Exploring the bioactive components of millets for their nutraceutical potential

Abstract

Millets have gained significant attention in recent days due to their potential as nutritious and bioactive-rich food sources. Beyond their macronutrient content such as carbohydrates, fats, protein and minerals, possess impressive array of bioactive compounds viz., polyphenols, flavonoids and antioxidants. Ferulic acid is predominantly found among all the millets followed by caffeic acid and soluble/bound fractions of whole grains contain flavonoids. Their prebiotic properties contribute to gut health by promoting growth of beneficial gut microbiota. Phenolic compounds contribute to their antioxidant, anticancer and antiviral properties. Millets are rich in dietary fibre (15–20%), which has water absorbing and bulking property thereby increases transit time of food in the gut and helps in reducing risk of inflammatory bowel disease and acts as detoxifying agent in the body. The bioavailability of minerals is however hindered by the antinutritional factors like tannins and phytates. This review focuses on the nutraceutical potential of millets by exploring its bioactive components and its enhancement through biofortification strategies which is essential for utilizing and harnessing their health-promoting properties for the benefit of global nutrition and well-being.

Introduction

Food security and malnutrition are major global issues in the current world. Nutritional insecurity is a major threat to the global population which is highly dependent on cereals-based diet, and deficient in micronutrients. Next to cereals, millets are the potent energy source in the semi-arid tropics and drought-prone regions of Asia and Africa. They are nutritionally superior as their grains contain high amount of proteins, essential amino acids, minerals, and vitamins and hence referred as a nutricereals (Malhotra, 2023) or cereals of the poor (Saini et al., 2021). They contain about 60–70% carbohydrates, 1.5–5% fat, 6–19% protein, 2–4% minerals and 12–20% dietary fibre (Annor et al., 2017) and serves as a potent energy source and abundant in B-vitamins as well as essential micronutrients like potassium, phosphorus, copper, magnesium, zinc, iron, and manganese (NIN- National Institute of Nutrition. Indian Foods Nutritional Value, 2003). Unlike other cereals, these nutricereals are known for its bioactive components and bioactive minerals (Dayakar et al., 2018) as well as significant quantity of amino acids such as methionine and cysteine (Dawson et al., 2019). They play vital role in maintaining weight, body mass index (BMI) and blood pressure (Radha et al., 2024). Despite their medicinal qualities and agroeconomic potential, they are still considered as “under-utilized,” “forgotten,” or “orphan” crops because of their coarse texture, low utilization in convenience foods, and lack of adequate phytochemical research and innovative food product development methods (Pradeep and Sreerama, 2015). In fact, world’s highest millets production is in India. When it comes to productivity, millets rank sixth among the grains, behind rice, wheat, maize, sorghum, and barley (Sharma et al., 2021). These crops are characterized by their resilience and small seeds and it is grown for mankind consumption and fodder purpose and referred as a “famine reserves” because of their capability to be stored for extended periods, lasting up to 2 years to 20 years (Ashoka et al., 2020). They are classified as C4 cereals and recognized as one of the most drought-resistant grains (Saleh et al., 2013) having a short seed maturation period of 6 to 8 weeks, contributing to their resilience, making them suitable as “famine reserves” (Hariprasanna et al., 2014).

In India, millet is typically eaten with legumes, which results in balanced nutrition, raises the quantity of amino acids and improves the protein’s overall digestibility. One of its notable properties is its gluten-free nature, offering a beneficial option for individuals managing celiac disease and gluten allergies (Nithiyanantham et al., 2019) and acts as an excellent antioxidant source (Odusola et al., 2013). To enable their usage in infant foods, the molecular foundation of waxy starch is found in foxtail, proso, and barnyard millets. Comparative genomics has been useful in identifying genes and quantitative trait loci associated with protein quality in finger millet due to its close genetic resemblance to cereals. High grain zinc has recently been developed through transgenic expression of zinc transporters, and transcriptomics has identified several calcium sensor genes involved in calcium absorption, translocation, and accumulation in finger millet (Vinoth and Ravindhran, 2017). However, biofortification in millets is still hindered by the presence of antinutrients like tannins, phytic acid and polyphenols. Moreover, protease and amylase inhibitors reduce the digestibility of millets. Improving the bioactive components and suppressing the antinutritional factors via., genome editing tools and RNA interference will help in increasing the bioavailability of minerals and favours biofortification which is limited by antinutritional factors (Vinoth and Ravindhran, 2017). This review article emphasises the importance of bioactive components in millets possessing enormous nutraceutical potential and effective utilization of these bioactive compounds in enhancing the quality of human lifestyle by preventing from deadly diseases.

