Egg-Derived Lipids and Phospholipids as Emerging Functional Ingredients: Molecular Structure, Bioavailability, Metabolic Effects, and Food Applications

Egg yolk is often treated in the functional-food literature as a single, compositionally uniform fat depot, but this simplification obscures its actual structure: the yolk is a compartmentalized lipid system in which egg-yolk lipids are distributed between a triglyceride-dominant, low-density-lipoprotein (LDL) plasma fraction and a phospholipid-enriched, high-density-lipoprotein (HDL) granule fraction. Prior reviews of egg-yolk lipid bioactivity, extraction methodology, and flavor chemistry have generally addressed these topics as separate studies; here, molecular architecture, extraction and characterization, gastrointestinal digestion and bioaccessibility, human bioavailability and metabolic fate, health outcomes, and technological functionality are synthesized as stages of a single pathway. The compositional asymmetry between the LDL plasma fraction and the HDL granule fraction is well established, and differences in their emulsifying behavior are supported by the technological literature; divergent metabolic effects of the isolated fractions have so far been shown only in animal models, whereas differences in digestion kinetics are inferred from structure and remain a working hypothesis because direct experimental comparison of the digestion of isolated HDL and LDL fractions is still lacking. On the human evidence side, a single randomized crossover trial in 18 healthy adults found a higher plasma choline response to phospholipid-bound choline than to choline delivered as a synthetic salt, a preliminary finding that awaits replication, while two crossover studies found no significant postprandial rise in plasma cholesterol area under the curve after whole-egg intake. Because this is an indirect, postprandial measure rather than a direct assessment of intestinal absorption, it is consistent with, but does not establish, limited cholesterol absorption from the whole-egg matrix, and whether whole-egg cholesterol should be distinguished from isolated dietary cholesterol exposure in the epidemiological literature remains a hypothesis. Large prospective meta-analyses report that egg and dietary cholesterol intake associate modestly with elevated all-cause and cancer mortality, but not with cardiovascular-disease-specific mortality or events; these observational associations describe health outcomes and do not, on their own, test a mechanistic distinction between whole-egg cholesterol and isolated dietary cholesterol absorption. Extraction and delivery-system technology for egg phospholipids is more advanced than commonly assumed, with pilot-scale supercritical-fluid purification already achieved and egg-yolk phosphatidylcholine reported to show formulation-specific advantages over soybean phosphatidylcholine in individual liposome studies, although these studies differ in cargo, preparation method, and outcome measures. This delivery-system evidence, however, remains at the level of isolated liposome formulations; finished food products incorporating egg-phospholipid liposomes, with accompanying sensory or consumer-acceptance validation, have not yet been reported. The principal barriers to validated functional-ingredient status are correspondingly narrower than a general shortfall in bioactivity or functionality evidence: a direct human bioavailability comparison between whole egg and an isolated egg-phospholipid ingredient, and dedicated techno-economic and life-cycle assessment of egg-lipid extraction, remain the two outstanding gaps.