Progress on water-in-oil-in-water (w/o/w) double emulsions: bioactive delivery, stability optimization, and sports nutrition applications

Abstract

There is a growing global demand for healthy and nutritious foods, especially among specialized groups such as athletes and bodybuilders. Water-in-oil-in-water (W/O/W) double emulsions have garnered significant interest due to their ability to simultaneously deliver both hydrophilic and hydrophobic bioactive compounds. This review discusses the cargo release mechanism of W/O/W emulsions, along with their inherent advantages and limitations. The primary preparation methods and fabrication techniques for such emulsions are subsequently summarized. Additionally, the instability mechanisms of W/O/W emulsions are verified, and corresponding strategies to enhance their stability are proposed. Furthermore, the application of double emulsions as nutritional supplements in sports nutrition is critically reviewed, alongside key directions for future research. This review provides a theoretical foundation for the design and development of stable, functionally tailored double emulsions intended for application in athlete-specific food nutrition.

Introduction

Sports food market has been growing fast in the health category over the past decades, and it is expected to continue growing in the foreseeable future. Appropriate selection of nutrient-rich foods can reduce the nutrients deficiencies risk, particularly when energy intake is restricted to reduce body mass/fat mass or recovery from exercises injury. Athletes and body builders can get benefit from the dietary food guidance in sports nutrition for individual need; meanwhile, nutritional supplements are also recommended. Although nutritional supplements are not as important as training, talent of athletes and resistance to injury, suitable food nutritional supplements can gain a competitive edge, optimize their performance and help the recovery from injuries. Sports nutritional supplements is originally designed for elite athletes and body builders to keep up with their unique nutritional demands, and it is now aimed to facilitate the performance, muscle building and excise recovery as well (Arenas-Jal, et al., 2020). Previous study stated that 40–100% of the trained athletes or body builders cconsume dietary supplements depending on their special requirement (Kaufman et al., 2022), while a study of collegiate athletes also indicated that 64–89% use at least one nutritional supplement (Murphy et al., 2022). Recently, not only the body builders and strength athletes are pushing for nutrients supplements, but other consumers including amateur athletes and active people concern these products, from muscle growth, performance enhancement and post-exercise recovery to health and wellbeing (Harrison and Smith, 2016).

Rapidly developed professional strategies and innovative technologies have strongly driven the advancement of sports foods. Among them, microencapsulation, such as microgel, nanoparticles, emulsion, liposomes, have been proven as promising tools to deliver diverse nutrients for athletes and body builders (Liu et al., 2020). These delivery systems provide multiple functional benefits, for instance, masking the unpleasant flavor, inhibiting component interactions, improving stability and bioavailability, and achieving controlled release (Bah, et al., 2020; Yan et al., 2025). Water-in-oil-in-water (W/O/W) double emulsions, also known as multiple emulsions, can be described as emulsions within emulsions which has one phase dispersed in another as tiny droplets that again are dispersed in a third phase (Li et al., 2025a). They are versatile delivery vehicles for encapsulating bioactive compounds in sports nutrition supplements. This article reviews recent literatures on fabrication methods of double emulsions, addresses their instability mechanisms, and discusses strategies to enhance stability during application. The main applications of double emulsions in sports foods and future research perspectives are also summarized.

General information on W/O/W double emulsions for bioactive compounds delivery

The formulations of W/O/W double emulsion

There are two types of double emulsions, namely oil-in-water-in-oil (O/W/O) and W/O/W emulsions (Fig. 1). The former consists of oil droplets dispersed in an water phase, which is further disperses in the second oil phase; the latter comprises an aqueous phase dispersed in oil droplets that are disperses in the second water phase (Kim et al., 2025). Most food ingredients and matrices are aqueous phase; thus, the W/O/W emulsion displays more potential application in the food industry and has gained more remarkable prominence amongst scientists. Hence, this review focuses on the W/O/W double emulsion.

Fig. 1.

Structure of double emulsions including oil-in-water-in-oil (O/W/O) emulsion and water-in-oil-in-water (W/O/W) emulsion

The W/O/W double emulsions are thermodynamically unstable and emulsifiers are usually added to double emulsions to maintain the structural stability (Muschiolik and Dickinson, 2017). The main composition of W/O/W emulsions includes the internal water phase, hydrophobic emulsifier, oil phase, hydrophilic emulsifier, and external water phase (Xie et al., 2026). The inner water phase contains hydrophilic compounds, such as sugar, salt, and nutrients, and the outer water phase consists of an emulsifier and stabilizer. The preparation of double emulsions requires two distinct emulsifiers with complementary properties: one stabilizes the water-in-oil (W/O) interface, while the other stabilizes the oil-in-water (O/W) phase (Su et al., 2022). The emulsifiers adsorb at the surfaces and inhibit aggregation within each phase. Three methods are commonly employed for double emulsion preparation, including the single-step phase inversion technique, two-step emulsification, and membrane emulsification; these approaches are discussed in the following sections.

Advantages and limitations of W/O/W double emulsion

W/O/W double emulsions exhibit numerous advantages when used for nutrient delivery. Firstly, double emulsions can enhance the taste and aroma of ingredients while masking the odor of certain volatile components by encapsulating these molecules within the inner water or oil phase. Secondly, they protect bioactive molecules from oxidation or hydrolysis during storage and digestion. Thirdly, encapsulation in double emulsions sustains the release of entrapped compounds. Finally, they improve the bioavailability of functional components during gastrointestinal digestion. However, there are several limitations of W/O/W double emulsions when they are used in food system. The inherent thermodynamic and kinetic instability, such as gravity separation, coalescence, flocculation, and diffusion of water from the internal water phase to the external water phase during processing and storage, have been observed (Heidari et al., 2022; Rakshit and Srivastav, 2022). Thus, the preparation of W/O/W double emulsions have greater challenges compared to the single O/W emulsions. Besides, the lack of food-grade stabilizers further hinders the wide application of double emulsions in food and nutrition (Li et al., 2025b). Consequently, the following section will elucidate the typical fabrication methods of double emulsions and the detailed instability mechanisms, as well as review strategies for achieving stable formulations.

Mechanism of bioactive compounds released from W/O/W emulsion

Three possible mechanisms govern the release of bioactive compounds from W/O/W double emulsion. First, swelling or shear forces induced by concentration gradients between the inner and outer phases cause rupture of the double emulsion, resulting in the release of nutrients from the inner to the outer phase (Alam and Al Riyami, 2018) Second, interphase coalescence between the middle oil phase and the inner aqueous phase destabilizes the middle layer and triggers cargo release (Han et al., 2022). Tthird, nutrients may diffuse or permeate from the inner phase across the middle phase to the outer phase owing to the permeability of the middle layer, without disrupting the overall double emulsion structure (Lamba et al., 2015).

Techniques for the preparation of W/O/W emulsions

One step/phase inversion technique

Single step technique is also known as the phase inversion technique. In this technique, the phase inversion occurs when the non-ionic emulsifier or a mixture of emulsifiers is heated, to form a double emulsion (Fig. 2a). When the double emulsion is prepared, the oil phase consisting of liquid paraffin and hydrophobic emulsifier is added to a mixer. The aqueous solution containing hydrophilic emulsifier is introduced at a constant rate to the oil phase. The continuous oil phase is replaced using the aqueous phase when the volume ratio of the water phase exceeds 70%, leading to a phase inversion and formation of W/O/W double emulsion (Neeraj et al., 2013). Compared to other techniques, one-step emulsification method prevents destabilization and thus simplifies the manufacturing process and needs less dosage of synthetic surfactants (Jin et al., 2024). This technique is robust since it has high production efficiency and is suitable for industrial application. A stable double emulsion was fabricated via one-step emulsification using edible chitosan/soy β-conglycinin complex nanoparticles as emulsifier, and when the chitosan/soy β-conglycinin ratio was 1:2 and oil phase contained 40 wt%, the most stable double emulsion was obtained (Chen et al., 2023a). However, the limitation of the one-step technique is the formation of large-sized emulsions and low encapsulation efficiency (Table 1).

Fig. 2.

Systematically representative methods for preparation of W/O/W emulsions. a One step/phase inversion technique. b Two-step/ double emulsification technique. c Membrane emulsification. d Microchannel emulsification. e Ultrasonic emulsification

Table 1.