Bioactive components in millets

Plants produce broad range of secondary metabolites among which phenolics including flavonoids make up a significant component (Andres-Lacueva et al., 2009). These secondary metabolites contain a number of known bioactive compounds which have an impact on different biological systems. Foods containing modest amounts of bioactive substances especially whole grains, fruits and vegetables, not only have higher nutritional value but also provide health advantages (Gokmen, 2015). Bioactive substances have the ability to reduce pro-inflammatory states, oxidative stress and metabolic disorders through their effect on energy intake (Siriwardhana et al., 2013). According to Azmir et al (2013), bioactive substances include three primary groups: (a) phenolic compounds (b) terpenes and terpenoids and (c) alkaloids. Furthermore, there are four main mechanisms via which the synthesis of bioactive chemicals or secondary metabolites occur: (1) the malonic acid pathway; (2) the shikimic acid pathway; (3) the non-mevalonate pathway (MEP) and (4) the mevalonic acid pathway. The pathway for synthesis of bioactive components is depicted in Fig. 1

Fig. 1.

Pathway for synthesis of bioactive components (Singh et al., 2019)

Alkaloids are synthesised through aromatic amino acids via shikimic acid pathway and aliphatic amino acids are derived from tricarboxylic acid cycle. Phenolic compounds are derived from shikimic acid pathway and malonic acid pathway. Terpenes are derived from Mevalonic acid pathway and non-mevalonate pathway. These phytochemical substances possess biological properties such as anti-oxidant, antibacterial, immune system stimulation, detoxifying enzyme modulation, anticancer activity, decreased platelet aggregation and hormone metabolism modification (Azmir et al., 2013).

Phenolic compounds (C₆H₅OH)

Phenolics are a significant source of antioxidants among all the phytochemicals found in millets. The term “phenolic compounds” refers to a broad group of compounds that have an aromatic ring with single or more OH groups and different substituents. It is one among the diverse classes of phytochemicals occurs in plant foods which are crucial to the diet of humans (Hoda et al., 2019). Brown finger millet has high concentration of polyphenol content (0.1%) than its white equivalent (0.003%) (Udeh et al., 2017). Malted millet flour has a total phenol level of approximately 38.36 mg/100 g (Ibidapo et al. 2019). 265 to 373.15 mg/100 g of total phenolic compound is contained in finger millet (Shahidi and Chandrasekara, 2013). Higher levels of antioxidant activity and phenolic content are reported in methanolic extracts of kodo millet grains (Sharma et al., 2017). The various classes of phenolic found in plants include flavonoids, phenolic acids, lignans and phytoestrogens (isoflavones) (Dey et al., 2022). Among them, phenolic acids and flavonoids are the two primary dietary phenolic components (Hassan et al., 2020).

Phenolic acids

According to Udeh et al. (2017), phenolic acids are derived from benzoic acid and cinnamic acids. Phenolic acids are of two types: hydroxycinnamic acids which are obtained from cinnamic acid and hydroxybenzoic acids which are formed from benzoic acid.

Hydroxycinnamic acids

Cinnamic acid (C6-C3) is the parent compound of hydroxycinnamic acids. The caffeic, chlorogenic, sinapic, coumaric (p-coumaric), cinnamic (trans-cinnamic) and ferulic (trans-ferulic) acids are among the hydroxycinnamic acids (Dey et al., 2022; Lorigooini et al., 2020; Udeh et al., 2017). According to Udeh et al. (2017), it can make up to 30% of all phenolic acids. According to Udeh et al. 2017, ferulic acid is the most prevalent bound phenolic acid in millet, which is followed by caffeic and coumaric acids which occur in the order of 18.6, 1.64, and 1.20–1.25 mg/100 g finger millet flour, respectively. Little millets are high in p-coumaric acid, ferulic acid and caffeic acid (Kaur et al., 2019).

Polyphenols mostly occur in finger millet are concentrated in the bran layer which includes hydroxycinnamic acids, protocatechuric, anilic, hydroxybenzoic, coumaric, sinapic and ferulic acids. It guards against neurological diseases, inflammation, epithelial neoplasms and arthritis (Priya et al., 2023). In the soluble fraction, little millet has greater concentrations of hydroxy cinnamic acid derivatives (303.12 mg/g).

Caffeic acid

Caffeic acid is one type of polyphenol, known for its antioxidant properties. It has anti-inflammatory, antiviral and anticancer abilities. The caffeic acid is a cinnamic acid based phenolic acid ranging from 5.9 to 10.4 μg/g in ragi (Hithamani and Srinivasan, 2014). The R₁, R₂ and R₃ of the cinnamic acid ring are substituted by OH, OH and H respectively (Udeh et al., 2017). The caffeic acid content in millets are depicted in Table 1

Table 1.

