity Index: 0.270 • Zeta potential: -30.8 mV • Entrapment efficiency of EPA and DHA: 99%	(Lin et al., 2020)
5	Nanostructured lipid carriers	Agitation followed by Ultrasonication/ Agitation: • Speed: 10,000 rpm • Time: 5 min Ultrasonication: • Power: 200 W • Time: 5 min	• Particle Size: 143.5 nm • Polydispersity Index: 0.153 • Zeta Potential: − 57.4 Mv • EE: > 96% • Physical and chemically stable during 70-day storage at room temperature and 4 °C	(Zhu et al., 2015)
6	Maltodextrin	Spray Drying	• Activation energies: EPA: 53.7 ± 9.2 kJ/mol DHA: 59.7 ± 12.0 kJ/mol	(Jimenez et al., 2021)
7	3,4-dihydroxyphenylacetic acid	Complex coacervation/ • Speed:154 rpm • Time: 24 h • Temperature: 60 °C	• Particle Size: Homogeniser: 626–677 µm Ultrasound: 608–515 µm • Oxidative stability- 25 days • EE: 80.86 ± 4.97 to 92.58 ± 0.38	(Kermasha et al., 2018)
8	Chitosan-TPP (tripolyphosphate)	Homogenization and agitation followed by freeze-drying Homogenization: • Speed: 13,000 rpm • Time: 1 min Agitation: • Time: 40 min Freeze-drying: • Time: 72 h • Temperature: -35 °C	• Loading Capacity (LC): 8.8–24.7% • EE: and 33.3–58.9% • Particle Size: 229.5–182.4 nm • Polydispersity Index: 0.199 • Zeta potential: 37.7 mV	(Haider et al., 2017)
10	Carboxymethyl chitosan (CMCS)	Homogenization followed magnetic stirring: Homogenization: • Speed: 19,000 rpm • Time: 2 min • Pressure: 800 bar Magnetic stirring: • Dripping speed: 1drop/s • Mixing speed: 300 rotation/min	• Oil extraction yield—86.02% • Zeta potential: -38.43—42.09 with CMCS concentration from 0.0–0.5% • Particle size: 185.07- 228.16 nm with CMCS concentration from 0.0–0.5% • EE- 85.4- 86.45% with CMCS concentration from 0.0–0.5%	(Zhou et al., 2020)
11	Maltodextrin and Gum arabic	Spray drying and freeze-drying Spray drying: • Flow rate: 2.5 m3/min • Pressure: 0.06 MPa • Inlet air temperature: 130℃ • Outlet temperature:71- 75℃ Freeze-drying: • Temperature: − 50 °C • Pressure: 0.04 mbar • Time: 48 h	• Bioaccessibility of oil: 8%(w/v) • EE: 62.2–78.8% • Particle Size: 153.9 nm -162.3 nm • Zeta Potential: 17.0 mV- 48.8 mV	(El-Messery et al., 2020)
12	Beef-hide gelatin	Homogenization followed by stirring for complex cocervation: Homogenization: • Speed: 20,500 and 30,000 rpm • Time: 5 min Stirring: • Time: 5 min • Temp: 45 ± 3℃ • Speed:400 rpm	• EE: 96% • Particle Size: 412.515 mm • Storage stability: 25 days at 25℃	(Aziz et al., 2014)