Comparison of different techniques for W/O/W emulsion fabrication

Technology	Instrument	Primary emulsification and secondary emulsification	Advantages	Limitations	References
One step/ phase inversion technique	Shear/high pressure or gentle pump combined with microfluidic device; Homogenizer	W1/O: homogenized at speed of 20 000 r/min for 3 min; W2 containing salt introduced to W1/O under stirring 150 r/min	Simplicity: one set of mixing conditions must be optimized; Robustness: two mixing steps do not destabilize each other	Obtain large sized emulsion	Jin et al., (2024)
Two step/ double emulsification technique	High speed/pressure homogenizer, colloidal mills, Ultra-Turrax	W1/O: homogenized for 5 min at 6 500–24 000 rpm; W/O/W: treated by homogenizing at 6 500–24 000 rpm for 5 min or 840 bar for 1 pass	Controllable droplet size and achievement of uniformity	Need to optimized many factors to form stable emulsion.; High shear stress may cause a rupture of the internal emulsion (W/O); wide size distribution of outer droplets or to a low encapsulation efficiency	Hu et al., (2025)
Membrane emulsification	Porous Glass Membrane	Rotating porous glass membrane at 100 to 900 rpm (wall shear stress 0.1 to 100 Pa, static pressure 0.025 to 0.1 MPa). Droplet size relies on rotation speed, static pressure and viscosity of outer water phase	Size can be accurately regulated by pore size, shear stress and ultrasonicator in batches; low energy input; suitable for low-molecular-weight component encapsulation	Low productivity and fouling	Muschiolik et al., (2007)
Microchannel emulsification	T-junction microchannels, double cross microchannels and 3D flow focusing microchannels	Consists of numerous 10 μm × 100 μm microslots with 30 μm depth, each connected to a 10-μm-diameter circular plate with a 70 μm depth. W1/O: 10,000 rpm for 5 min (droplet size < 4 μm); W1/O in W2 at breakthrough pressure of 5 to 6 kPa, forming monodisperse W/O/W droplets of size 30 μm	Superior monodispersity and precisely controlled size	Hard to fabricate monodispersed double emulsion with particle size larger than 100 μm	Khalid et al., (2014)
Ultrasonic emulsification	Ultrasound equipment, Ultra-Turrax and sonicator	Vortex mixing for 5 min followed by sonication at 20 kHz, amplitude 40% for 90 ti 150 s; W/O/W for by amplitude of 20% for 15 to 120 s	Obtain small sized and narrow distributed double emulsion, low cost	Not apply in the industry and still under lab conditions	Tang and Sivakumar, (2012)

Two-step/ double emulsification technique

Two-step emulsification is the most applied method to obtain a double emulsion. The primary emulsion step is fabricated by finely mixing the emulsifiers to obtain small inner droplets (usually smaller than 1 μm) using high shearing force (Fig. 2b). Then low energy input in the secondary emulsification step is used to prevent the liquid membrane between the inner and outer phases from rupture (Hu et al., 2025). Two surfactants with notable differences in the Hydrophile-Lipophile Balance (HLB) values are used when it is applied to the two-step emulsification technique (Shukai, 2019). The main advantage of this method lies in its controllable droplet size and uniform size distribution. However, there is a difference in the laboratory-scale preparation and industrial scale-up as the former uses laminar flow while the latter applies turbulence flow (Håkansson, 2017). Besides, the production of W/O/W emulsion is high cost, including conversions of the production room (Table 1). Moreover, the second step is often carried out at a low shear rate per pressure, which leads to a wide size distribution of outer droplets or low encapsulation efficiency (Neeraj et al., 2013). Therefore, the industrial-scale production of double emulsions via the two-step method remains challenging.

Membrane emulsification

The principle of the membrane emulsification method is the dispersion of the dispersing phase into a continuous phase under constant pressure using a porous glass membrane (Konovalova et al., 2023). There are two methods to prepare double emulsion using membrane emulsification, including cross-flow membrane and pre-mix membrane (Fig. 2c) (Zhi et al., 2023). In the former method, a to-be-dispersed coarse W/O emulsion is pressed through a microporous membrane into a continuous phase (water phase containing hydrophilic emulsifier), and the droplets grow at pores and detach at required size, resulting in forming a W/O/W double emulsion (Left in Fig. 2c). A critical step in the preparation process is the generation of fine W/O droplets smaller than 1 μm using high shear, high pressure, or ultrasonication. For the membrane emulsification method, a coarse W/O/W emulsion is first pre-mixed using a conventional homogenizer or rotor, then passed through a cellulose acetate membrane to reduce oil droplet size (Right in Fig. 2c). Coarse droplets break down into smaller ones and disperse into the outer aqueous phase; these are encapsulated within oil droplets under the stabilization of hydrophilic emulsifiers, thereby forming uniform double emulsions. The particle diameter of the double emulsion can be regulated by selecting an appropriate membrane (e.g., narrow pore size and microporous glass) to produce stable double emulsions (Wang et al., 2022a). As an illustration, a nano emulsion with a diameter of 0.26 μm was obtained using the Shirasu Porous Glass membranes with a pore size in the range of 0.2–0.8 μm (Alliod et al., 2018). Further, the rotating membranes are another technique to regulate the droplet size of double emulsion. This technology is mainly used in the medical field; however, it is also applied for the commercial production of very low-fat spreads in Japan (Vladisavljević et al., 2014). Overall, membrane emulsification features low energy consumption and enables superior control over droplet size and size distribution. However, low productivity and membrane fouling remain the primary limitations (Table 1).

Microchannel emulsification

Microchannel emulsification technology uses exquisitely designed microchannels to divide the fluid into separate microflows (Fig. 2d). The dispersed phase is passed through microchannels to form another insoluble continuous phase, which gathers in the form of collision geometry to form droplets (Alkanawati et al., 2018). Many microchannels have been designed according to this principle to prepare double emulsions, such as double T-junction microchannels, double cross-microchannels, and 3D flow-focusing microchannels (Yang et al., 2022). Three-dimensional flow-focusing microchannels can be categorized into two configurations: parallel flow and reverse flow structures (Table 1). In the parallel flow structure, the inner and middle phases are introduced through separate inlets to form a coaxial jet focused at the capillary outlet, which subsequently interacts with the outer phase to generate double emulsions (Francisco et al., 2023). In the reverse flow structure, the injection direction of the outer phase is reversed. The three phases meet at the capillary outlet at the same time and emerge together, forming a double emulsion (Chong et al., 2015). Double emulsions fabricated via microchannel emulsification exhibit exceptional monodispersity, with coefficients of variation below 5%. This characteristic is highly favorable for food applications, as it enables precise control over droplet size and the release kinetics of target bioactive compounds (Oh and Kim, 2024). However, the pressure range for the constant droplet diameter is narrow, indicating it is challenging to fabricate monodispersed double emulsion with a particle size larger than 100 μm, as the interfacial tension is subservient for large scale.

Ultrasonic emulsification

There are two steps in the preparation of double emulsion using the ultrasonication method. Firstly, the inner phase is added to the interphase containing an emulsifier, followed by the preparation of primary emulsion using ultrasonication with higher amplitude (Fig. 2e). Secondly, the primary emulsion is added to the external phase containing the emulsifier, and the double emulsion is obtained using ultrasonic treatment with a low amplitude (Tang et al., 2013). When the ultrasonic waves travel through the liquid, it leads to the occurrence of acoustic cavitation, and the bubbles nucleate, grow, and collapse (Khadem and Sheibat-Othman, 2019). The ultrasonic wave destroys the water–oil interface structure, which causes the dispersed phase to spray into the continuous phase in the form of droplets (Wang et al., 2022b). The physical effects of high-pressure shock wave and high shear force generated by cavitation bubbles create small droplets (Shanmugam and Ashokkumar, 2017). This method aids in the preparation of small size and narrow distribution of double emulsion and is also simple and lower in cost (O’Sullivan et al., 2014). It was found that the technique can save up to 73% of emulsification energy compared to high-pressure homogenization, additionally, the stability and formation ability are also enhanced (Gavahian et al., 2018). Ultrasound treatment can reduce the protein aggregate size, which helps to stabilize the emulsion and to obtain tiny shell-stable emulsion droplets (Table 1). It is important to have food safety, shelf-life extension, and cost saving in the food industry. However, this method is currently studied under laboratory conditions, and further study needs to be performed to scale up for industrial utilization.