Caffeic acid and Gallic acid content in Millets

Millets	Caffeic acid (µg/g)		Gallic acid (µg/g)
	Free	Bound	Free	Bound
Pearl millet	43.88 ± 0.20	45.12 ± 0.97	3.87 ± 0.09	–
Finger millet	38.91 ± 0.15	31.86 ± 0.20	11.93 ± 0.11	56.12 ± 0.31
Foxtail millet	13.59 ± 0.40	36.27 ± 2.73	6.23 ± 0.05	7.35 ± 0.13
Little millet	17.59 ± 0.26	23.77 ± 1.28	5.21 ± 0.05	2.54 ± 0.07
Barnyard millet	23.65 ± 0.18	10.66 ± 0.40	10.56 ± 0.39	13.91 ± 0.08
Kodo millet	12.45 ± 0.36	20.79 ± 0.30	24.81 ± 0.25	20.52 ± 0.31
Proso millet	8.69 ± 0.36	–	4.23 ± 0.07	23.44 ± 0.18

Source: Goudar et al. (2023), Pradeep and Sreerama (2017)

Sinapic acid (3,5-Dimethoxy-4-hydroxycinnamic acid)

Sinapic acid is a derivative of phenyl propanoid which have anti-inflammatory, antioxidant, antibacterial, anti-mutagenic and anticancer qualities in addition to its role as a radical scavenger (Connelly et al., 2011; Priya et al., 2023). In finger millet, it makes up roughly 11.0–24.8 μg/g (Hithamani and Srinivasan, 2014).

The whole grain of foxtail millet has 4.13 ± 0.12 μg/g of bound sinapic acid and approximately 85.36 ± 2.08 μg/g of soluble sinapic acid. About 5.69 ± 0.13 μg/g of bound sinapic acid and 121.96 ± 1.56 μg/g of soluble sinapic acid are present in foxtail millet bran. The whole grain and bran of little millet contains 63.24 ± 1.04 μg/g and 113.37 ± 2.01 μg/g of soluble sinapic acid respectively. According to Pradeep and Sreerama (2017), the bound sinapic acid content of the whole grain and bran of small millet is 2.59 ± 0.07 μg/g and 6.37 ± 0.18 μg/g, respectively. Sinapic acid prevents diseases like arthritis, epithelium neoplasms and mucosa skeletal system by inhibiting the synthesis of enzymes like cyclooxygenase (COX-2), inducible nitric oxide synthase (iNOS) and tumour necrosis factor alpha (TNF-α) (Tan et al., 2022).

p-coumaric acid

The coumaric acid is (2E)-3-(4-hydroxyphenyl) prop-2-enoicacid. p-Coumaric acid is the isomers of coumaric acid and it is a hydroxy derivative of cinnamic acid (Dey et al., 2022). The R₁, R₂ and R₃ position of the cinnamic acid is occupied by H, OH and H respectively (Udeh et al., 2017). It possesses anti-inflammatory, anti-cancer and antioxidant potential. Low-density lipoprotein, cholesterol oxidation and lipid peroxidation are also decreased by it (Boz, 2015; Kannan et al., 2013). In finger millet, p-coumaric acid makes up around 1.81–41.1 μg/g. The soluble p-coumaric acid content in foxtail millet is 3.66 ± 0.12 μg/g, while the bound form is 196.62 ± 4.9 μg/g. According to Khare et al. (2020), kodo millet contains 1.38 ppm of p-coumaric acid which help to reduce obesity and 2.71 ± 0.16 μg/g of soluble and 70.36 ± 1.2 μg/g of bound p-coumaric acid is present in the little millet (Pradeep and Sreerama, 2017).

Ferulic acids

A phenolic derivative of cinnamic acid is ferulic acid (4-hydroxy-3-methoxy cinnamic acid). Numerous health advantages of ferulic acid have been documented including its ability to boost sperm viability and exhibit antioxidant, antithrombotic, anti-allergic, antimicrobial, anti-inflammatory, antiviral, hepatoprotective and vasodilatory effects (Dey et al., 2022). The antioxidant activity was reported by Kumar and Pruthi (2014). The R₁, R₂ and R₃ position of the cinnamic acid is substituted by CH₃O, OH and H respectively (Udeh et al., 2017). Finger millet contains 41–405.0 μg/g of ferulic acid (Shahidi and Chandrasekara, 2013). According to Goudar et al. (2023), the amount of free and bound ferulic acid in pearl millet is 47.03 ± 0.08 μg/g and 988.78 ± 8.29 μg/g, respectively. Within foxtail millet ferulic acid that is free and bound makes up approximately 54.65 ± 0.42 μg/g and 254.20 ± 3.72 μg/g, respectively. Ferulic acid concentration in little millet is 29.94 ± 0.15 μg/g and 133.58 ± 3.85 μg/g respectively for free and bound forms. Ferulic acid content in the barnyard and kodo millet is 27.88–231.84 μg/g and 99.35–1445.06 μg/g respectively. According to Pradeep and Sreerama (2017), the whole grain of foxtail millet has 83.72 ± 2.51 μg/g of bound ferulic acid and 93.4 ± 2.94 μg/g of soluble ferulic acid. Similarly, the whole grain of little millet has 121.02 ± 2.63 μg/g of bound ferulic acid and 129.31 ± 15.07 μg/g of free ferulic acid. Khare et al. (2020) reported 20.45 ppm of ferulic acid present in kodo millet. Previous reports indicated that the ferulic acid provides renal protective benefits by enhancing glycemic management and causing structural alterations in the kidneys that reduce oxidative stress (Choi et al., 2011).

