Structurally, a chicken egg comprises the shell with membranes, albumen, and yolk, which account for approximately 9.5, 63, and 27.5% of the whole egg, respectively (11). The edible portion primarily consists of water (~74%), protein (~12%), lipid (~12%), carbohydrates (<1%), and ash (~1%), with proteins distributed across both albumen and yolk, while lipids, fat-soluble vitamins, and most minerals are concentrated in the yolk (12). Egg proteins, including ovalbumin, ovotransferrin, lysozyme, phosvitin, and other yolk-specific proteins such as lipoproteins, provide high-quality, digestible amino acids and exhibit multiple biological functions, such as antioxidant, antimicrobial, and anti-inflammatory effects (4, 13–15). Proteomic analyses have identified nearly a thousand distinct egg proteins, highlighting their functional and therapeutic potential for health maintenance and disease management (14). Egg yolk lipids are primarily triacylglycerols (~65%), phospholipids (~30%), and cholesterol (~4%), with phospholipids such as phosphatidylcholine and phosphatidylethanolamine constituting 28–30% of total yolk lipids (16–18). These phospholipids are highly bioavailable (>90%), efficiently incorporated into plasma high-density lipoproteins, and play key roles in lipid metabolism regulation. The fatty acid composition of yolk lipids can be modulated via maternal diet to enrich polyunsaturated fatty acids (PUFAs), including ω-3 s α-linolenic acid (ALA), EPA, and DHA, thereby improving the ω-6: ω-3 ratio, which is critical for cardiovascular and cognitive health (19, 20). Eggs are also a natural source of bioactive carotenoids, primarily lutein and zeaxanthin, which exhibit higher bioavailability than plant-derived forms (21, 22). These carotenoids preferentially accumulate in the retina, enhance macular pigment density, and protect against age-related macular degeneration (23). Furthermore, eggs provide fat-soluble vitamins (A, D, E, K), choline, and trace minerals such as selenium and iodine, contributing to antioxidant defense, immune modulation, neurodevelopment, and other physiological functions (6, 24). Collectively, eggs provide a compact, nutrient-dense matrix of high-quality proteins, lipids, vitamins, minerals, and bioactive compounds. Their structural and biochemical characteristics, combined with the ability to modulate nutrient composition through feed enrichment, make eggs an ideal natural delivery system for functional nutrients and bioactive compounds.

The absorption and bioavailability of egg-derived nutrients are enhanced by the egg’s unique biochemical matrix. Lipophilic compounds, including phospholipids, carotenoids, fat-soluble vitamins, and ω-3 PUFAs, are efficiently solubilized within yolk lipids, improving intestinal uptake and systemic availability (25, 26). For example, phosphatidylcholine is preferentially incorporated into high-density lipoprotein (HDL) particles, promoting favorable lipid profiles and supporting cardiovascular health (27).

Egg proteins are highly digestible, with cooked egg white achieving ~91% digestibility compared to ~51% for raw egg, providing rich essential amino acids that support muscle protein synthesis (28, 29). Specific bioactive proteins, such as lysozyme and ovotransferrin, can partially resist gastrointestinal degradation, allowing them to exert systemic antioxidant and antimicrobial effects (30, 31). The bioavailability of lutein and zeaxanthin is influenced by cooking methods, with boiling optimizing absorption (32, 33), and their incorporation into micelles is further enhanced by yolk phospholipids (34). Likewise, maternal dietary enrichment of eggs with ω-3 PUFAs, vitamins, or trace elements ensures that these nutrients are deposited in forms that are efficiently absorbed, maximizing the functional benefits of enriched eggs (35, 36). In summary, the unique yolk matrix and protein composition of eggs enhance the bioavailability of lipophilic and hydrophilic nutrients, ensuring that proteins, fatty acids, vitamins, and carotenoids are efficiently absorbed and functionally active.