S.no	Emulsification technique/ Type of emulsifier	Process conditions	Results	Reference
1	Agitation/Tween 80 and Span 80	• Agitation speed: 800 r/min • Agitation temperature: 25 ± 1 ºC • Surfactant (Tween80 & Span 80) ratio: 4:6, 5:5, 6:4, 7:3, 8:2, 9:1 and 10:0 • Surfactant phase: oil phase (Krill oil and solvent) = 8:2 Surfactant phase: oil phase = 5:5	• Particle size: 27.47 nm • Droplet shape: spherical • Size of droplets: ≤ 100 nm • Pseudoplastic fluid • Stability: 1 month (refrigeration)	(Zhao et al., 2020)
2	Ultrasound /Chickpea protein isolate and Ginseng powder	• Sonication power: 600 W • Probe diameter: 13.0 mm • Frequency: 20 kHz Ultrasound time: 5,10,15,20 and 25 min	• Particle size: 183.70–232.20 nm • Zeta potential: -28.00 to -35.75 mV • Peroxide value: 41.37 to 232.01 mg/L (24 days)	(Xu et al., 2021)
3	Homogenization and micro fluidization/ Krill oil	• Homogenization time: 4 min • Microfluidizer pressure: 9 Kbar pH: 7	• Particle size: 150–165 nm • Zeta potential: -25 to -17 mV • Phospholipid concentration (Krill oil): 60% • Phospholipid concentration (emulsion-aqueous phase): 60%	(Wu et al., 2016)
4	Isoelectric solubilization and homogenization/Krill oil	• Isoelectric solubilization/precipitation followed by homogenization • Isoelectric solubilization temperature: 4 ºC • Processing time = 60 min • Centrifugation: 10,000 × g for 10 min Homogenization • Speed: 3000 rpm • Time: 5 min Time for refrigeration: 24 h	• Enacpsulation efficiency: 15–80 g/100 g • Loading efficiency: 4–15 g/100 g Fatty acid profile • Polyunsaturated fatty acid (PUFAs): 43.0–59.8 g/100 g EPA: 28.4–40.8 g/100 g • DHA: 8.6–11.3 g/100 g	(Shi et al., 2018)
5	Homogenization/ Fish gelatin and protein species	Heat treatment • Speed: 1650 rpm • Temperature: 60 ºC • Time: 7 min Homogenization KO-in- water emulsion • Speed: 7000 rpm Time: 15 min	• Zeta potential: -25.9 ± 1.0 mV • EPA and DHA: 98.4–102.3% • Particle size: 10–16 µm • EPA: 11.30–11.59 mg/g • DHA: 5.22–5.29 mg/g • Stability: 25 days	(Shen et al., 2014)
6	Agitation/ HPMC (hydroxypropyl methylcellulose), lysolecithin and glycerine	• Agitation and vortex (room temperature) • Formulations: KO (57, 60 and 95) + lysolecithin (29,30 and 0), glycerine (9,10 and 0) and HPMC (5,0,5)	• Droplet mean diameter: 260-270 nm • Zeta potential: -36 to -32 mV	(Seto et al., 2018)
7	Mixing/Chlorella protein hydrolysate (CPH)	• Formulation: CPH in two forms neutral (NCPH) and alkaline (ACPH) proteases + Krill oil (1.25, 2.5, 5, 10 and 20 mg/mL) concentrations Tween 20 emulsions used as control	• Particle size NCPH: 257 nm ACPH: 201 nm • Zeta potential: -30 mV to -45 mV • Stability: 30 days	(Liu et al., 2022)
8	Mixing/ Soyabean oil	Mixing • Speed: 8000 rpm • Time: 60 s • 10 ml of casein suspension + oil phase Formulations: • Oil phase: soyabean oil (70, 75,80,85,90,95 and 100%) and AKO (Antarctic krill oil) (5,10,15,20 and 30%) concentrations	• Particle size: 10.47–16.07 µm • DHA: 8.73–174.62 mg/g • EPA: 115.77–5.79 mg/g	(Liu et al., 2021)
9	Homogenization/Tween 80, lecithin and span-20	• Homogenization • Speed: 18,000 rpm Time: 3 min	• Particle size: Tween-80: 145.1 nm Lecithin: 378.6 nm Span-20: 184.2 nm • Zeta potential: Tween-80: -40.6 mV Lecithin: -68.2 mV • Span-20: -35.1 mV	(Zheng et al., 2021)
10	Mixing/ Glycerol	• Three separate batches of a 5% KO-in-water aseptically + glycerol (25.0 g) • pH = 6.89	• KO emulsion inhibited the macrophage binding of LPS to the TLR4 by 50% (at 12.5 g/mL) • 75% at 25 g/mL • 50 g/mL, completely abolished the LPS binding	(Bonaterra et al., 2017)

EE encapsulation Efficiency

Ultrasonication used for modifying proteins and improving their functional properties, such as emulsifying, solubility and foaming properties. In the context of krill oil emulsions, ultrasound method could be employed to enhance the formation and stability of emulsions Xu et al. (2021). The author demonstrated that ultrasound treatment with a 15-minute duration could be used to prepare microemulsions with chickpea protein isolate and ginseng powder as co-emulsifiers, which could potentially increase the exposure of buried binding sites like hydrophobic residues and thiol groups, thus enhancing the binding affinity of chickpea protein isolate to ginseng saponin.

Furthermore, isoelectric solubilization/ precipitation is another isolation technique used for proteins, which can then be employed as a wall material in krill oil microemulsions. To exemplify, Shi et al. (2018) worked on the feasibility of Krill protein isolated with isoelectric solubilization/precipitation approach, achieving good emulsion efficiency and loading efficiency of 4-15 g/100g. These efficiency parameters significantly depend on the kind of oil used for microcapsule manufacturing and are affected by pH, stirring speed and cross-linking agents as well as process variables such as core material to the wall ratio (Aziz et al., 2014).