Hydroxybenzoic acids

Benzoic acid is considered as the source of hydroxybenzoic acids. About 70–71% of the phenolic acids present in grains are hydroxybenzoic acids among which protocatechuic, p-hydroxybenzoic, vanillic, gallic, gentisic and syringic acids are noteworthy. The primary free phenolic acid among them is protocatechuic acid. In fingermillet, the hydroxybenzoic concentration is around 45 mg/100 g (Chandrasekara and Shahidi, 2012a, b; Hithamani and Srinivasan, 2014). Benzoic acid levels are greater in little millet (45.56 mg/g). Little millets are high in bioactive substances such as gallic acid, sinapic acid, p-hydroxybenzoic acid and vanillic acid. (Kaur et al., 2019). They possess antioxidant, anti-inflammatory, antibacterial, and hepatoprotective activities.

Gallic acid

Gallic acid is the hydroxy substituents of benzoic acid’s R₁, R₂, and R₃ (Udeh et al., 2017). The gallic acid makes up around 3.91–30.0 μg/g of finger millet (Hithamani and Srinivasan, 2014). Gallic acid content of various millets is given in Table 1. The possible benefits of gallic acid include anti-inflammatory, antibacterial, antioxidant and anti-cancer properties (Kahkeshani et al., 2019).

Protocatechuic acid

According to Udeh et al. (2017), in protocatechuic acid the H, OH and OH occupy the R₁, R₂, and R₃ position of the benzoic ring respectively. Protocatechuic acid has antioxidant and anti-inflammatory properties. The most common phenolic acid in finger millet is protocatechuic acid which ranges from 119.8 to 405 μg/g. The pearl millet does not contain protocatechuic acid (Hithamani and Srinivasan, 2014). It has been stated that protocatechuic acid has pharmacological qualities that include antiatherosclerosis, antiviral, antifibrotic and anticancer effects (Chandrasekara and Shahidi, 2011; Kakkar and Bais, 2014).

p-hydroxybenzoic acid

The phenolic derivative of benzoic acid, i.e., p-hydroxybenzoic acid is reported in finger millet cultivars exclusively with mean values of 13.5 and 16.8 mg/kg. This compound is a monohydroxy benzoic acid (Hassan et al., 2020) and 6.3–370.0 μg/g of p-hydroxybenzoic acid has been reported in finger millet (Hithamani and Srinivasan, 2014). The soluble and binding forms of p-hydroxybenzoic acid in a whole foxtail millet are 1.07 ± 0.05 and 1.06 ± 0.02 μg/g, respectively. In terms of soluble and binding p-hydroxybenzoic acid, the whole grain little millet contains around 4.05 ± 0.13 μg/g and 1.32 ± 0.03 μg/g, respectively. H, OH and H in that order occupy the benzoic acid’s R₁, R₂ and R₃ and are reported to have antibacterial and anti-inflammatory properties.

Syringic acids

Through shikimic acid pathway, syringic acid (SA) a phenolic molecule is synthesised. According to Srinivasulu et al. (2018), it has anti-oxidant, antibacterial, anti-inflammatory, antiendotoxic, neuroprotective and hepatoprotective properties. The R₁, R₂ and R₃ position of the benzoic ring are replaced by CH₃O, OH and CH₃O respectively. About 10.0–60.0 μg/g are found in finger millet (Hithamani and Srinivasan, 2014). As per the findings of Goudar et al. (2023), 2.44 ± 0.38 μg/g of free syringic acid is found in pearl millet, 30.05 ± 0.52 μg/g in finger millet, 8.59 ± 0.14 μg/g in foxtail millet, 21.56 ± 0.29 μg/g in barnyard millet, 28.22 ± 0.46 μg/g in kodo millet and 15.72 ± 0.23 μg/g in proso millet.

Flavonoids

Flavonoids represent the most extensive and varied category of phenolic compounds found in plants. They have oxygenated heterocycle with two benzene rings joined by three carbons (Papoutsis et al., 2021). Bioactive qualities of flavonoids include anti-cancer, antidiabetic, anti-inflammatory, neuroprotective, cardio-protective, immunomodulatory, antibacterial, antiparasitic, antiviral and anti-aging properties (Fraga et al., 2019; Jucá et al., 2020;Saini et al., 2017). Malted pearl millet flour contains about 34.73 mg/100 g of flavonoid (Ibidapo et al., 2019).