Eggs represent a uniquely efficient vehicle for nutrient delivery compared with other common dietary carriers such as dairy products, dietary supplements, or functional beverages. Their compact matrix naturally combines high-quality proteins, lipids, fat- and water-soluble vitamins, minerals, and bioactive compounds in a balanced form that supports both stability and bioavailability. Lipophilic nutrients, including carotenoids, fat-soluble vitamins, phospholipids, and ω-3 PUFAs, are solubilized within yolk lipids, enhancing intestinal absorption and systemic distribution, often exceeding the bioefficacy of isolated supplements (5, 37). Eggs also allow co-delivery of enriched nutrients with endogenous components such as phospholipids, choline, and antioxidant peptides, enabling synergistic physiological effects that are difficult to achieve with pill-based formulations. Moreover, egg proteins are highly digestible and provide bioactive peptides that can exert systemic antioxidant, immunomodulatory, or metabolic effects. As a widely consumed staple food, eggs combine high consumer acceptance, culinary versatility, and sustained dietary compliance, strengthening their translational potential as functional foods. Collectively, these characteristics position eggs as a superior and practical platform for functional fortification, capable of delivering multiple nutrients simultaneously while maintaining stability, bioavailability, and physiological efficacy, in contrast to many conventional supplements or fortified products. This capacity underpins their central role in developing functional eggs enriched with ω-3 PUFAs, antioxidants, and essential minerals, as discussed in subsequent sections (Figure 1).

ω-3 PUFAs, primarily ALA, EPA, and DHA, play essential roles in cardiovascular, metabolic, neurological, and immune health (38). ALA is an essential fatty acid that must be supplied by the diet, whereas EPA and DHA are directly incorporated into cellular membranes, supporting membrane fluidity, eicosanoid balance, and neurocognitive function (19). Extensive evidence showed that EPA and DHA provide cardiometabolic protection by lowering circulating triglycerides, improving endothelial function, enhancing vascular compliance, and reducing arrhythmic events (39). The involvement in reverse cholesterol transport, modulation of LDL particle size, and maintenance of arterial elasticity collectively contributes to a reduced risk of atherosclerosis and insulin resistance. In metabolic syndrome, ω-3 PUFAs have demonstrated benefits in attenuating hyperlipidemia, hypertension, hyperglycemia, hepatic steatosis, and endothelial dysfunction (39, 40). In the central nervous system, DHA is a key structural component of neuronal membranes and supports synaptic plasticity, cognitive performance, and early-life neurodevelopment (41). Additionally, ω-3 PUFAs promote the resolution of inflammation through the formation of specialized pro-resolving mediators, including resolvins and protectins, thereby exerting systemic anti-inflammatory and immunomodulatory effects (42).

Eggs naturally supply high-quality protein, phospholipids, fat-soluble vitamins (A, D, E, K), B vitamins, minerals, and other bioactive lipids (13). Enriching eggs with ω-3 PUFAs offers an accessible and cost-effective dietary strategy to improve population-level intake. Flaxseed supplementation in layers is widely employed to increase yolk ALA and DHA content, producing ω-3-enriched eggs with up to five times the ω-3 PUFAs content of conventional eggs (39). Consuming these enriched eggs has been shown to reduce serum cholesterol and triglyceride levels and to shift LDL particles toward less atherogenic forms, thereby improving cardiometabolic risk profiles (43).

Eggs respond rapidly and predictably to dietary manipulation of their fatty acid composition, making laying hens an efficient biological system for enriching ω-3 PUFAs, including ALA, EPA, and DHA. Because birds lack Δ12- and Δ15-desaturases, both n-6 and ω-3 PUFAs must be supplied through the diet, and the endogenous conversion of ALA to EPA/DHA is inherently limited, with desaturation at the Δ6 position representing a major metabolic bottleneck (44). Consequently, efficient yolk enrichment depends on dietary sources that provide preformed long-chain ω-3 PUFAs or highly bioavailable precursors.