Oxidative stability

Krill oil, a known source of omega-3 fatty acids and antioxidants, requires effective encapsulation to enhance the oxidative stability and prolong its shelf life. Various encapsulation techniques have been developed to protect krill oil, such as microencapsulation, coacervation, and inclusion complexes (Takashige et al., 2018). All these methods aim to provide a physical barrier between the krill oil and the external environment aiming to reduce the risk of oxidation. The type of wall materials used in encapsulation plays a significant role in the determining the oxidative stability of krill oil. Wall materials like proteins and polysaccharides can form a protective layer around the krill oil, preventing it from coming into contact with oxygen and other oxidizing agents. For instance, Fu et al. (2021) investigated the oxidative stability of krill oil microcapsules prepared using yeast cells with respect to the krill oil prepared from water emulsion. Initially, both the samples showed a peroxide value of 0.70 ± 0.01 and 0.74 ± 0.08 meqvO₂/kg oil, respectively. After 15 days of storage, the peroxide value of microcapsules obtained using yeast cells increased to 8.78 ± 0.07 meqvO₂/kg oil, however the value is significantly lower than that of microcapsules prepared using water-emulsion (16.83 ± 0.91 meqvO2/kg oil. Similar results were obtained for the thiobarbituric acid reactive substances values (similarly accelerated during the storage). The particle structure of yeast cells, which included the mechanically strong cell wall with the reticulated network of mannoproteins and fibrous β-1,3-glucans, along with the lipid bilayer, inhibited the destructive effects of moisture, heat, light, and oxygen (Paramera et al., 2023). Similar production of nanoliposomes of krill oil using carboxymethylcellulose showed a decrease in peroxide value and thiobarbituric acid reactive substances values indicated that the addition of carboxymethylcellulose can effectively prevent the oxidation, improve the oxidative stability and inhibit the increase of turbidity, which is otherwise caused by leakage as well as aggregation of lipids (Zhou et al., 2020). In continuation to this, Haider et al. (2017) prepared encapsulates of krill oil using chitosan nanoparticles, and the oxidative stability of the encapsulates were determined by using Fourier-transform infrared spectroscopy. The study revealed that chitosan nanoparticle loaded krill oil exhibited a modest decrease in band shifts even after the prolonged storage as compared to bulk krill oil, indicating the enhanced oxidation prevention. This may be due to the reduced availability of hydroperoxides in chitosan nanoparticles, limiting their conversion to aldehydes and ketones (Kaur et al., 2024). Despite of the wall material; the consequent method used for the encapsulation, concentration of the material, along with the storage temperature plays an important role in determining the oxidative stability of the encapsulated material. However, despite of the selection of wall materials other factors like; the method of encapsulation, and material concentration, along with storage conditions, significantly influence the oxidative stability of encapsulated krill oil. Emulsification followed by the use of spray drying technique has been shown to be a particularly effective method in reducing oil droplet size and increasing surface area, thereby enhancing the efficiency of encapsulation and reducing susceptibility to oxidation. Particularly, spray drying enables the production of fine powder particles with enhanced dispersibility and stability, making it a preferred method for encapsulating omega-3 fatty acids like EPA and DHA present in krill oil (Sultana et al., 2021b). Similarly, El-Messery et al. (2020) recommended the spray drying method for the dehydration of krill oil nanoemulsion instead of freeze-drying. However, spray-dried and freeze-dried krill oil nanoemulsions demonstrated comparable oxidative stability at the 8% (w/v) concentration during the 15 days storage period but the author found that the sample with the same concentration of krill oil spray drying exhibited greater encapsulation efficiency of 72% in spray-dried nanoemulsion as compared to the freeze-dried nanoemulsion (67%). Moreover, storage temperature was found to be a critical factor influencing the long-term stability of encapsulated krill oil. Elevated temperatures accelerate oxidation reactions which leads to the degradation of EPA and DHA over time, as indicated by the approximately 5% retention of EPA and 0% retention of DHA after 6 months of storage at 50 °C by Sultana et al. (2021a). The Avrami equation, as utilized in the study, provides valuable insights into the kinetics of PUFA retention, with retention rates exceeding 80% after 6 months of storage at room temperature (25 °C). Thus, careful consideration of storage conditions, along with encapsulation parameters, is essential to maximize the shelf life and efficacy of encapsulated krill oil products. Similarly, Zhu et al. (2015) stated that low temperature favored the physicochemical stability of the nanocarriers containing krill oil. Ortiz Sánchez et al. (2021) analyzed the degradation kinetics of the astaxanthin, a high value biomolecule present in krill oil microcapsules. The study revealed that astaxanthin degradation in krill oil microcapsules follows a first-order reaction kinetics, with the lowest oxidation rate observed at an a_w of 0.108 and the rate increased as a_w levels rise up to 0.743. The lowest oxidate rate was observed at a_w of 0.1, which may be attributed to reduced water diffusion and the protective film formed by the microcapsule’s wall material, limiting radical attacks on astaxanthin. Furthermore, with increasing a_w above monolayer moisture content, more water and oxygen molecules penetrate the microcapsule wall, affecting the oxidation rate and decreasing stability. At an a_w of 0.743, the uptake of moisture leads to solubilization of the wall material, exposing krill oil to the environmental conditions and accelerating the degradation of astaxanthin. In conclusion, effective encapsulation of krill oil is crucial for enhancing its oxidative stability and the prolonged shelf life. Various encapsulation techniques, such as microencapsulation, coacervation, and inclusion complexes, along with different wall materials like proteins, polysaccharides, and chitosan nanoparticles, offer promising alternatives for the protection of krill oil from oxidation. Additionally, methods like emulsification followed by spray drying have proven effective in reducing oil droplet size and increasing surface area which further improves the encapsulation efficiency. Storage conditions, including temperature and water activity also plays a critical role in determining the long-term stability of encapsulated krill oil, with low temperatures generally favoring physicochemical stability. Understanding the degradation kinetics of valuable components like astaxanthin provides further insights into the optimization process of encapsulation. Overall, careful consideration of encapsulation parameters and storage conditions is essential for maximizing the efficacy and shelf life of encapsulated products of krill oil.