Depending on the level of saturation of central heterocyclic ring flavonoids are classified into two main groups (Saini et al., 2017). Unsaturated flavonoids include anthocyanidins, flavones, flavonols and isoflavones; saturated flavonoids include flavanones, dihydroflavonols and flavan-3-ols (Dias et al., 2021). Millets are reported to have bioactive components including three flavonols (quercetin, kaempferol and myricetin), two flavones (apigenin, and luteolin) and one of each of flavan-3-ol (catechin), flavanone (naringenin) and isoflavone (daidzein). Major flavonoids found in millets include catechin, gallocatechin, quercetin and apigenin.

There is a high concentration of catechin ((2R,3S)-2-(3,4-dihydroxyphenyl)-3,4- dihydro-2 h-chromene-3,5,7-triol) in the millet seed coat (Sharma and Niranjan, 2018; Shekhar et al., 2023). According to Nazni and Shobana (2016), it aids in the prevention of cancer, diabetes, cardiovascular disorders, aging as well as oxidative stress. Catechin concentration in kodo millet was determined by Khare et al. (2020) and it has an anti-obesity benefit at 1.10 ppm. The foxtail millet contains 11.79 ± 0.34 and 31.65 ± 2.63 µg/g of free and bound catechin respectively. Apigenin (5,7-Dihydroxy-2-(4-hydroxyphenyl)-4Hchromen-4-one) has antidiabetic, antirheumatic and anticancerous property (Thakur and Tiwari, 2019). Foxtail millet has a higher amount of apigenin. According to Pradeep and Sreerama (2017), it accounts about 91.32 ± 1.9 µg/g in a free manner and 125.16 ± 4.68 µg/g in a bound manner. The most prevalent flavonoid found in finger and little millet in all fractions is quercetin. There is 3 µg/g of quercetin found in finger millet (Chethan et al., 2008). Kaempferol is more in little millet which reduces the risk of chronic diseases. Little millet consists of about 24.61 ± 1.73 µg/g of free and 38.45 ± 1.23 µg/g of bound kaempferol (Pradeep and Sreerama, 2017).

Finger millet is a rich source of flavonoid which has been reported to a maximum of 2100 µg/g and primarily occurs in the soluble form (Chandrasekara and Shahidi, 2012a; Shahidi and Chandrasekara, 2013). According to Pradeep and Sreerama (2017), the total amount of flavonoids found in soluble and bound fractions of whole grain millets are 244.34 and 368.28 µg/g (foxtail) and 108.97 and 323.23 µg/g (little millet) respectively.

Nutritional profiling in millets

Millets are group of small grained cereal food crops that are highly nutritious. It includes pearl millet (Pennisetum glaucum (L.)), finger millet (Eleusine coracana (L.)), proso millet (Panicum miliaceum (L.)), foxtail millet (Setaria italica (L.)), barnyard millet (Echinochloa frumentacea (L.)), kodo millet (Paspalum scrobiculatum (L.)), and little millet (Panicum sumatrance). These crops have long been cultivated under dryland in India and World. These have a short growing season (70–80 days) and provide nutritious grain and forage in a short span of time. Each millet has an importance of its own as discussed hereunder:

Pearl millet

Pearl millet has a notable concentration of unsaturated fatty acids, folate, copper, zinc, iron, magnesium, calcium and also a great source of antioxidants (Rao et al., 2018). It also constitutes 11.5% of dietary fibre. The inclusion of magnesium in pearl millet contributes to the potential treatment of migraines and can alleviate respiratory issues in individuals with asthma (Ambati and Sucharitha, 2019). The phytonutrients found in pearl millets such as lignin, myricetin, apigenin and flavonoids have anti-ulcerative and anti-fungal qualities in addition being protective against cardiovascular disease and breast cancer (Thakur and Tiwari, 2019). The lipid content of pearl millet consists of free lipids ranging from 5.6 to 6.1% and bound lipids ranging from 0.6 to 0.9% (Hassan et al., 2021). Essential Fatty acids are beneficial for cardiovascular health. The principal fatty acids in pearl millet are linoleic, oleic, palmitic, and stearic acids. It contributes to the maintenance of stable blood sugar levels over an extended period making it beneficial for individuals with diabetes (Pei et al., 2022).