Fish oil is the most direct source of preformed EPA and DHA and consistently elevates yolk ω-3 PUFAs. For example, inclusion of 1.5% fish oil increased total ω-3 PUFAs from 205 to 327 mg/100 g egg (45). However, higher inclusion levels can introduce undesirable fishy flavors and increase lipid oxidation susceptibility, requiring careful formulation and antioxidant support (46). Microalgae and microalgae-derived oil offer a sustainable, high-purity source of DHA while avoiding the sensory issues associated with marine oils (47). Comparative studies show that algal oil can achieve DHA deposition comparable to or exceeding that of fish oil, with improved flavor stability, making algal DHA an increasingly attractive option (48). Flaxseed or linseed oil, rich in ALA, effectively raises yolk ALA concentrations. Reported increases range from 1.5% to nearly 10% of total fatty acids when 1–4% flaxseed oil is included in the diet. Although hens convert ALA to EPA and DHA only inefficiently, flaxseed remains a widely used, cost-effective plant source for increasing total ω-3 PUFAs (49). Other emerging sources, including chia, perilla, and camelina seeds, as well as encapsulated ω-3 lipids, provide additional dietary ALA or stabilized EPA/DHA forms while enhancing oxidative stability during storage and cooking (47). Overall, evidence from multiple feeding trials demonstrates that dietary inclusion of these oil sources can increase yolk ALA, EPA, and DHA by 2- to 10-fold while substantially reducing the ω-6/ω-3 ratio. Comparatively, fish oil achieves the highest DHA/EPA deposition but may negatively impact flavor and oxidative stability; algal oil offers sustainable DHA enrichment with improved sensory quality; and flaxseed provides a cost-effective, plant-based source of ALA, albeit with limited conversion to long-chain ω-3 PUFAs. Therefore, the choice of feed source must balance enrichment efficiency, sensory quality, oxidative stability, and production performance (47).

Dietary fatty acids, especially ω-3 PUFAs such as EPA and DHA, as well as their precursor ALA, must undergo a series of coordinated digestive, metabolic, and selective deposition processes before being incorporated into the developing yolk (50). Following ingestion, these lipids are released and absorbed in the gastrointestinal tract, transported to the liver for metabolic routing and limited bioconversion, assembled into yolk-targeted lipoproteins, and ultimately taken up by ovarian follicles via highly selective receptor-mediated pathways. A comprehensive understanding of each step along this metabolic axis is critical for optimizing nutritional strategies aimed at producing ω-3 PUFA-enriched eggs. Overall, the process can be conceptualized in three interrelated stages, as illustrated in Figure 2: (1) intestinal digestion, absorption, and transport, (2) hepatic uptake, enzymatic conversion, and lipoprotein assembly, and (3) ovarian follicle uptake and selective deposition into the yolk.

Antioxidant-enriched eggs represent an important category of functional eggs aimed at improving both egg quality and human health through targeted enhancement of yolk antioxidant capacity. Approximately 36% of the edible portion of an egg consists of lipids, nearly all of which are concentrated in the yolk and organized into lipoprotein structures rich in triacylglycerols, phospholipids, and cholesterol (69, 70). Although carotenoids account for less than 1% of total yolk lipids, they are the primary determinants of yolk pigmentation and contribute substantially to the antioxidant and anti-inflammatory properties of eggs (69). Dietary supplementation of laying hens with vitamin E, carotenoids, and plant-derived polyphenols is widely employed to enhance yolk antioxidant content. This targeted enrichment improves oxidative stability, enhances nutritional value, and confers physiologically relevant health benefits, thereby positioning antioxidant-enriched eggs as a promising strategy for the development of value-added functional foods.

Eggs, with their high lipid content, provide an ideal matrix for lipophilic antioxidants, including carotenoids (lutein, zeaxanthin), vitamin E, selenium, iodine, lycopene, and ω-3 PUFAs (92). Dietary supplementation of laying hens with ω-3-ich ingredients such as fish oil, flaxseed, microalgae effectively increase yolk ω-3 PUFAs content. However, elevated levels of PUFAs also increase susceptibility to lipid peroxidation, highlighting the importance of co-supplementation with antioxidants, such as vitamin E and carotenoids, to enhance both oxidative stability and the nutritional benefits of enriched eggs (10).

Carotenoids, the main lipophilic pigments in yolks, are highly dependent on the hen’s diet. Natural sources such as marigold (Tagetes erecta) and alfalfa (Medicago sativa) provide lutein, while maize (Zea mays) and red peppers (Capsicum annuum) supply zeaxanthin and capsanthin. Chemically synthesized carotenoids, including canthaxanthin, β-apo-8′-carotene, and β-apo-8′-carotenoic acid esters, are also used to modulate yolk pigmentation and antioxidant capacity (72, 93). Lutein and zeaxanthin play key roles in mitigating oxidative stress and supporting retinal and skin health. Biofortified eggs can achieve up to a 15-fold increase in yolk lutein (~1.9 mg per egg) with higher bioavailability than dietary supplements or plant sources such as spinach (83, 94). Natural carotenoids are derived from microalgae (Spirulina, Chlorella), yeast (Phaffia rhodozyma, Rhodotorula rubra), bacteria (Paracoccus spp.), plants (marigold, basil, Stevia, and spinach), plant by-products (tomato pomace, carrot derivatives, and grape seed powder), and animal by-products (crab meal, golden snail egg powder) (95–100). Additionally, biofortified crops, such as lutein- or zeaxanthin-enriched maize, are also employed to enhance yolk carotenoid content (101). While synthetic carotenoids remain common in conventional feed, consumer preference for natural sources has increased, particularly in organic egg production (102).