Technological hurdles for mass production

Currently, many preparation methods referred above have technological hurdles when they are moved from laboratory to mass production. For example, membrane emulsification is time consuming, and only for the fabrication of very high value products like drug delivery systems is feasible. Moreover, Polyglycerol polyricinoleate (PGPR) is the most commonly used emulsifier for the making and stabilizing of the primary W/O emulsion, while PGPR is strictly regulated by international regulatory authorities in many foods (Muschiolik and Dickinson, 2017). Therefore, future research should focus on developing techniques or machines that can produce well size distribution and controllable structure at large scales, such as microfluidic systems, with double-T-junction microchannel, double-cross-shaped microchannel and several three-dimensional (3D) microchannels included (Chong, et al., 2015). Besides, lower cost, more natural and higher efficiency stabilizer, e.g., complex of protein, polysaccharides and polymeric surfactant, or highly lipophilic soy lecithin, can be developed to obtain stable double emulsions (Shukai, 2019).

Understanding the mechanism of the instability of W/O/W double emulsions

General theory of instability of W/O/W emulsion

The stability of the double emulsion is a relative concept. The droplet size of W/O/W emulsions are in the range of 20–100 μm, and they possess a larger interfacial area than the single emulsions (W/O and O/W), resulting in their inherent instability (Neumann et al., 2018). The instability of double emulsion typically involves one or more phenomena, including leakage of the encapsulated compound from the inner phase, phase separation caused by coalescence of the inner aqueous or oil phase, high packing density inducing the rupture of the oil phase, and migration or diffusion of different phases. Stoke’s law (Eq. 1) can be used to describe the phase separation of unstable double emulsion:

$$\text{V}=\frac{2g{r}^2(d_1-d_2)}{9\eta }$$ 1

where V is the velocity of separation (cm/s), g is the acceleration due to gravity (980 cm/s²), r is the droplet size, d₁ and d₂ are the density of continuous and dispersed phase (g/cm³), respectively, and η is the viscosity of continuous phase (g/cm s).

From the equation, it can be found that the factors influencing the stability of double emulsion include droplet size, the difference in density between the continuous and dispersed phase, and the viscosity of the continuous phase (Lamba et al., 2015).

Emulsifiers distribution and osmotic active substrates

The W/O/W emulsions are stabilized using two emulsifiers with varied HLB values (Fig. 3). The lipophilic surfactant stabilizes the inner water phase of the W/O emulsion, and the hydrophilic surfactant is responsible for stabilizing the oil to disperse uniformly and prevents them from coalescence (Hua et al., 2022). The lipophilic emulsifier absorbs on the inner interface, while the hydrophilic substrates absorb on the outer layer. The free lipophilic emulsifier can disperse in the oil phase and the hydrophilic surfactant dissolves in the outer water phase (Fig. 4a). Hence, both the emulsifier molecules do not contact each other and have the responsibility of stabilizing the double emulsion.

Fig. 3.

Emulsifier distribution of W/O/W emulsion. a Idealized distribution with stable structure. b Realistic distribution with unstable structure

Fig. 4.

Possible instabilities of W/O/W emulsion inducing by swelling-breakdown, coalescence, diffusion and Ostwald ripening

Since emulsifiers possess amphiphilic properties, they can disperse in both water and oil phases, as well as at the interfaces (Chevalier et al., 2022). Thus, the hydrophilic emulsifier adsorbed at the outer O/W phase may partition into both the outer and inner water phases (Fig. 4b). Lipophilic emulsifiers not only occupy the inner W/O interface and diffuse within the oil phase, but also can migrate to the outer interface. Demulsification occurs when the hydrophilic surfactant in the W/O phase separates the interfaces from each other (Hippmann et al., 2018). Thus, the partitioning of emulsifier between the water and oil phases is governed by oil droplet characteristics and the molecular structure and properties of the emulsifiers.

In addition, incorporating osmotically active components into the inner aqueous phase of W/O emulsions helps balance capillary pressure across the two aqueous phases, thereby inhibiting Ostwald ripening (Matos et al., 2022). The osmotic active molecules move across the interface and dissolve in the outer water phase through diffusion or coalescence. The delivered molecules can also increase the osmotic activity of the water phase at higher concentrations (Kwak et al., 2022). Within the emulsion system, when a high concentration of encapsulated molecules is included, the uniformity of the droplets cannot be maintained with wide size distribution, high polydispersity and low encapsulation efficiency, and then rupture may occur during emulsification. In a relevant study, Choi et al. (2020) found that the optimal loading ratio of bioactive peptides for double emulsion preparation was 5%. When the concentration of bioactive peptide exceeds 10%, it indicated that the osmotic value was considerably high, causing premature release of encapsulated molecules from the inner to the outer aqueous phase during preparation and storage, ultimately leading to emulsion destabilization.

As shown in Eq. (2), Laplace pressure is a function of the size of the inner droplets and the surface tension, which is inversely to the droplet size. On the other hand, according to Eq. (3), the osmotic pressure is independent of the droplet size and is correlated to the concentration gradient of the two continuous internal and external phases. Thus, the balance between Laplace pressure and osmotic pressure is important for the stability of double emulsion. When the osmotic pressure is smaller than the Laplace pressure, shrinkage of the inner droplets occurs; while, if the osmotic pressure is lager, the inner phase swell (Khadem et al., 2020). If the droplet is over swelled, it is followed by a rupture in the oil membrane, which is later call swelling-breakdown (Fig. 4).

$$\mathrm{\Delta }\text{P}=\frac{4\mathrm{\sigma }}{\text{d}}$$ 2

where σ is the surface tension and d is inner droplet size.

$$\mathrm{\Delta }\mathrm{\pi }=\text{iRT}\mathrm{\Delta }\text{C}$$ 3

where i is the van’t Hoff factor, R [8.314 J/(mol·K)] is the universal gas constant and T (K) is temperature.

Coalescence

Coalescence is an irreversible phenomenon in double emulsions (Cowell et al., 2024). Other instability mechanisms such as creaming and agglomeration are reversible, and hence it is not featured in this review. The inner water-water coalescence occurs when the droplet merges together (Fig. 4). The behavior of the emulsion remains constant, though the particle size and size distribution of the inner W/O emulsion is altered. The characteristics of the W/O/W emulsion primarily depend on the oil droplet diameter and percentage volume of the oil loaded in the emulsion. However, the water-water coalescence changes the behaviour of double emulsion slightly, since they manly depend on the oil droplet size distribution and volume ratio of the oil droplets. On thing should be noted is that larger inner aqueous droplet may behave differently in water-water coalescence. For example, when the inner water droplet size increase, the capillary pressure decrease, which may lead to diffusion. Apart from water-water coalescence, oil-oil coalescence also often occurs (Fig. 4). It is found that larger oil droplets could cream faster. When the oil droplet size increases, the viscosity of double emulsion decreases, accelerating more creaming. Thus, the double emulsion could become unstable due to the separation of the water phase and the inner W/O emulsion in an extreme case (Chen et al., 2023b). The third type of coalescence occurs between the inner water droplet and the outer water, and the double emulsion may even disrupt to form a single O/W emulsion. It results in instability as the encapsulated ingredient is released along with this type of coalescence. Under these conditions, two interfaces with completely different curvatures and distinct surfactant components come into contact with each other (Leister and Karbstein, 2020).

Pays et al. (2001) hypothesized that coalescence rate per unit time is proporational to the number of inner droplets adsorbed on the droplet surface. Kang et al. (2016) proposed a dewetting model for double emulsion that the viscosity of the continuous phase is sufficiently small and the escape time is estimated based on the balance between the viscose forces and the interfacial tension. Khadem and Sheibat-Othman. (2019) thought the escape is related to the presence of inner droplets in a critical area close to the surface of outer droplets. They developed a model (Eq. 4 and 5) to evaluate the escape rate and the volume of the critical region was obtained experimentally.

$$\text{Coalescence}\left(P\right)={(\frac{\pi \sigma r_m^2}{2A_H})}^{-\frac{4}{9\pi t_{\mathit{cir}}}(1+\frac{\mu M}{\mu x})}$$ 4

$$\text{Volume fraction of critical region}\left({\propto }_{\mathit{cr}}\right)=1-2exp\left(-\frac{r_M}{r_m}[2.22+1.51{\left(\frac{{\mu }_M}{{\mu }_{\mathit{ex}}}\right)}^{-0.57}\right)$$ 5

where $\sigma$ interfacial tension (N/m), r is droplet radius (m), A_H is Hamaker constant, μ is viscosity (Pa.s), t_cir is total circulation time, m is micro-droplet, M is macro-droplet.