Finger millet

Finger millet is known for its high concentration of calcium, protein, and well-balanced essential amino acids, complemented by significant amounts of Vitamin A, Vitamin B, and phosphorous. Remarkably, it contains a notably higher level of calcium of about 300–350 mg/100 g (10 times higher than rice and wheat) and phosphorous contributing to the management of high blood cholesterol, alleviating constipation and potentially reducing the risk of intestinal cancer (Jacob et al., 2024). It contains bioactive substances that are significant for the treatment of cardiovascular disease and diabetes including luteolin, taxifolin, gallocatechin, daidzein, gallocatechin, pyrocyanidin B1, apigenin and pyrocyanidin B2 (Saini et al., 2021). Additionally, finger millets exhibit antimicrobial and antitumorigenic properties (Thakur and Tiwari, 2019). The hemicellulose in finger millet includes hemicellulose A and B, constituting approximately 1.4 and 1.9%, respectively (Udeh et al., 2017). It also constitutes about 18% of dietary fibre (Devi et al., 2014). The majority of non-starch polysaccharides are composed of arabinoxylans. Finger millet arabinoxylans are more complicated or branching in structure (Fig. 2) than those of wheat and barley and they resemble those of sorghum, rice and maize bran. Because of the side chains of the polymer, it contains large amount of terminally connected arabinose along with high amounts of uronic and ferulic acids, galactose and small amounts of xylose which are extremely branched in nature (Udeh et al., 2017). Finger millet arabinoxylans bound phenolic acids have been demonstrated to have strong antioxidant properties. Bound ferulic acids have antioxidative property (Udeh et al., 2017).

Fig. 2.

Arabinoxylan structure of Finger millet (Udeh et al., 2017)

Proso millet

Proso millet is gluten free and high in calcium, protein, and dietary fiber. Most notably, it serves as a cost-effective source of manganese. It is high in bioactive phytochemicals like caffeic acid, ferulic acid, syringic acid and chlorogenic acid which all contribute to a number of health advantages. It is made up of about 65% phenolic compounds. (Zhang et al., 2014). One notable advantage is its ability to release energy gradually over extended period of post-consumption, enabling individuals to put in more hours at work without experiencing fatigue (Dayakar et al., 2018).

Foxtail millet

Foxtail millet, distinguished by its non-acid-forming and non-glutinous nature, offers easy digestibility. Gupta et al. (2013) highlights its role in gradual release of sugars in the body, facilitating energy release without impeding metabolic processes. Enriched with catechin, quercetin, apigenin and kempherol, foxtail millet contributes for addressing conditions such as diabetes, cardiovascular diseases and aids in maintaining healthy lipid levels. Furthermore, Lin et al. (2020) indicated that foxtail millets exhibit an anti-ulcer response and possess a robust antioxidant effect providing protection to the gastric mucosa.

Barnyard millet

Barnyard millet, particularly the dehulled varieties are most beneficial for people with type 2 diabetes, because of its low glycemic index (Ugare et al., 2014). Additionally, barnyard millet exhibits a notable retrogradation of amylose, resulting in the formation of increased amounts of resistant starches (Dayakar et al., 2018). This millet variety is known for its antioxidative phenolic compounds such as serotonin and flavonoids derivatives. (Watanabe, 1999). Major compounds such as N-(p-coumaroyl), luteolin, tricin and serotonin in barnyard millet contribute to its anti-rheumatic, anti-cancer and anti-diabetic properties (Thakur and Tiwari, 2019).

Kodo millet

Presence of high lecithin in kodo millet favours the crop for its easy absorption and act as a beneficial supplement for strengthening the neurological system. It is also rich in vital minerals including iron, calcium, magnesium, potassium and zinc. It contains 14.3% of dietary fibre. Besides it is rich in B vitamins especially B6, folic acid and niacin. It satisfies the dietary requirements of people who are intolerant to gluten because of its gluten-free nature and it is easily digestible due to its coarse texture. Beyond its nutritional benefits, various kodo millet plant parts have been utilized for medicinal purposes. The kodo millet stem can serve as poultice for treating conditions like beriberi, while kodo roots concentration has been employed as a diuretic and galactagogue (Saini et al., 2021). Furthermore, it is recognized as good source of polyphenols, flavonoids and antioxidant compounds as highlighted by Bunkar et al., (2021).

Little millet

Little millets stand out as nutritional powerhouses, particularly rich in phosphorous, iron and also contains significant quantity of vitamin B. Germinated little millets are recognized for their excellent α-amylase source, offering specific yield and higher purity in comparison (Usha et al., 2011). Little millet’s soluble fraction is well-known for its high concentration of phenolic compounds which include ferulic, caffeic and sinapic acids making up about 80% of the total as well as flavonoids like kaempferol and luteolin (Pradeep and Sreerama 2018). Apigenin, a bioactive substance found in little millets has been linked to a number of health advantages. Apigenin is known for its potential in combating conditions such as diabetes, celiac disease, high cholesterol levels, cardiovascular diseases and exhibiting anti-cancer properties (Thakur and Tiwari, 2019). Notably, little millet is reported to contain 37 to 38% dietary fibre earning it the status of a nutraceutical (Saini et al., 2021). Referred to as “cool food” due to its cooling impact, little millet also has a total carotenoid content of about 51 to 104 µg/100 g (Guha et al., 2015).