Other antioxidants, including vitamin E, folate, selenium, and iodine, can also be enriched through maternal diet modification. Vitamin E protects membrane lipids from peroxidation and can reach ~20 mg per egg, exceeding daily human requirements while preserving unsaturated fatty acids (103). Folate, particularly in the form of 5-methyltetrahydrofolate (5-MTHF), is a potent antioxidant that remains stable during cooking, and can supply up to 12.5% of the recommended daily intake per egg (104, 105). Selenium and iodine are efficiently deposited in yolks, potentially providing 50% of daily human requirements (106). In summary, antioxidant enrichment of eggs relies on natural feed ingredients, synthetic compounds, and maternal diet modification to achieve targeted deposition of bioactive compounds in yolk lipids or phospholipid fractions. These strategies improve oxidative stability and enhance the nutritional and functional properties of eggs for human consumption.

Dietary carotenoids are absorbed and transported to egg yolks through a multistage physiological process (107). In the duodenum, digestive enzymes release carotenoids from the food matrix, which are then solubilized in mixed micelles composed of fatty acids, phospholipids, cholesterol, bile salts, and monoacylglycerols to facilitate intestinal absorption (108, 109). Carotenoids enter enterocytes via passive diffusion across the unstirred water layer and active transport mediated by scavenger receptor class B type 1 and cluster of differentiation 36 (110, 111). Certain carotenoids, such as lutein esters, are hydrolyzed by intestinal lipases to free forms for more efficient uptake (112). Within enterocytes, carotenoids may be enzymatically cleaved by β-carotene-15,15′-oxygenase or β-carotene-9′,10′-oxygenase to generate bioactive metabolites. While unprocessed carotenoids are incorporated into chylomicrons along with lipids and subsequently released into the lymphatic system, ultimately reaching the circulation (108, 111, 113, 114).

In the liver, carotenoids are incorporated into lipoproteins, VLDL, LDL, and HDL, for transport to peripheral tissues and the ovary (115–117). Polar carotenoids like lutein and zeaxanthin distribute more evenly between LDL and HDL, whereas nonpolar carotenoids, including β-carotene, are mainly carried by VLDL and LDL (109, 118). The ovary selectively uptakes VLDL particles carrying VLDLy (~30 nm), which are resistant to lipoprotein lipase-mediated hydrolysis and recognized by LR8 receptors on developing oocytes, enabling targeted carotenoid deposition into the yolk (115, 119, 120). Carotenoid deposition occurs rapidly, yet yolk pigmentation stabilizes over 6–14 days depending on yolk size and oocyte growth stage (121). Studies indicate that yolk color reaches a plateau after 7–12 days of supplementation, independent of dosage, reflecting the physiological kinetics of carotenoid accumulation (122–125). Overall, carotenoid deposition in eggs involves coordinated release from the diet, micellar solubilization, receptor-mediated intestinal uptake, lipoprotein-mediated hepatic transport, and selective ovarian incorporation. Understanding these mechanisms is essential for optimizing dietary strategies to produce antioxidant-enriched eggs with enhanced carotenoid content and improved nutritional and functional properties (107, 126, 127).

Global production approaching 80 million tons and an average per capita consumption of approximately 161 eggs per year. Their broad accessibility and habitual consumption make eggs a practical dietary carrier for micronutrient delivery (128). Experimental and observational studies indicate that eggs can efficiently accumulate and maternally transfer essential trace elements, including selenium (Se), iodine (I), iron (Fe), and zinc (Zn), into the yolk (129). Although the physicochemical characteristics of the yolk lipid-protein matrix are often cited as supporting high micronutrient bioavailability, quantitative comparisons with other fortified foods or supplements remain limited.