Bioavailability of krill oil

The unique composition of krill oil, where EPA and DHA are predominantly bound to phospholipids, has been hypothesized to influence its bioavailability and pharmacokinetics. Bioavailability refers to the proportion of a consumed substance that enters the bloodstream and becomes available for use by the body. In the context of krill oil, it signifies how efficiently EPA and DHA are absorbed by the digestive system and integrated into circulation. Pharmacokinetics, on the other hand, encompasses the entire journey of these fatty acids within the body, including absorption, distribution, metabolism, and excretion. Several studies have investigated the bioavailability and pharmacokinetics of EPA and DHA from krill oil compared to fish oil. Ulven & Holven, (2015) reviewed all the studies done on the assessment of the bioavailability of krill oil in human beings with respect to the other comparable sources present in the market up to 2015. Seven human randomized trials – five double-blind (Banni et al., 2011; Bunea et al., 2004; Maki et al., 2009; Ramprasath et al., 2015; Schuchardt et al., 2011) and two open-label (Laidlaw et al., 2014; Ulven et al., 2011) ones – investigating the effects of krill oil compared with fish oil were identified. Out of the seven studies analyzed, five of them reported the impact of these oils on the bioavailability and/or plasma levels of EPA and DHA fatty acids. Ramprasath et al., (2015) performed a crossover investigation involving 24 healthy participants who were given either krill oil or fish oil, each containing the concentration of EPA and DHA (600 mg EPA and DHA) over four-week. They observed that both krill and fish oil supplementation increased the plasma levels of EPA and DHA and total omega-3 fatty acids, level of red blood cells, (omega-3 index) as compared to the control group. However, the Intake of krill oil led to significant increase in plasma EPA levels, total omega-3 PUFAs levels, the level of red blood cells EPA, and the omega-3 index as compared to the fish oil. Furthermore, the change in omega-3 index after consumption of krill oil was two-fold higher than that with fish oil. Similarly, Maki et al., (2009) conducted a comparable trial, administering the same amount of krill oil and fish oil, but varying the amounts of EPA and DHA, in a randomized controlled trial of 4-week involving 76 overweight and obese individuals. The subjects received daily doses of 2 g of krill oil (providing 216 mg of EPA and 90 mg of DHA), menhaden oil (providing 212 mg of EPA and 178 mg of DHA), or olive oil. The rise in plasma concentrations of EPA and DHA was comparable between the krill oil and menhaden oil consuming groups, both significantly differing from the control group administered olive oil. Ulven et al. (2011) conducted a 7-week trial randomized trial involving 113 subjects with normal and slightly elevated total blood cholesterol and triglycerides levels. Subjects received either 3 g per day of krill oil (providing a total of 543 mg of EPA + DHA) or 1.8 g per day of fish oil (providing a total of 864 mg of EPA + DHA). A third group did not receive any supplementation and observed significant boosts in plasma EPA, DHA, and docosapentaenoic acid levels in both krill oil and fish oil groups compared to controls, but there were no significant differences in the changes in any of the omega-3 PUFAs between the fish oil and the krill oil groups despite the difference in dose levels.