Nutraceutical significance of bioactive components in Millets

Millets are rich in vitamins, protein and minerals and also a superior source of iron and calcium with low glycemic index. It is gluten-free and possess dietary properties which do not contribute to acidity. An old saying states, “One who eats rice stays as light as a bird, one who eats jowar is robust like a wolf and one who eats ragi never gets sick.” They contain various physiologically functional components, including phytochemicals such as dietary fibre, polyphenol compounds and antioxidants (Devi et al., 2014). Polyphenol compounds exhibit health-promoting properties, play a vital role in preventing or reducing the occurrence of degenerative diseases including osteoporosis, cancer, diabetes, neurological and cardiovascular disease (Rahman et al., 2021). Different types of millets (Fig. 3a and b), their nutritional and B vitamin profile are depicted in Table 2 and Fig. 3c respectively.

Fig. 3.

a Types of millets. b Grains of millets. https://www.fssai.gov.in/upload/uploadfiles/files/Guidance_Notes_Version_2_Millets_29_01_2020.pdf. c B- Vitamin profile of Millets Source: Indian Food Composition Tables, NIN—2017 and Nutritive value of Indian foods, NIN—2007

Table 2.

Nutritional profiling and nutraceutical potential of millets

(Source: Longvah et al., 2017; Amadou et al., 2013)

Millets	Carbs (g)	Protein (g)	Total dietary fibre (g)	Energy (Kcal)	Nutrient content				Significance
					Mg (mg)	Ca (mg)	Fe (g)	P (mg)
Pearl millet	67.5	11.6	11.3	361	137	42	11	296	It provides relief from breathing difficulties in asthma patients and lessens migraine severity By lowering the excess bile secretion in our systems, the insoluble fiber content of pearl millet aids in the prevention of gallstone formation (Ambati and Sucharitha, 2019).
Finger millet	72	7.3	11.5	328	137	344	3.9	283	It aids in bone strengthening and lower the chance of fractures Because of its high calcium content, it also helps breastfeeding moms by increasing breast milk secretion and improving the milk quality
Proso Millet	70.4	12.5	–	341	153	14	2.9	206	It lowers cholesterol and lowers the risk of heart disease It prevents Pellagra
Foxtail millet	60.9	12.3	2.4	331	81	31	2.8	290	It lowers the chance of diabetes It is a heart-healthy food that has an anti-ulcer effect
Kodo millet	69.9	8.03	6.39	302	122	22.0	1.7	101	It strengthens the nerve system and further aids in lowering joint pain and menstrual cycle regulation
Barnyard millet	65.5	6.2	1.98	307	82	20	18.6	280	Act as antioxidant and also help in reducing blood lipid level
Little millet	65.55	10.13	7.72	1449 kJ	91.4	16.0	9.3	220	Reduces the polydipsia in diabetes mellitus

Millets help to prevent celiac disease and cancer. People with celiac disease eat only gluten-free grains which include rice, quinoa, kodo millet, small millet, sorghum, oats and pearl millet, rather than products made of wheat, rye or barley (Thompson, 2009). A methanolic extract of kodo and finger millets was found to suppress the glycation reaction, a critical process in anti-aging. It might be caused by antioxidants such as tannins, phenols and phytates (Dey et al., 2022). Due to their high dietary fibre content, kodo and small millets can be added to drinks that promote healthy digestion by acting as prebiotics. The acceptability of millets is low and the food industries are also not utilizing the millets to develop functional foods. Optimizing the processes for the extraction, quantification and purification of bioactive components from millets is therefore essential. The nutraceutical potential of various millets is presented in Fig. 4. The knowledge on millet’s nutritional content provided by the current review will be useful in the creation of functional foods and nutraceuticals. For effectively utilizing the nutraceutical importance of millets, it is essential for harnessing biofortification in millets.

Fig. 4.

Nutraceutical potential of Millets (Saini et al., 2021; Priya et al., 2023)

Biofortification in millets through genetic modification

Biofortification is the process by which the nutritional quality of food crops is improved through agronomic practices, conventional plant breeding, or modern biotechnology, according to the World Health Organization (2016). It is a financially effective strategy for addressing micronutrient deficiency. In millet, biofortification is accomplished by lowering antinutrient levels and increasing nutrient accumulation. Biofortification of millets includes modification in macronutrients viz., starch, protein and amino acid and micronutrients such as iron, zinc and calcium. Genome editing tools such as transcription activator-like effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR) and zinc finger nucleases (ZFNs) has to be employed in the biofortification process to address the malnutrition.