The concept of micronutrient-enriched eggs has evolved alongside the recognition of eggs as functional foods capable of addressing dietary inadequacies. In low- and middle-income regions, where micronutrient deficiencies are highly prevalent, eggs have been promoted as a cost-effective and culturally acceptable intervention (130). Several randomized and quasi-experimental studies have reported improvements in growth indicators and overall nutritional status in infants and young children following regular egg consumption, supporting the physiological relevance of egg-derived micronutrients during early development (131–133). However, these benefits appear context-dependent, varying with geographic region, baseline dietary patterns, and coexisting nutrient deficiencies. Evidence from high-income countries further indicates that insufficient intake of certain micronutrients, including vitamin E, iron, and zinc, persists despite generally adequate food availability, suggesting that egg-based strategies may also have relevance beyond undernourished populations (133, 134). However, long-term population-level evidence linking egg-derived micronutrient intake to clinically meaningful health outcomes remain limited.

Among the trace elements targeted for egg fortification, selenium and zinc have received particular attention due to their fundamental roles in antioxidant defense, immune regulation, thyroid hormone metabolism, and growth (135). Selenium-enriched eggs predominantly provide selenium in organic forms, which exhibit superior absorption and biological efficacy compared with inorganic selenium sources (136). Similarly, zinc-enriched eggs contribute to immune competence, enzymatic function, and early growth and neurodevelopment (137). In addition, iron- and iodine-enriched eggs have been explored as nutritional strategies to prevent iron-deficiency anemia and iodine deficiency disorders, respectively, with particular relevance for vulnerable groups such as infants, pregnant women, and adults.

Eggs are naturally rich in essential nutrients, and their mineral composition can be substantially enhanced through targeted dietary interventions and advanced nutritional technologies. Current poultry nutrition strategies aim to increase the concentrations of minerals, such as chromium (Cr), iron (Fe), zinc (Zn), selenium (Se), iodine (I), and manganese (Mn), in eggs through feed-based interventions, thereby improving their nutritional value. The efficiency of mineral deposition into eggs is determined by multiple interacting factors, including the chemical form of the mineral (organic vs. inorganic), feed formulation, hen physiology, mineral-nutrient interactions, and production systems (e.g., conventional, free-range, or organic housing) (138, 139). Compared with conventional fortified foods or dietary supplements, mineral-enriched eggs offer several advantages: they are affordable, widely consumed, culturally accepted, and provide minerals within a natural, highly bioavailable biological matrix (140). Consequently, micronutrient-enriched eggs have attracted increasing attention as a scalable and food-based strategy to address “hidden hunger,” particularly in low-income populations, while remaining relevant in high-income countries where suboptimal micronutrient intake persists.

Among available approaches, the enrichment and biological conversion of selenium and zinc into organic forms via plant- or animal-derived feed ingredients has emerged as an effective strategy to enhance mineral efficacy (141). The egg matrix itself further facilitates organic mineral transformation and stabilization, which may improve intestinal absorption and metabolic utilization in humans. Despite progress in enhancing mineral deposition, the nutritional functionality and biological activity of selenium- and zinc-enriched eggs, as well as mineral-enriched eggs more broadly, remain insufficiently evaluated, with most studies focusing on compositional outcomes rather than long-term health effects (129, 142). This synthesizes current feed-based fortification approaches, including the use of inorganic salts, organic chelates, mineral-enriched plants and yeasts, and emerging nano- and biologically transformed mineral sources. In addition, key determinants of mineral deposition efficiency, such as mineral chemical form, dosage, interactions with other nutrients, hen physiological status, and production system, are critically considered.