All these findings collectively support the hypothesis that EPA and DHA from krill oil exhibited a superior bioavailability as compared to the fish oil. Other than this, several factors influence the bioavailability of EPA and DHA from krill oil. The chemical form of EPA and DHA, with phospholipids in krill oil and triglycerides in fish oil, plays a significant role in absorption mechanisms. In support to this, Schuchardt et al. (2011) similarly analyzed the comparative bioavailability of similar doses of EPA + DHA (1,680 mg) from krill oil with respect to their other chemical forms in a double-blinded crossover postprandial study. Lasting up to 72 h post-intake, 12 healthy male participants given a single dose of fish oil capsules containing either of reesterified triglyceride or ethyl ester or krill oil capsules containing EPA + DHA mainly as phospholipids. While the authors observed higher levels of EPA, DHA, EPA + DHA, and total omega-3 fatty acids in plasma phospholipids after krill oil treatment compared to reesterified triglyceride and ethyl esters treatments although the treatment was not statistically different. However, a trend was observed suggesting a potential variance in EPA bioavailability between reesterified triglyceride and krill oil. Thus, the study suggests a possible higher bioavailability of EPA and DHA in plasma phospholipids from krill oil compared to fish oil. In contrast to these studies, Laidlaw et al. (2014) conducted a 28-day crossover trial involving 35 healthy subjects, administering varying amounts of oil, EPA and DHA from four different omega-3 supplements. The four doses and the supplements included reesterified triglyceride fish oil (providing EPA at a level of 650 mg and DHA at a level of 450 mg), ethyl ester fish oil (providing EPA at a level of 756 mg and DHA at a level of 228 mg), Phospholipid krill oil (providing EPA at a level of 150 mg and DHA at a level of 90 mg), and triglyceride salmon oil (providing EPA at a level of 180 mg and DHA at a level of 220 mg). The author found that the supplementation of reesterified triglyceride fish oil statistically increased the whole-blood omega-3 fatty acids levels as compared to the other products. Furthermore, the increase in whole-blood DHA, EPA + DHA, and EPA levels was also found to be greater than that observed with the phospholipids and triglyceride products. When comparing the krill oil-phospholipid and the salmon oil-triglyceride groups, which certainly had the same amount of daily intake of EPA i.e. 150 mg and 180 mg, respectively, the mean percentage increase in whole-blood EPA was nearly the same in both the groups, suggesting that the structural form of EPA does not appear to influence the bioavailability. Interpretation of studies on the bioavailability of EPA and DHA from krill oil and other similar sources such as fish oil is challenging due to the variations in amount of EPA and DHA, intervention durations, and study populations. One postprandial study mentioned above used the same amount of EPA and DHA doses from krill oil and fish oil, and the study suggests that at the same amount of doses the bioavailability of krill oil’s EPA and DHA was found to be higher than the fish oil, thus strengthening the hypothesis that PUFAs from krill oil are superior than the fish oil. On the other hand, Laidlaw et al. (2014) demonstrated that at the equal amounts of EPA quantities from phospholipid krill oil and triglyceride salmon oil led to similar increases in whole-blood EPA levels, implying no significant difference in DHA bioavailability between fish oil and krill oil. However, comparing these current studies is challenging due to the fact that the different blood components were analyzed as whole-blood fatty acids were examined in one study versus plasma phospholipids and red blood cells in the others. Further, the bioavailability of EPA and DHA from krill oil, krill meal, and fish oil has been studied by Köhler et al. (2015) which compare the acute bioavailability of these materials in healthy subjects. A randomized, single-dose, single-blind, cross-over, active-reference trial was conducted with 15 subjects who ingested krill oil, krill meal, and fish oil, with each containing an approximate concentration of 1700 mg for EPA as well as DHA. The compositions of fatty acids in plasma triglycerides and phospholipids were measured repeatedly for 72 h. The primary efficacy analysis was based on the 72-h incremental area under the curve of EPA and DHA in plasma phospholipid fatty acids. The study found that a larger incremental area under the curve for EPA and DHA in plasma phospholipid fatty acids was detected after krill oil (mean 89.08 ± 33.36% × h) than after krill meal (mean 44.97 ± 18.07% × h, p < 0.001) or after fish oil (mean 59.15 ± 22.22% × h, p = 0.003). Mean incremental area under the curve after krill meal and after fish oil were not different. A large inter-individual variability in response was observed. The study concluded that EPA and DHA in krill oil had a higher 72-h bioavailability than in fish oil. The chemical form of EPA and DHA, with phospholipids in krill oil and triglycerides in fish oil, plays a significant role in absorption mechanisms. Phospholipids are amphipathic molecules, meaning they possess both a hydrophilic (water-loving) and a hydrophobic (water-hating) region. This unique structure allows them to readily form micelles in the digestive tract, facilitating the absorption of EPA and DHA. The digestion of fatty acids esterified as phospholipids is carried out mainly by pancreatic phospholipase A2 (pPLA2) and other pancreatic lipases. pPLA2 interacts with phospholipids at the sn-2 position, yielding free fatty acids and lysophosphatidylcholine, which are then absorbed by the enterocytes as part of mixed micelles. The presence of phospholipids is essential for the formation of these mixed micelles, which enhances the absorption of lipids (Schuchardt et al., 2011). In support to the above statement, Ahn et al. (2018) investigated the absorption rate and bioavailability of EPA and DHA from krill oil compared to fish oil in the blood and brain of rats and observed that krill oil had a higher content of EPA and DHA bound to phospholipids compared to fish oil, which had these fatty acids bound to triglycerides. After the short-term administration, krill oil showed higher EPA and DHA levels in the blood compared to fish oil. Krill oil maintained higher levels of these fatty acids over time, while the levels of fish oil decreased rapidly after 8 h. Despite the total unsaturated fatty acid content was lower in krill oil compared to fish oil but the content was found to be higher in brain in the krill oil groups over time. After long-term (1–2 weeks) oral administration, the triglyceride and phospholipid contents in the blood were slightly higher for the krill oil groups as compared to the groups administered with fish oil. EPA and DHA levels in the brain were also slightly higher with long-term supplementation of krill oil, though the difference was not statistically significant. Weather this improvement of EPA and DHA concentration is an advantage in terms of health effect it is yet to be fully demonstrated.