Waxy gene in millets

Generally, millets are the major source of carbohydrates in some parts of India and Sub-Saharan Africa. The two polymers that make up grain starch are usually amylopectin (70–85%) and amylose (15–30%). Millet accessions are categorized into two main phenotypes: waxy and non-waxy, based on the amount of amylose present. It is advised to feed waxy grains to infants under six years old since they are easily digested, have no amylose, and almost 100% amylopectin (Englyst et al., 1992). Grain starch amylose concentration is reduced by 7.2 folds in waxy lines. Dominant single waxy allele (Wx) controls the synthesis of amylose in millets, but a recessive loss-of-function allele (wx) produces a waxy phenotype with almost 0% amylose. The traditional method of breeding millets for the waxy characteristic is a labor intensive and time-consuming process. The essential enzyme facilitating the synthesis of amylose is granule-bound starch synthase 1 (GBSS1) (Vinoth and Ravindhran, 2017). The Wx genes are found to have 13 introns and 14 exons in millets. They stated that in Setaria, two polymorphisms happened in wx allele viz., transposable elements TSI-2 inserted in intron 1 and TSI-7 inserted in exon 3. In barnyard millet, a loss-of-function mutation in one allele was discovered that results in waxy cultivars (Hoshino et al., 2010). In the waxy Panicum exon sequence, three SNPs were observed. The coding region (exon) sequence of waxy Panicum has three SNPs viz., deletion of 15 bp in exon 10, adenine residue inserted in exon 9, substitution of G/A in exon 7 (Fig. 5).

Fig. 5.

Structure of GBSS 1 gene in foxtail millet and proso millet in association with waxy phenotype (Vinoth and Ravindhran, 2017)

Opm genes for protein biofortification

The essential amino acid content in finger millet is higher than that of any other cereals (Vinoth and Ravindhran, 2017). Chandra et al. (2016) and Amadou et al. (2013) reported that the lysine and tryptophan content of ragi is 220 and 100 mg/100 g respectively. Finger millet’s high tryptophan and lysine content is due to Opaque 2 (o2). In endosperm, o2 modifiers (Opm) cause free tryptophan and lysine via., upregulating aspartate kinase and downregulating lysine ketoglutarate reductase dehydrogenase (Vinoth and Ravindhran, 2017). The fine mapping of Opm genes, which are linked to QTLs, may lead to the genetic improvement of seed protein quality in cereals and small millets. 16 prolamin-encoding genes known as setarins have recently been discovered in foxtail millet (Muthamilarasan and Prasad, 2015). Least homology in sequence with other millets and cereals indicates the unique protein quality in foxtail millet.

OsZIP1 for high zinc biofortified millets

Iron and zinc are the major micronutrients to be biofortified. It is reported that about 56% of women and 79% of pre-school children are affected by iron deficiency. Half of the world’s population suffers from zinc (Zn) deficiency, which can lead to diarrhoea, stunted growth, and weakened immune system (Vinoth and Ravindhran, 2017). In India,”Dhanashakti (ICTP 8203 Fe 10.2)” is the high iron biofortified pearl millet variety which was introduced in 2012. It has 71 mg/100 kg of Fe which is 9% higher than others. Rai et al. (2013) reported that moderate to high correlation existed between Fe and Zn. It is also reported that the association of Fe and Zn with grain yield is negative and weak but it is not always significant. Use of large segregating populations can increase both micronutrients without considerably affecting grain output. Recently, constitutive (35S) and endosperm-specific promoters (Bx17) were used to over express OsZIP1, resulting in transgenic ragi plants with significant zinc accumulation (Ramegowda et al., 2013). The strategy for producing high zinc biofortified millets is the overexpression of ZIP transporters, which leads to zinc accumulation in seeds.

Generally, millets are proven to be repository of dietary antioxidants, vitamins, minerals, dietary fibres and phytochemicals and often considered as underutilized crops. They are particularly rich in polyphenols like syringic acid, flavonoids, protocatechuic acid, catechins, gallic acid, coumaric acid, sinapic acid and tocopherols which are known for their positive impact on health and their ability to combat degenerative diseases. They have anti-diabetic, anti-inflammatory, antimalaria, anti-cancerous and antioxidant properties. The exploration of bioactive components in millets underscores the immense potential of these grains as nutritional power houses. Research on molecular characterization of small millets for nutritional traits is limited. It is imperative that small millets be thoroughly evaluated for nutritional traits for generating genotypes suitable for biofortification. In order to develop biofortified varieties of millets, it is crucial to characterize their germplasm. Additionally, omics techniques can be used to improve grain-nutrient density, and the use of genetic engineering and genome editing tools can help facilitate nutrient accumulation in edible portions and prevent the biosynthesis of antinutrients. Furthermore, millets could be employed as a staple crop in the global economy and promoted as a model system for advancing quality traits by employing a collaborative approach that leverages all of the omics tools, such as transcriptomics, proteomics, metabolomics, and genomics.

Declarations

Conflict of interest

The authors have no conflicts of interest to declare. All co-authors have seen and agree with the contents of the manuscript and there is no financial interest to report.