Egg.

Lutein in egg yolk sits in a phospholipid and triglyceride matrix, and carotenoids need lipid present to form the mixed micelles that carry them across the gut wall. Yolk-derived lutein is absorbed more efficiently gram for gram than lutein from leafy greens, where it is bound inside plant cell structures. The absolute lutein content of an egg is small, so the advantage is in efficiency rather than in amount.

Zeaxanthin travels with lutein in the yolk and depends on the same micellar route into the body. Adding a fat-containing food to a carotenoid dose reliably raises how much is absorbed. Egg yolk supplies that fat in the same bite as the carotenoid.

Beta-carotene absorption from vegetables rises substantially when eaten with a fat source, and whole egg has been used in feeding studies as that source. The yolk phospholipids improve emulsification in the small intestine. This is about the vehicle, not about anything unique to egg.

Vitamin D3 needs dietary fat to be absorbed well, and egg yolk supplies both a small amount of the vitamin itself and the fat to carry it. Yolk D3 content tracks what the hen was fed and how much light she saw, so it varies widely between production systems. Egg is a modest contributor to intake rather than a main source.

Most of the choline in egg is bound as phosphatidylcholine rather than as the free base, and a single large yolk supplies a meaningful share of daily choline intake. Phosphatidylcholine is hydrolysed and absorbed, then feeds acetylcholine synthesis, phospholipid membrane turnover and the betaine methyl pool. Stacking a choline supplement on top of a high-egg diet is duplication rather than addition.

Egg yolk phosvitin is a heavily phosphorylated protein that binds iron tightly, and eggs eaten alongside a non-haem iron source lower how much of that iron is absorbed. The effect is measurable in absorption studies. Separating an iron supplement from an egg-containing meal sidesteps it.

Ascorbate reduces ferric to ferrous iron and keeps it soluble, which offsets part of the inhibition that phosvitin and other binders impose. It is the standard way to rescue non-haem iron absorption from an inhibitory meal. The offset is partial, not complete.

Whole egg protein carries roughly eight to nine percent leucine and scores at the top of protein quality rankings for digestibility and amino acid pattern. Leucine is the amino acid that triggers the muscle protein synthesis signal. Adding free leucine to an already leucine-adequate protein dose adds less than adding it to a low-quality protein.

Egg yolk lecithin and sunflower lecithin serve the same emulsifying role and share the phosphatidylcholine backbone, differing in their fatty acid profile and in allergen status. Formulations rarely need both. The pairing is a substitution decision rather than a stack.

Egg supplies B12 in a protein-bound form that must be released by gastric acid and pepsin before intrinsic factor can pick it up. Absorption from egg is less efficient than from other animal foods. For anyone with low stomach acid, supplemental crystalline B12 does not depend on that release step.

Collagen is rich in glycine and proline but short on several indispensable amino acids, while egg protein covers the full pattern. Using collagen as the only protein source leaves gaps that egg protein fills. This is complementation, not a special interaction between the two.