Diffusion

Two types of diffusion occur generally, including the water diffusion from the outer to the inner phase and the inner phase diffusion to the outer water (Fig. 4). The differences in droplet size leads to differences in capillary pressure, which is the basis for Ostwald ripening. Larger droplets grow and smaller ones shrink. The diffusion in emulsions is influenced by the type and concentration of surfactant. The lipophilic (HLB < 9.0) and hydrophilic (HLB > 11.0) surfactants are helpful to stabilize emulsion, with the former being typically applied to stabilize W1/O interfaces and the latter for O/W2 interfaces (Heidari et al., 2022). The most common used emulsifiers are PGPR, proteins, soluble polysaccharides and protein-polysaccharide complexes, which are going to discuss at the following section. Owing to the thickening or gelling properties, the viscosity of the continuous phase of W/O/W emulsion increase, and thus the Brownian motion of droplets and creaming rate decrease. Besides, the high shear stress during emulsion preparation can lead to the internal flow of the droplet which increases the collision frequency between the inner and outer water phases (Xia et al., 2024). There is a pressure gradient between the small and the large droplet, and the small droplet diffuses into the large droplet when the emulsion viscosity decreases or the internal environment changes duing to a greater dispersed phase (Kumar et al., 2022). Water and water-soluble bioactive compounds diffuse from the inner water phase through the oil phase to the outer water phase in a reverse micelle transport way, leading to the loss of the inner water phase and the destruction of the double emulsion structure (Lamba et al., 2015).

Stability under gastrointestinal conditions

W/O/W emulsion face a big challenge on the stability when it goes through the gastrointestinal track since the hostile environment. The rapid pH changes, the strong ionic strength and the various enzymic hydrolysis are all pose significant obstacles to its stability. At low gastric pH and high ionic strength environment, the osmotic pressure of the medium induces the emulsion droplet coalescence and increase in particle size; when the pH is in alkaline environment at small intestine, the emulsion system droplets are smaller and uniformly dispersed (Huang et al., 2022). Pancreatic lipase in the small intestine can enter the oil phase to hydrolyze it with free fatty acid release, while, single emulsion (e.g., W/O) released markedly higher fatty acids than that of W/O/W emulsion (Huang et al., 2022). Apart from the changes in the structure of emulsion, the delivered cargos also display obviously changes, mainly saying release of entrapped compounds. Generally, four different release manners happen for the cargos (Heidari et al., 2022): First, Brownian motion of inner droplets induces coalescence of different phases, leading to the escape of the cargo release. Second, the breakage of the outer droplets makes the release of entrapped molecules from inner droplets to the outer continuous phase. Third, swelling-breakdown happens when the osmotic pressure overcomes the Laplace pressure, causing the release of cargos from inner phase to the outer one. Fourth, cross the intermediate layer without film rupture through molecular diffusion or permeation by Fick’s law.

How to improve the stability of W/O/W emulsions?

Osmotic pressure tailoring

Destabilization of W/O/W emulsions can be induced by water diffusion from the inner to the outer aqueous phase driven by osmotic pressure gradients (Djaber et al., 2024). Osmotic pressure arises from substantial concentration differences between the inner and outer aqueous phases. When the solute concentration of the inner aqueous phase is lower than that of the outer phase, water molecules migrate outward into the W/O emulsion. In contrast, when the inner aqueous phase exhibits a higher solute concentration, the W/O droplets absorb water from the outer phase (Sameh et al., 2012).

A considerable body of theoretical and experimental evidence supports the role of osmotic pressure in governing the behavior of W/O/W emulsions. Owing to their complex structure and large oil–water interfacial area, W/O/W emulsions are inherently unstable. Water molecules exchange across the inner aqueous phase boundary, and water migration between the inner and outer aqueous phases is mainly driven by osmotic pressure. This migration induces shrinkage or swelling of the oil phase, thereby modulating the stability of W/O/W emulsions. The direction of water diffusion is especially critical. For instance, swelling of the inner phase can lead to pronounced changes in the rheological properties of W/O/W emulsions. Furthermore, such swelling may compress inner droplets or induce droplet aggregation, thereby promoting the release of water-soluble compounds.

Thus, to fabricate stable double emulsions, osmotic balance between the two aqueous phases must be carefully regulated. The incorporation of salts into the internal aqueous phase can enhance the stability of W/O/W emulsions. The swelling and shrinkage behavior of W/O/W double emulsions can be modulated by adjusting the ratio of osmotically active components in the two aqueous phases, thereby counteracting the Laplace pressure derived from droplet surface curvature. For example, the addition of sodium favors the enhancement of W/O/W emulsion stability (Raviadaran et al., 2019), since salts play a dual role in double emulsions, acting not only as osmotic regulators but as interface modifiers by significantly altering interfacial properties. To address the structural collapse of W/O/W emulsion in the study of Chang et al. (2026), NaCl, KCl, MgCl₂, and CaCl₂ were added in the outer aqueous phase to coarsened the particle diameters and decreased the bulk viscosity, with the structural stability of emulsion enhanced. They found that salts in the outer aqueous phase could inhibited water migration and enhanced encapsulation efficiency, with CaCl₂ demonstrating optimal encapsulation retention.

In addition, polysaccharides are used to control the internal droplet diameter by altering the chemical potential of the water phases. The equilibrium of the internal droplet size is achieved through Laplace pressure equalization, as an osmotic pressure difference exists between the two water phases after the addition of neutral linear polysaccharides, resulting in a relatively stable double emulsion system (Wei et al., 2024). For instance, carrageenan was a suitable stabilizer for the external water phase in the double milk emulsion during storage, as it prevents the aggregation and coalescence of the internal droplet (Klojdová et al., 2018). In the study of Deng et al. (2026), they used sodium alginate and propylene glycol alginate to strengthened interfacial film by forming hydrogen bonding and reduce W/O/W droplets size, then a stable yolk-based W/O/W emulsion system was obtained. 1.0% concentration of xanthan gum and carrageenan in the inner phase of W/O/W emulsion can slowed down droplet floating and coagulation, especially at acidic external aqueous phase (pH 4) led to larger and more uniform particle size distribution, displaying an enhanced stability by decreasing the viscosity and delaying molecular exchange in the oil phase (Jiang et al., 2024).

In addition, proteins such as sodium caseinate, gelatin, and whey protein also function most commonly as emulsifiers to regulate the stability of double emulsions by altering osmotic pressure. The stabilization mechanism can also be attributed to an increase in the viscoelastic barrier formed by the interaction between proteins and emulsifiers, thereby preventing coalescence of emulsion droplets and loss of ingredients delivered into the inner droplets (Zhu et al., 2018). Hybrids, complexes, or combinations of proteins and polysaccharides can enhance the stability of double emulsions and the migration of inner water to the outer water phase (Benichou et al., 2007). In addition, whey protein isolate (WPI) and pectin are dissolved in the outer water phase and serve as hydrophilic emulsifiers during the preparation of W/O/W emulsions (Mezzenga et al., 2004).

Selection the right emulsifiers

Hydrophilic emulsifiers

The stability of W/O/W emulsions is influenced by many factors, such as the types of emulsifiers, fat crystal addition, and the droplet size of the W/O emulsion. The HLB value of an emulsifier depends on the balance between the strength and size of the hydrophobic and hydrophilic moieties of the molecule. A lower HLB value indicates that the emulsifier has greater lipophilic characteristics, while a higher HLB value means the emulsifier has stronger hydrophilic ability. Conventionally, hydrophilic emulsifiers have HLB values around 8–18, and hydrophobic emulsifiers have HLB values ranging from 4 to 6.

Tween 80 is one of the most used hydrophilic emulsifiers to formulate emulsions. For instance, Kim et al. (2016) used soybean oil-Tween 80 to fabricate the delivery system of apigenin-loaded W/O/W emulsions, which increased the bioavailability in the digestive tract. A stable double emulsion containing bioactive peptide was formulated using Tween 80 or mixture of Tween 80 and chitosan as hydrophilic emulsifiers in the study of Choi et al. (2020), who showed that double emulsion stabilized by Tween 80 layers displayed well physical stability without phase separation. Tween 80 was used as a control hydrophilic emulsifier in the study of Sobti et al. (2023), and they found that double emulsions prepared from plant proteins (such as soy protein isolate and pea protein isolate) had higher encapsulation efficiency and better physical stability than those prepared from animal proteins (WPI). However, the safety of Tween 80 used in food and nutrition is a concern, which limits its addition in actual production processes.