The deposition and enrichment of minerals in eggs is a tightly regulated, compartment-specific process that integrates dietary mineral supply, hen physiology, and embryonic developmental demands. Following ingestion, minerals are absorbed in the gastrointestinal tract of laying hens, with absorption efficiency strongly influenced by mineral chemical form (organic vs. inorganic), interactions with other dietary components, and the presence of anti-nutritional factors such as phytates (143). Organic mineral sources, including selenium-methionine, zinc-amino acid chelates, and mineral-enriched yeasts, generally exhibit superior intestinal bioavailability compared with inorganic salts, thereby facilitating more efficient systemic transport and subsequent deposition into eggs (144). After absorption, minerals are transported via the bloodstream and selectively deposited into different egg compartments through two anatomically and functionally distinct maternal pathways: ovarian transfer into the yolk and oviductal secretion into the albumen, shell membranes, and eggshell (145). This dual-pathway system underlies the pronounced mineral heterogeneity observed between yolk and albumen. For example, phosphorus (P), Zn, Cu, Mn, and Fe are predominantly localized in the yolk, whereas sodium (Na) and potassium (K) are mainly associated with the albumen. Quantitative analyses of fresh eggs from 50-week-old Cobb 500 breeder hens indicate that the yolk accounts for over 94% of total Mn and P, while contributing less than 20% of total Na and approximately one-third of total K, highlighting the yolk’s primary function as a storage reservoir for trace minerals essential to embryonic development (146).

The accumulation of minerals in the yolk reflects its unique lipid- and protein-rich matrix, which enables the stabilization and long-term retention of protein-bound and lipophilic mineral complexes (147, 148). Albumen primarily serves as a medium for water-soluble and protein-associated electrolytes that support early embryonic homeostasis (149). Importantly, once minerals are deposited into the yolk, their concentrations remain relatively stable and are respond only weakly to short-term dietary changes, indicating that egg mineral composition is governed by stringent physiological control rather than simple proportional transfer from the maternal diet (144). Mineral deposition efficiency is further modulated by hen-specific and production-related factors, including age, laying stage, endocrine regulation, stress status, and housing system (144). As maternal deposition constitutes the sole source of minerals available to the developing embryo during incubation, both the quantity and distribution pattern of minerals within the egg are critical determinants of embryonic viability, skeletal development, immune competence, and post-hatch performance. Collectively, effective egg mineral enrichment depends on the interplay of dietary fortification, maternal absorption and transport, and compartment-specific deposition. Understanding these coordinated mechanisms is essential for designing strategies to optimize egg mineral content and support both embryonic development and post-hatch performance.

A major challenge in functional egg development is the substantial variability in bioactive component deposition, arising from differences in feed formulation, hen physiology, production systems, and environmental conditions. This compositional heterogeneity not only hampers product standardization and reproducibility, but also complicates health claim substantiation, regulatory approval, and large-scale commercialization. In addition, enrichment strategies, particularly those involving polyunsaturated fatty acids, often face trade-offs between enrichment efficiency, oxidative stability, sensory quality, and cost-effectiveness. Recent advances in artificial intelligence-assisted feeding systems, digital nutrition management, and real-time physiological monitoring offer promising solutions to these limitations. AI-driven precision fortification could enable dynamic adjustment of nutrient inputs according to hen status, production stage, and targeted egg functionality, thereby improving deposition consistency while minimizing oxidative risk and unnecessary supplementation. Such approaches represent a critical shift from empirical fortification toward data-driven and mechanism-informed nutrient delivery.

Functional eggs have traditionally been regarded as passive carriers of enhanced nutrients. However, growing evidence indicates that maternal gut microbiota can modulate nutrient absorption, metabolic conversion, and subsequent deposition into eggs, giving rise to the concept of a “gut-egg axis.” While microbial variability introduces additional complexity and inter-individual variation in enrichment outcomes, it also presents new opportunities for targeted intervention. Microbiome-oriented strategies, including probiotics, prebiotics, fermented feeds, and dietary fiber modulation, may enhance the bioavailability, metabolic retention, and stability of egg-derived bioactive compounds. Incorporating gut microbiota considerations into enrichment strategies could therefore move functional eggs beyond simple nutrient accumulation toward biologically active systems with improved functional efficacy.

Future progress in functional egg development will likely require a transition from population-level fortification to mechanism-driven and personalized nutrition strategies tailored to specific life stages and health conditions, such as aging, pregnancy, and metabolic disorders. Advances in multi-omics technologies, including nutrigenomics, metabolomics, lipidomics, and microbiomics, provide powerful tools to elucidate the pathways governing nutrient deposition and egg bioactivity, identify responsive biomarkers, and link egg composition to health-relevant outcomes. Integrating multi-omics insights with precision feeding systems and consumer stratification may ultimately redefine functional eggs as targeted nutritional interventions rather than generalized fortified foods.