Side effect safety

Krill products, derived from the minute shrimp-like crustaceans known as krill, have emerged as popular dietary supplements due to their distinctive nutritional composition. These supplements have drawn attention due to their potential health advantages because they are rich in astaxanthin and packed with omega-3 fatty acids (Burri & Johnsen, 2015). The growing number of people using krill-based supplements to promote their general health makes it necessary to investigate the potential negative impacts and safety issues related to their use. The investigation is essential for giving people an in-depth understanding of the advantages and disadvantages of krill products, enabling them to make well-informed decisions about adding them to their health regimens as there is a scarcity of thorough research analyzing the safety profile of krill oil in the literature currently available on the toxicity of the oil. Kim et al. (2018) conducted a cytotoxicity investigation on the viability of RAW 264.7 cells using ozonated krill oil. 7 macrophages were assessed using the 3-(4,5-Dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide test at various concentrations (10, 50, 100, and 200 μg/mL). The ozonated krill oil did not cause substantial cytotoxicity at doses of 100 μg/mL and also inhibited nitric oxide synthase, which is known to cause death in a host by creating an inflammatory mediator. However, the cytotoxicity became apparent after increasing the further concentration upto 200 μg/ml. Furthermore, an animal study has raised some concerns about potential krill oil kidney damage (Gigliotti et al., 2013). In this study, krill oil was compared to various fish oils as well as a flaxseed oil and a corn oil as control, after being fed to 4 weeks old rats on a 12% fat diet for an additional 8 weeks. The authors concluded that the rats fed krill oil showed renal calcification and tubulo-interstitial injury. This was attributed to increased urinary phosphorous excretion caused by the phospholipids content of these oil sources. However, it’s worth noting that the results obtained in both the above studies were using a diet with 200 μg/ml and 11.8 wt% as krill oil concentration respectively and these are the doses that is unlikely to occur in human diets and no statistical analyses were performed and the results concludes that krill oil is generally considered safe for human consumption. Furthermore, Spindler et al. (2014) studied the effects of a daily, isocaloric intake of pharmaceutical-grade fish oil, Lovaza (Omacor; 4.40 g/kg diet), and krill oil (1.17 g oil/kg diet), on the lifespan and mortality-associated pathologies in long-lived male B6C3F1 mice beginning at 12 months of age and the results revealed that krill oil (constituting 3% of the diet) and Lovaza (11% of the diet) significantly reduced the span of life by 6.6 prcent (P = 0.0321; log-rank test) compared to controls. Individually, krill oil showed non-significant reductions in median life span by 4.7% and also elevated the occurrences of lung tumors (4.1- and 8.2-fold) and hemorrhagic diathesis (3.9- and 3.1-fold). Further analyses of mice treated with serum indicated that krill oil modestly triglycerides, raised bilirubin and blood glucose levels. However, it is crucial to note that a more comprehensive investigation into this aspect is imperative. Some studies have indicated a decrease in triglycerides and cholesterol levels in mice, even at doses as high as 10 times greater (12.5 g/kg diet) than those employed in the aforementioned study (Tandy et al., 2009). Similarly, despite of above few studies which mention the negative effect there are various other studies which have investigated the positive effects of consumption of krill oil. For instance, a prolong study done by Gart et al., (2021) for 28-weeks with supplementation of 3% krill oil in an obesogenic diet exhibited significant anti-inflammatory effects. Krill oil induced favorable changes in fatty acid composition, adipose tissue morphology, and adipokine levels, contributing to reduced inflammation and improved metabolic parameters. These findings align with few more studies (Bonaterra et al., 2017; Costanzo et al., 2016; Grimstad et al., 2012; Hwang et al., 2022; Liu et al., 2020; Sistilli et al., 2021) that also investigated the anti-inflammatory effects of krill oil consumption. Moreover, the beneficial effects of krill oil consumption extended to cardiovascular health (Jayathilake et al., 2019; Lobraico et al., 2015; Zhou et al., 2017) as an anticancer agent (Jayathilake et al., 2016; Jing et al., 2021; Zhou et al., 2021), antidiabetic agent (Rossmeisl et al., 2020), against Osteoarthritis (Ku et al., 2023; Zhan et al., 2020), protective effects on cognitive function (Li et al., 2018), neuroinflammatory process (Andraka et al., 2020; SenGupta et al., 2022; Zhang et al., 2021). In conclusion, the aggregated findings from various studies predominantly favour positive effects associated with krill oil consumption. However, to strengthen these observations and establish a stronger understanding, further confirmatory research is needed. Additional studies exploring long-term effects of krill oil on all the health aspects, detailed assessment of toxicity at varying concentrations, and comprehensive investigations which delves into the potential interactions with other medications would contribute to a more thorough evaluation of the safety and benefits of krill oil supplementation.