Apart from Tween 80, proteins, soluble polysaccharides, and protein-polysaccharide complexes are frequently used as hydrophilic emulsifiers. This is because they provide thickening and gelling properties, which result in a high viscosity of the continuous phase and a reduction in droplet Brownian motion and creaming rate (Heidari et al., 2022). WPI and whey protein concentrate (WPC) are the commonly used hydrophilic emulsifiers to prepare double emulsion. WPI was used as hydrophilic emulsifier to prepare W/O/W emulsion in the study of Eisinaite et al. (2018), and it was found that higher content of PGPR lead to higher viscous of double emulsions, which had high creaming, thermal stability and encapsulation efficiency (˃90%). WPC, either alone or combined with gum arabic and high methoxyl pectin, was used to fabricate a double emulsion for delivering arbutin and coumaric acid. Compared to WPC alone, the WPC-high methoxyl pectin stabilized emulsion exhibited the highest viscosity and stability, resulting in the highest encapsulation efficiency: 91% for arbutin and 80% for coumaric acid (Huang et al., 2019). Moreover, a WPC-high methoxyl pectin complex was also used as a hydrophilic stabilizer in the W/O/W emulsion for co-delivering brassinolide and cinnamon essential oil. When the ratio of WPC to pectin was 1:3, the double emulsion exhibited the best storage stability and highest encapsulation efficiency (Huang et al., 2021). Nanoparticles formed by the covalent conjugate of WPI and epigallocatechin gallate (EGCG) were used as a stabilizer to prepare a double emulsion, which serves as a colon-targeted release carrier for probiotics. This offers a promising strategy for the encapsulation of probiotic strains in liquid form for oral applications in foods and pharmaceuticals (Qin et al., 2021).

Recently, the demand for alternative protein sources mainly from plants is increasing, since the food sustainability requirement, particularly in terms of the climate change and economic implications. Plant protein from soy, lupin, wheat gluten, oilseed and beans are the commonly used emulsifiers, and more information is discussed in the following section for the soy source. It is found that soy protein isolate with the globular structure had high-rate inhibition of mass transport in W/O/W double emulsion (Liu, et al., 2021). However, plant-derived proteins often have limitations in their functional properties (Brito-Oliveira et al., 2025): (i) several aspects of soy protein physicochemical behavior still need to be clarified; (ii) alternative plant proteins as emulsifiers (not obtained from soy, pea or more common beans) are needed more deeply investigated to understand their emulsifying properties. Thus, most current studies like to combine plant and animal protein to improve the functionality of double emulsions. For example, Paximada et al. (2021) applied rice protein, pumpkin seed protein and whey protein isolate together to produce the primary W/O emulsion with milk fat as the oil phase. Double emulsion prepared from 100% plant-based protein are very susceptible to destabilizations, resulting in a limited studied and it would be necessary and suggested for more future investigated.

Hydrophobic emulsifiers

PGPR is one of the most effective hydrophobic emulsifiers for fabricating W/O/W emulsions. The stability of W/O (20/80, v/v) and W/O/W (4/16/80, v/v/v) emulsions is significantly improved at PGPR concentrations above 2%. This hydrophobic surfactant induces structural rearrangements within the oil phase, thereby strongly facilitating the spontaneous formation of small water droplets in the oil phase (Kocaman et al., 2020). When the concentration of PGPR is lower than 2.0%, the emulsion becomes unstable and undergoes phase separation. This is attributed to an increase in the internal water content of the primary W/O emulsion and reduced viscosity, leading to coalescence and expulsion of the internal water. Sausages, a typical food containing a double emulsion, exhibit varied physical properties and textures when PGPR is used at a high concentration (Adams et al., 2011).

Soybean lipophilic protein (LP), a hydrophobic molecule with a core–shell structure, can reduce the interfacial tension of oil/water emulsions. It prevents lipid droplet coalescence through the physical barrier formation at the interface. Soy protein isolate is classified into three components, namely β-conglycinin (46%), lipophilic protein (LP, 31%), and glycinin (23%). A study has reported that the surface tension decreases to 10 mN/m after adding 0.0001% LP to water. However, no significant changes were observed with the addition of the same amount of β-conglycinin or glycinin (Matsumura et al., 2017).

Tween 20 is a synthetic surfactant consisting of three polar polyoxyethylene heads and a non-polar saturated hydrocarbon (C12) tail, and it stabilizes the emulsion mainly by steric repulsion (Shen et al., 2022). Besides, Tween 20 is relatively insensitive to pH, and it can reduce the hydrophobic interactions of oil droplets, resulting in the formation of a stable emulsion (Zhang et al., 2021). Span 80 is an advanced lipophilic emulsifier that has limited solubility in water. However, it can dissolve in hot oils and organic solvents. Multiple W/O/W nanoemulsions composed of sesamol and retinol were developed in the study of Yousefi et al. (2023), who used Span 80 as the emulsifier in the oil and water phase. They found that the highest encapsulation efficiency and lower particle size were obtained when the concentration of Tween 80 was 10.84%.

Amphiphilic emulsifiers

Casein is an amphiphilic protein emulsifier composed of αs1 − , αs2 − , β − , and κ-casein, which have regions that are predominantly hydrophobic and hydrophilic. As a result, it can coat on the surface of oil droplets to prevent aggregation, thanks to the presence of steric hindrance and electrostatic repulsion (Silva et al., 2020). However, casein is strongly influenced by the changes in ionic strength and pH, which are not beneficial for the formation of a stable emulsion. Currently, high-pressure homogenization has been used to improve the emulsifying ability of casein (Han et al., 2020). This technique unfolds casein molecules and introduces steric hindrance, thereby inhibiting intermolecular aggregation. Such unfolding exposes additional hydrophilic and hydrophobic groups, thereby improving the emulsifying performance of casein. Besides, pH adjustment, ultrasonic and enzymatic treatments are also employed to enhance the emulsifying capacity of casein (Wang and Zhao, 2022). Moreover, double emulsions have been prepared using casein and whey protein, with milk protein isolate and whey protein isolate serving as the internal and external water phases. It has been found that the stability and nutritional properties in complex dairy-based food systems have been significantly improved (Silva et al., 2020).

Soy lysophospholipid is a phospholipid-based emulsifier obtained by removing a fatty acid from the phospholipid molecule. This process increases its polarity and enhances emulsion stability. In lysophospholipid, the fatty acid chain is non-polar, while the phosphate group is a polar moiety responsible for emulsification (Mayar et al., 2020). However, lysophospholipids exhibit lower emulsifying capacity than proteins and many other surfactants. Moreover, they are unstable under acidic conditions and high salt concentrations, owing to reduced electrostatic repulsion.

Gum Arabic is an amphiphilic polysaccharide emulsifier, which consists of polar polysaccharides covalently linked to the non-polar polypeptide chains, enabling them to absorb onto the surface of the oil phase to prevent the aggregation of each phase (Zhao et al., 2025). However, once gum Arabic coats the oil droplet surface, it forms a thick layer that acts as oil resistant due to the aggregation under various conditions through steric repulsion. The ratio of small and large arabinogalactan units in gum Arabic has a remarkable emulsification ability (Liu et al., 2021). Anionic pectin is another polymer emulsifier, and the stabilization potential of pectin is primarily because of its ability to increase the viscosity of the water phase in the emulsion (Huang et al., 2022). Meanwhile, the emulsifying performance of pectin also arises from its hydrophilic and hydrophobic moieties. Pectin adsorbs at the interface via hydrophobic groups, whereas its hydrophilic segments extend into the aqueous phase, forming a protective layer around oil droplets that inhibits coalescence and aggregation (Niu et al., 2025).

Quillaja saponin is a natural and amphiphilic emulsifier with a small and ellipsoid molecular structure, and there are non-polar (quillaja acid moiety) and polar (sugar moiety) groups on its surface (Reichert et al., 2019). Generally, the molecule is negatively charged (pH > 3) under most food conditions, indicating that the substance can stabilize emulsions through steric and electrostatic repulsion. As an emulsifier, saponins are naturally degradable and can quickly reduce interfacial tension while increasing the negative charge of the emulsion, thereby effectively stabilizing it (Riquelme et al., 2019). Prichapan et al. (2020) studied the influence of Quillaja saponin on the formation and stability of iron-delivered W/O/W emulsion, and they found the size of the oil droplets in double emulsions decreased with an increase in the saponin concentration. Currently, food-grade Q-Naturale® has been isolated from the barks of the Quillaja saponaria Molina tree and used as a surfactant in emulsions, suggesting the replacement of synthetic surfactants in food and beverage products (Yang et al., 2013).

Quillaja saponin have several advantages over other biosurfactants and synthetic surfactant like PGPR. For one thing, proteins are often unstable to aggregation at their isoelectric point, and at high temperatures and ionic strengths. Besides, it is usually difficult to produce double emulsion with very small droplet sizes applying polysaccharide-based emulsifiers. For natural lecithin, it is had to be chemically or enzymatically modified to lysolecithin before it can stabilize double emulsion, which increase the cost of production (Yang et al., 2013). Quillaja saponin are more effective to produce smaller droplet size at lower concentrations, attributing to low molecular weight (1.67 kDa) and high tendency to adsorb to the droplets surface and create thin interfacial layers. A previous study showed that emulsions prepared with Quillaja saponin displayed high stability in a wide pH ranging from 3 to 7 (Ahmadi and Jafarizadeh-Malmiri, 2020). For the other thing, PGPR is not a natural emulsifier and its use is very limited in food products. Meanwhile, the concentration of PGPR higher than 5% generating an unpleasant taste. Conversely, Quillaja saponin are natural, bioavailable, biodegradable, environment friendly, high foaming, and low-cost, which are very important for the practical and commercial application (Panagopoulou et al., 2017).

Selection of biopolymers

The selection of a suitable emulsifier for double emulsion is critical. Various food-grade biopolymers have been reported to stabilize the primary emulsion droplets in the W/O/W emulsion. There are two technical advantages of using biopolymers as stabilizers. First, biopolymers do not diffuse from one phase to the other, thus they are better suitable for the stabilization of W/O/W emulsion than the short-chain hydrophilic emulsifiers (Schuch et al., 2015). Second, biopolymers can readily form stable network structures both in continuous aqueous phases and on the outer oil–water interface (Dickinson, 2011).

In the study of Kavitake et al. (2019), galactan exopolysaccharides, extracted from Weissella confuse KR780676, were optimized to stabilize W/O/W emulsion. They found galactan exopolysaccharides had higher emulsifying activity for the vanilla and pineapple flavor emulsion, which demonstrated reliable thermal stability between − 20 and 60 ℃. Liu et al. (2021) compared three plant-based emulsifiers including soy protein isolate, pectin, and gum Arabic, and they found that pectin was most effective in producing small droplets of W/O/W emulsion at lower concentrations. An interesting study employed a one-step method for double emulsion preparation, in which two oppositely charged biopolymer particles-zein nanoparticles and cellulose nanocrystals-stabilized the interface via electrostatic interactions across different curvatures (Li et al., 2021). β-Carotene and epigallocatechin gallate were encapsulated in this W/O/W emulsion, and the combined use of both biopolymer particles was shown to synergistically enhance double emulsion stability under diverse conditions. Beyond single biopolymers used as stabilizers, protein–polysaccharide complexes provide considerable potential for designing tailored interfaces in double emulsions for targeted applications. For example, Hernández-Marín et al. (2013) utilized soluble whey protein concentrate-carboxymethylcellulose complexes to stabilize W/O/W emulsions and achieved more stable multiple emulsions at pH 4.3. More recent study from Cui et al. (2023) used whey protein fibril-cellulose (WPIF-CNC) complexes to prepare a double emulsion. Resistant to heat and salt stress ability, and higher bioaccessibility of delivered agents, curcumin and EGCG, were observed for the double emulsion.

In addition to the above-mentioned, the emerging Pickering stabilizer for double emulsion fabrication is attracted much attention. The traditional W/O/W emulsion is easily destabilized due to the excess free energy at the two oil–water interfaces and the imbalance between osmotic pressure and Laplace pressure in the inner and outer aqueous phases (Zhu et al., 2026). While, the Pickering emulsifier with above certain size and an intermediate contact angle tend to adsorb strongly at the oil–water interface, resulting in a high energy barrier to droplet shrinkage and drop-drop coalescence. The emulsifiers stabilized W/O/W Pickering emulsion are usually solid biopolymer particles such as cellulose, protein assemblies and starch particles. In the study of Marefati et al. (2015), they developed a food-grade oil filled powders from modified starch granules stabilized W/O/W Pickering emulsions, which showed increased process stability than that of the traditional emulsions. Nie et al. (2025) formulated a Pickering emulsion stabilized by whey protein microgel particles combined with hydroxypropyl methylcellulose at the external interface, and they found low concentrations of hydroxypropyl methylcellulose could enhance emulsion stability by promoting faster molecular rearrangement and increasing viscoelastic modulus of the interfacial membrane.

Reduction of droplet size

The droplet size, specifically for the inner W/O emulsion, is an important property for the stability of the double emulsion. Numerous factors can influence particle sizes, such as preparation method, oil phase composition, interfacial properties, and viscosity of the phases (Aditya et al., 2015). The small-sized particle possesses higher stability, primarily because the Brownian motion may be sufficient to overcome gravity. In other words, creaming or sedimentation should not occur even on prolonged storage, and the small-size emulsion prevents flocculation and coalescence (Bai et al., 2021). Liu et al. (2023) found that the representative size D50 has a positive linear correlation with turbiscan stability index value, suggesting the smaller the particle size, the more stable the emulsion. They also used Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and Stokes-Richardson-Zaki model to elucidate that the smaller droplets have the greater energy barrier to overcome before coalescing and lower rising velocity to move to the upper oil phase, and hence better stability. However, the results from the study by Yildirim et al. (2017) contradicted the observation. Though the high-speed homogenization created a reduction in particle size of the primary emulsion, it did not drastically affect the stability and delivery efficiency of the double emulsion. However, there is a limitation in the study of effect of double emulsion size on the stability of W/O/W emulsion, and most of them are related to the encapsulation efficiency.

Sports nutrition application using W/O/W double emulsion: an overview and trends in brief

The classifications of supplements for sports nutrition

As consumer demands have increased, the diversity of sports nutrition supplements has also grown. Currently, there are four main types of supplements categorized by their functions (Fig. 5). First, supplements for speed and strength. Certain nutrients can be directly utilized to enhance the body’s energy reserves and explosive power. At present, creatine, glutamine, β-alanine, calcium β-hydroxy-β-methylbutyrate and fructose 1, 6-diphosphate are the key ingredients widely used to improve speed and strength. Second, supplements for endurance. B vitamins (especially VB₁, VB₂ and VB₆) and caffeine can improve metabolism and facilitate the energy production. L-carnitine helps to promote fat oxidation, reduce the accumulation of lactic acid in muscle cells, and delay the occurrence of fatigue (Posridee et al., 2025). Arginine and citrulline both lead to NO production and thus improve subjective feelings of fatigue and soreness for endurance sportsman (Suzuki et al., 2019). Third, supplements for sports recovery. L-glutamine can promote protein synthesis and to provide energy for immune cells, and thus inhibit the decline of immune function after a long-term and high-intensity exercise. Branch chain amino acids (BCCAs), including valine, leucine and isoleucine, play an important role in exercise recovery such as reducing muscle protein breakdown and increasing lean body mass, promoting anabolism, relieving fatigue after exercise and reducing muscle breakdown (Yang et al., 2025). Besides, calcium and vitamin D can support the bone health and muscle function (Abushamma and Adeeb, 2022). Fourth, antioxidant and anti-inflammation. Exercise-induced oxidative stress can lead to muscle damage and fatigue. Antioxidants and anti-inflammatory agents like vitamin C and E, selenium, alpha-lipoic acid (ALA), resveratrol, ginger root powder, can neutralize free radicals and reduce soreness and inflammation after intense workouts, accelerating recovery (Liu et al., 2024).

Fig. 5.