Market product

There are several market-available krill oil products from various brands. Krill oil supplements are typically sold as soft-gel capsules which offers various health benefits, including promoting heart and brain health and reducing inflammation. These products typically contain omega-3 fatty acids such as EPA and DHA, phospholipids and astaxanthin as mentioned in Table 3. Moreover, the nutrients available in krill oil claimed to improve blood lipid levels. In addition, products such as Aker biomarine and Canadian Neptune Biotech claims to provide benefits in skin and sports segments and supports joint segments. Further, unlike fish oil, krill oil’s DHA and EPA are bound to phospholipids, potentially enhancing absorption. Here are some popular brands that offer krill oil supplements. Krill oil emerges as a nutritional powerhouse, boasting a rich composition of omega-3 fatty acids, phospholipids, choline, and astaxanthin, which collectively offer an innumerable health benefit. Its unique phospholipid-bound omega-3 fatty acids, particularly EPA and DHA, exhibit superior bioavailability compared to the traditional omega-3 supplements like fish oil. Beyond its cardiovascular benefits, krill oil also demonstrates efficacy in managing various diseases like: dyslipidemia, inflammation, glucose metabolism, and even premenstrual symptoms. Moreover, synergistic combinations of krill oil with other bioactive compounds, such as probiotics and vitamins, further enhance its therapeutic potential, showcasing a promising approach for the mitigation of inflammation and improving gut health. Encapsulation techniques shield krill oil from the degradation, preserves its efficacy in food products. Emulsions, tailored with suitable emulsifiers, strengthen stability. These methods offer a potent means to harness the benefits of krill oil in various culinary applications as these methods helps to increase its oxidative stability. Additionally, the unique composition of krill oil, rich in phospholipids, enhances its bioavailability compared to fish oil, as demonstrated in several studies. However, while existing research indicates promising advantages of krill oil over the existing sources in terms of EPA and DHA bioavailability, several gaps in knowledge exists. Variability among studies, joined with the need for more rigorous investigation into the long-term effects on human health and potential clinical outcomes, highlights the necessity for further research in the current field. Future studies should focus on standardizing the dosage and intervention protocols, exploring the diverse populations, and employing superior analytical methods to elucidate the full mechanism behind the benefits offered by krill oil supplementation. Furthermore, similar to the safety concerns of krill oil supplementation which were addressed in the review were not satisfactory as some studies indicating potential cytotoxicity and renal damage in animal models while some showed the positive effects of krill oil consumption on inflammation, cardiovascular health, and cognitive function. Additional research is important to comprehensively assess the safety profile of krill oil, including long-term implications and interactions with medications. Additionally, the market landscape of krill oil supplements showed several brands offering products claiming various health benefits.

Table 3.