Four types of supplements (speed and strength, endurance, muscle recovery and both health, and antioxidant and anti-inflammatory) based on their functions

Why W/O/W double emulsion?

The training ability, performance, and recovery time of athletes can be enhanced by the appropriate consumption of a diet that boosts energy and balances the nutrients optimally (Bu et al., 2025). To avoid disturbance in immune function, loss of fat-free mass, increase in susceptibility to injury, and decrease in bone mineral density, sufficient intake of nutritional supplements is essential for the sportsperson. The W/O/W emulsion is widely used as a delivery system for nutrients, and many studies have shown the application of double emulsion in masking unpleasant tastes, control release of nutrients in the GI tract, and enhancement of the shelf-life and bioavailability of food and beverages (Zhi et al., 2023). W/O/W emulsion shows better slow release of glucose to the body than some of other carriers since its double-layered water phase and one-layered middle oil phase, which exhibits double physical barriers for gastrointestinal tract environment and thus could be beneficial for those with athletes who require a more sustained release of energy during exercise (Norton et al., 2015). Besides, double emulsion can simultaneously deliver both hydrophilic and hydrophobic bioactive compounds in the inner water phase and oil phase with high delivery efficiency, and this property is rare found in other carriers. Referred to the design of emulsion parameters (droplet size, phase ratio, and emulsifier selection) relate to athlete-specific physiological conditions, W/O/W emulsion exhibits obvious advantages. When eating to promote high-performance exercise, functional proteins and fat are required. These macronutrients normally have large molecules that hard to entrapped in other nanotechnologies. Whereas, W/O/W emulsion possess large size with ‘huge’ entrapped volume in both aqueous phases and oil phase, and the emulsifiers used for double emulsion formulations can be edible like protein and lecithin, which are also good for muscle development and post-exercise recovery. Last but the most, the W/O/W emulsion can be tailored to fit for many requirements, such as controllable size for abundant delivering amount, double emulsifiers used for regulating release speed.

Currently, food-grade W/O/W emulsions have been formulated currently to encapsulate functional compounds, such as minerals, vitamins, amino acids or peptides, and polyphenols to mask the unpleasant flavor, enhance the cargo’s stability, solubility, anti-oxidation and bioavailability, and controlled release (Fig. 6). These formulae that assure the delivery of nutrients could be applied in sports nutrition.

Fig. 6.

Potential applications of W/O/W emulsion in sports nutrition including mask flavor, controlled release, enhance environmental stability and improve bioavailability

Flavor masking for athletes

Flavor masking emulsions enable the delivery of nutrients into inner water or oil phases in drinks and chews without their characteristic bitter, fishy, and astringent tastes. Medium-chain triglycerides (MCT), which consist of C8 and C10 fatty acids, are transported directly to the liver without the need for re-synthesis of triacylglycerols, providing quick energy with minimal accumulation in adipose tissues. They are quite good source for the people doing exercise, while the weak stability and the followed gastrointestinal discomfort side effect restrict their application. Jeong et al. (2025) therefore prepared xanthan/locust bean gum–based W/O/W double emulsions to carry MCT and canola oil, and they found that the double emulsions exhibited superior stability under extreme conditions, with xanthan gum/locust bean gum further enhancing thermal stability. Caffeine is commonly supplemented by athletes before or during competitive events, owing to its increasing mobilization of intracellular calcium, enhancing in free fatty acid oxidation, and serving as an adenosine receptor antagonist in the central nervous system (Silva et al., 2025). Whereas, it includes two drawbacks of pronounced bitterness and astringency and fast absorption into blood (de Souza et al., 2022). Thus, W/O/W emulsions was designed in the study of Hernández-Marín et al. (2016) to encapsulate caffeine to helped to overcome these problems. Branched-chain amino acids (BCAAs: leucine, valine, isoleucine) are oxidized in the muscle and then stimulate anabolic signals, which in return optimized activities and recovery from injury (Martinho et al., 2022). Despite the advantages of BCAAs in sporting, they are hard to consume duing to the uniquely bitter taste (Lee and Shin, 2023). Liposomes prepared from phospholipid and particles used gum arabic and maltodextrin as wall material have been reported to deliver BCAAs (Giroldi et al., 2022; Jo et al., 2020), while hardly study was used W/O/W emulsion to carry BCAAs. However, compared to other delivery systems, W/O/W emulsion shows higher encapsulation efficiency since it has two aqueous phases, and better controlled release ability due to the bilayer structure and tunable properties of each layer, especially for the particles solidified from double-emulsion droplets (Chong et al., 2015). Thus, these advantages suggest W/O/W emulsion could be a big potential application in sport area for loading more ingredient and better masking unpleasant flavor and controlling release at target region.

Delayed, sustained and targeting release of nutrients in sports

Vitamins serve primarily as regulators of metabolic functions, and a survey showed that most elite athletes take 50–100 times higher of dosages than the US recommended dietary allowances (Peeling et al., 2023). Vitamin C is an antioxidant that can improve tolerance by mitigating against excessive tissue damage (Close et al., 2022), so its sustainable supply in the body could improve muscle strength during long-term exercise. W/O/W double emulsions stabilized by gelatin-EGCG-high methoxyl pectin ternary complex were fabricated to load vitamin C in the study of Huang et al. (2022), and only 25% of vitamin C released during 2 h of simulated intestinal digestion, indicating the sustained-release function of the double emulsion. In another study on double emulsions followed by complex coacervation in gelatin-sodium caseinate, vitamin C was also encapsulated in the system and exhibited controlled release behavior in vitro (Fraj et al., 2021). Bou et al. (2014) focused on the influence of lipid sources on the physicochemical properties of riboflavin (vitamin B₂) plays essential roles as cofactors in enzymatic reactions of anabolic pathways to produce energy during metabolism. In this study, olive oil, sunflower oil, chia oil, or rendered pork backfat were used as oil phase to fabricate food-grade riboflavin-loaded W/O/W double emulsion, respectively. The authors illustrated that the double emulsion containing chia oil was the most efficient in delivering riboflavin, however, all formulas could slow release of riboflavin at room temperature for 8 days.

Curcumin is a safe antioxidant that is beneficial for sports and physical exercise in the body. During training, curcumin supplementation appears to reduce inflammation and oxidative stress, decrease pain and muscle damage, and enhance recovery and muscle performance (Suhett et al., 2021). However, its poor solubility and high susceptibility to oxidation have restricted its practical application. Microencapsulation represents a promising strategy for targeted delivery, yielding substantially higher bioavailability than free curcumin. A novel pH-responsive double emulsion exhibited favorable sustained release of curcumin in MCF-7 cell models, thereby helping to prevent overdosage and mitigate adverse effects (Pourmadadi et al., 2023). More study focused on the co-delivered curcumin with other bioactive compounds in the double emulsion. Aditya et al. (2015) fabricated a W/O/W emulsion system for the co-delivery of hydrophobic curcumin and hydrophilic catechin via two-step emulsification. The authors achieved encapsulation efficiencies of 88–97% for both compounds, and co-encapsulation exerted no adverse effects on emulsion stability while enabling sustained release of the two components in beverage matrices. In another study, curcumin and EGCG were co-encapsulated in W/O/W double emulsions stabilized by whey protein isolate fibrils and cellulose nanocrystals (Cui et al., 2023). This composite system exhibited strong resistance to thermal and salt stress, allowing controlled release of curcumin and EGCG during in vitro digestion (Cui et al., 2023). A W/O/W emulsion stabilized by genipin-cross-linked sugar beet pectin-bovine serum albumin nanoparticles was created to co-deliver curcumin and betanin (Tang et al., 2021). Simulated digestion assay revealed that the robust double emulsion could delay nutrients release and improve their bioavailability.

Nutrient stability enhancement

Energy metabolic enzymes, such as CCO, SDH, Ca²⁺-ATPase, and H⁺-ATPase, were activated, along with higher levels of energy charge and ATP, which can be applied in sports beverages or nutrient foods for athletes. However, enzymes are typically very unstable during storage and application, indicating that carriers could be a good choice to protect and maintain their activity. Coenzyme Q10 (CoQ10), a fundamental catalyst in cellular energy production, can be produced in humans and it is existing in cardiac muscle tissues, skeleton, and organs. CoQ10 can increase muscle mitochondria energy production thereby reducing fatigue and increasing muscular performance, therefore it has an important connection to cellular energy production (Talebi et al., 2024). Recently, there are a couple of studies reported the fabrication of CoQ10 emulsions, such as CoQ10 in a simple emulsion, CoQ10 emulsions delivered along with oleyl polyethylene acetic acids (Sato et al., 2020), soybean-soluble polysaccharide layered potato protein-CoQ10 nano-emulsion (Xu et al., 2025), and CoQ10-loaded emulsions for milk enrichment (Wang et al., 2025). However, no W/O/W emulsion has been reported to deliver CoQ10, indicating CoQ10 in double emulsion has a broad research and application prospect in sports food.