Market products of krill oil and the related health claims

S.no	Manufacture	Brand Name	Health claims	References
1	Aker biomarine	SUPERBAKrill & MegaRedR	• Krill oil’s phospholipid complex of omega-3 and choline provides support to the heart, brain, liver and eyes, with recent research showing benefits in skin and sports segments	(https://www.superbakrill.com/)
2	Canadian Neptune Biotech	Doctor’s nutrition Neptune Krill Oil	• It can help to support joint comfort, healthy blood lipid levels already within normal range, and may help to maintain healthy feminine balance • The Phospholipid-bound forms of EPA and DHA from Krill Oil have also demonstrated exceptionally high bioavailability	(https://doctorsnutrition.com/store/neptune-krill-oil-1000-mg-60-softgels/)
3	Enzymotec	K•REAL®Krill Oil	• It is always fresh like the first day it was produced. The benefits of Krill oil are that it reduces LDL (bad cholesterol), reduces triglycerides, increases HDL (good cholesterol), anti-inflammatory and supports healthy liver function	(https://www.ulprospector.com/en/na/Food/Detail/6563/218428/KREAL-Krill-Oil)
4	Carlyle	Antarctic Krill Oil	• Contains 2,000 mg of krill oil per serving with naturally occurring EPA & DHA plus Astaxanthin • Made with premium Antarctic krill oil that is naturally free of gluten, wheat, milk, lactose, artificial color, flavor, sweetener, and non-GMO • Provides heart health support, brain health support, joint support, and memory support	(https://carlylenutritionals.com/products/antarctic-krill-oil-2000mg-omega-3-epa-dha-astaxanthin-120-softgels)
5	Reckitt Benckiser	MegaRed Krill Oil Supplement	• Provides 2X more Omega-3 s in just 1 pill • Excellent source of EPA and DHA Omega-3 fatty acids which may reduce the risk of coronary heart disease • Easy absorption due to being carried to the body's cells in phospholipid form	(https://www.seafoodsource.com/features/reckitt-launches-usd-8-4m-krill-oil-promotion-in-the-uk)
6	Bronson Vitamins	Bronson Antarctic Krill Oil	• Provides Omega-3 Fatty Acids, Omega-3 Phospholipids and Natural Astaxanthin • 100% Pure Antarctic Krill Oil • Heavy Metal Tested, Non GMO, Gluten Free and Soy Free	(https://www.bronsonvitamins.com/en-in/products/antarctic-krill-oil-omega-3-epa-dha-with-astaxanthin-non-gmo-2000-mg-120-softgels)
7	NatureBell	Antarctic Krill Oil	• Contains high levels of EPA, DHA, and astaxanthin for joint, heart, and immune health support • Non-GMO and non-irradiated, with no soy, dairy, gluten, preservatives, or additives • Phospholipids make it easier for the body to digest and absorb nutrients	(https://www.naturebellusa.com/search?type=product,page,article,collection&q=krill%20oil*)
8	Sports Research	Sports Research Krill Oil Supplement	• This advanced formulation provides superior support for brain health, heart health, and joint health • One bottle of Sports Research Krill Oil Supplement contains sixty soft gels, which is enough for a two-month supply	(https://www.sportsresearch.com/products/krill-oil-superba2tm-1000mg)
9	Native Path	Antarctic Krill Oil	• It is a great supplement for those looking to support their overall wellness • It contains omega-3 fatty acids, EPA, DHA, and astaxanthin, which can help fight free radicals and support joint, heart, brain, and immune health • The softgels are easy to take and should be consumed once a day with a meal. No fishy aftertaste	(https://www.nativepath.com/search?q=krill%20&blogs=5)
10	Kori Krill Oil	Kori Krill oil	• It delivers Omega-3 s in their most natural phospholipid and triglyceride form, just as you get in a healthy fish diet, for superior absorption vs fish oils and no fishy aftertaste • It is naturally a good source of the essential nutrient choline, found in foods such as eggs and broccoli, for brain and nervous system health • It is certified sustainable from the #1 global krill oil supplier that is the only global fishery to earn an ‘A’ rating, and for seven years running	(https://www.korikrilloil.com/health-benefits)

Author contributions

Nidhi Attri: Writing-original draft, Preparation, Visualization, Writing-review and editing. Diksha Arora: Preparation, Visualization, Writing-review and editing. Rajni Saini: Writing-original draft, Preparation, Visualization, Conceptualization, Supervision, Writing-original draft, Writing- review and editing. Mamta Chandel: Writing-review and editing. Priyanka Suthar: Writing-review and editing. Atul Dhiman: Conceptualization, Writing-original draft, Writing-review and editing.

Funding

None.

Declarations

Conflict of interest

Author and co-authors declare no conflict of interest.

Declaration of generative AI and AI-assisted technologies

Authors declare that they haven’t used any AI or assisted technology.