The dietary antioxidants, including vitamin C, vitamin E, resveratrol, beetroot juice, quercetin, spirulina and other food-derived polyphenols, are believed to reduce muscle damage, immune dysfunction and fatigue, and thus can improve the performance (Braakhuis and Hopkins, 2015). A shortcoming of the antioxidants at use is the poor stability during production, storage and intake. Vitamin E supplementation plays an important role in preventing the free radical damage and repairing muscle damage associated with endurance exercise (Higgins et al., 2020). Thus, W/O/W double emulsion systems have been explored to deliver vitamin E in the study of Kižytė et al. (2025) and Xie et al. (2024), where water-soluble vitamin (VB2) and functional peptide were simultaneously delivered in the water phase of the emulsion. Both research groups confirmed that double emulsions enable high encapsulation efficiency and enhance the antioxidant activity of vitamin E. Previous studies have also demonstrated that quercetin effectively improves physical endurance and performance in competitive sports (Nagai et al., 2024; Racinowski et al., 2022). Couples of studies have also focused on the delivery of quercetin in W/O/W double emulsions, which have shown that the encapsulation improves the quercetin stability and enhances the bioaccessibility of the delivered cargo (Chouaibi et al., 2019; Liang et al., 2022). Astaxanthin is a potent free-radical scavenger and has been found to enhance cognitive function, which may improve the exercise performance of athletes (Oharomari et al., 2021). Currently, astaxanthin has been encapsulated in W/O/W double emulsions or double emulsion-filled hydrogels, and the results have illustrated that the stability and intestinal targeted release capacity of astaxanthin were significantly improved (Wei et al., 2023; Yang et al., 2021). Moreover, pterostilbene and pycnogenol, emerging ingredients in sports nutrition, show potential to alleviate oxidative damage and muscle fatigue. However, they have not yet been encapsulated in double emulsions, suggesting that further research is required to optimize delivery formulations and validate their efficacy in exercise settings.

Bioavailability improvement

Iron is abundant in red meats and leafy green vegetables, and serves critical functions in oxygen transport by erythrocytes and the regulation of energy metabolism. Nevertheless, the World Health Organization (WHO) has identified iron deficiency as the most prevalent nutritional disorder, mainly driven by its low bioavailability and inadequate dietary intake (Buyukkestelli and El, 2019). Therefore, iron supplements for the sportsmen are strongly recommended to improve the performance. Couples of studies have developed and characterized W/O/W double emulsion to deliver iron (Duque-Estrada, et al., 2019; Koohenjani and Lashkari, 2022), and Hong et al. (2022) even fabricated double emulsion-based iron (II) sulfate heptahydrate-solid lipid nanoparticles. Their results all illustrated that the W/O/W double emulsion showed a high iron release in small intestine and the iron bioavailability was remarkable enhanced. Vitamin D has a well-established function in bone health, muscular function and may be important for stress fracture prevention in athletic populations (Wilson-Barnes et al., 2020). Vitamin D3 and calcium were co-encapsulated in W/O/W double emulsions stabilized with biopolymers in the study of Dima et al. (2020), who found the presence of calcium ions decreased the bioaccessibility of vitamin D3. However, Díaz-Ruiz et al. (2021) co-delivered vitamin D₃ and trans-resveratrol using a W/O/W double emulsion, and observed that vitamin D₃ retarded the release of trans-resveratrol and improved the utilization efficiency of the encapsulated bioactives.

To date, probiotic-enriched sports foods have garnered increasing attention. Probiotics are active microorganisms that modulate the colonic microbiota and strengthen intestinal immune function. Several nutrients, including probiotics and glutamine, have been shown to alleviate exercise-induced gastrointestinal injury and barrier permeability. Notably, probiotics exert moderate mitigating effects on exercise-associated gastrointestinal symptoms. However, a key challenge in the probiotic industry lies in preserving the bioactivity of probiotics under gastric pH conditions and within high-viscosity food matrices (Hughes, 2019). Thus, W/O/W double emulsion provides an alternative way to overcome the drawbacks and improve probiotics bioavailability. Milk-based W/O/W emulsions containing Lactobacillus paracasei subsp. paracasei DC 412 probiotic cells were used in the yogurt, and the viability of Lactobacillus paracasei was increased when stored and ingested in a simulated gastrointestinal tract (Chong-Cerda et al., 2020). Pour et al. (2022) fabricated Lactobacillus rhamnosus and L. plantarum-loaded individually and as W/O/W emulsions. The emulsions were incorporated into the milk products to form probiotic yogurts. They found that the free microbes had poor survival ability in the yogurt samples, while the W/O/W emulsion displayed stable viability during its storage. Besides, they concluded the double emulsion provided an assured delivery system for carrying the probiotic bacteria in the dairy field. In the study of Ding et al. (2022), a W/O/W double emulsion was incorporated into calcium-alginate hydrogel beads to enable intestinal-targeted delivery of probiotics (L. reuteri). The enhanced survival of L. reuteri during long-term storage, freeze-drying, and sustained release at the appropriate site indicates that this delivery system could be applied to supplements for modulating the gut health and metabolism of athletes.

Currently, W/O/W double emulsions have emerged as functional foods tailored for athletes and bodybuilders to support nutritional balance. A variety of techniques have been developed for preparing W/O/W emulsions, yet numerous factors influence their release behavior and stability. Accordingly, this review summarizes widely used methods for double emulsion fabrication, elucidates mechanisms underlying droplet instability, and proposes strategies to enhance their stability. Most significantly, the recent progress of W/O/W emulsions in sports nutrition is systematically reviewed. Although extensive studies have investigated the encapsulation of nutrients and supplements via W/O/W double emulsions, a substantial challenge remains in translating laboratory findings into practical efficacy. Further research is therefore required to connect fundamental studies on the structural design of double emulsions with their real-world applications in sports nutrition. This will enable more precise formulation design of W/O/W emulsions and help prevent excessive or unnecessary supplementation. Additionally, current research on double emulsions focuses mainly on preparation methods and physical characterization, while stability issues still represent a critical bottleneck limiting their industrial and practical application. Therefore, there is an urgent need for technologies to prepare stable double emulsion systems, as well as for the development of scalable tools. For instance, interfacial design, combination of multiple emulsifiers, and development of novel surfactants are recommended approaches. Microfluidics instruments with lower cost can applied to the food-grade double emulsion fabrication. Meanwhile, food-grade ingredients should be investigated to replace the currently used surfactant, such as PGPR. Natural emulsifiers can be explored, and other small-molecule emulsifiers or biopolymers might be applied to the double emulsion. Additionally, the oral rheology, sensory properties, gastrointestinal release kinetics, and exercise-related effectiveness and bioavailability of these emulsions need to be assessed and monitored to facilitate their translation into practical sports food applications. Animal models or clinical studies also need to be used to validate the biological effectiveness of double emulsions as bioactive compound delivery systems. Finally, there are very few sports nutrition recommendations and guidelines for sustainable diets that support athletes’ health and performance. More plant-centric double emulsions and research aimed at reducing packaging and food waste in sports foods are needed for a sustainable future.

Abbreviations

BCCAs

Branch chain amino acids

EGCG

Epigallocatechin gallate

HLB

Hydrophile-Lipophile Balance

LP

Lipophilic protein

MCT

Medium-chain triglycerides

O/W

Oil-in-water

O/W/O

Oil-in-water-in-oil

PGPR

Polyglycerol polyricinoleate

W/O

Water-in-oil

W/O/W

Water-in-oil-in-water

WPC

Whey protein concentrate

WPI

Whey protein isolate

Funding

Research project of basic scientific research business expenses of central universities in 2025: An empirical study on weight management and obesity risk based on multi-dimensional health assessment, 2025XJTY02, Junru Du

Declarations

Competing interests

All authors have no personal or financial conflicts of interest.