Sleep duration, sleep quality, and daytime napping in relation to incident cardiometabolic multimorbidity across three national aging cohorts: a prospective, interpretable machine learning study.

Agro-industrial by-products are increasingly used as feed resources to support circular animal production systems and ecosystem protection, but their value should not be reduced to the replacement of conventional feed ingredients alone. Fruit pomaces, olive mill residues, cereal brans, brewer’s spent grain, oilseed cakes, citrus pulp, tomato pomace and other secondary streams can recover nutrients that would otherwise remain underused, while also reducing disposal pressure and partially easing feed–food competition [,,]. Beyond their nutritional contribution, these matrices may contain polyphenols, including phenolic acids, flavonoids, tannins and stilbenes, as well as carotenoids, terpenes, tocopherols, unsaturated lipids, fermentable fibres and residual proteins able to interact with digestion, microbial metabolism, host physiology and product quality [,,]. They should therefore be interpreted as complex molecular matrices, not only as low-cost or alternative feed ingredients, but also as potential modulators of animal metabolism and the quality of milk, meat, eggs, fish and aquaculture products. However, the inclusion of a by-product in an animal diet does not automatically generate a functional animal-derived food. In this review, the term functional animal-derived food is used in an operational scientific sense, rather than as a regulatory health claim. It refers to milk, cheese, meat, eggs, fish or derived products in which feeding strategies based on agro-industrial by-products produce measurable nutritional, molecular, oxidative, technological, sensory or shelf-life-related improvements, or lead to the presence of specific dietary, microbial or host-derived metabolites. These dimensions should not be considered equivalent: direct enrichment with a defined compound, metabolite-mediated transfer and indirect modulation of product quality represent different levels of evidence. Many feeding studies evaluate by-products mainly through intake, digestibility, growth performance, feed efficiency or milk yield, whereas fewer studies connect the chemical profile of the by-product with measurable changes in milk, cheese, meat, eggs or fish products [,,]. Productive outcomes remain essential to define the feasibility of a feeding strategy, but they are not sufficient to demonstrate that the final product has acquired additional nutritional, antioxidant, sensory, technological or health-related value [,]. A further limitation of the available literature is that relatively few studies simultaneously evaluate by-product characterization, processing stability, microbiome-mediated transformation, host metabolism and final product traits within the same experimental design. This limitation is central because the biological activity of a by-product also depends on what happens after processing, ingestion and digestion. After ingestion, polyphenols may be released from the plant cell wall, degraded by ruminal or intestinal microorganisms, transformed into smaller phenolic acids, conjugated by the host, or bound to proteins and fibres before they can influence edible tissues or secretions []. Carotenoids and other lipophilic compounds may be affected by matrix structure, dietary lipid supply, oxidative stability and species-specific absorption mechanisms before they are deposited in milk fat, egg yolk or animal tissues [,]. Thus, the functional potential of a by-product depends less on its nominal richness in bioactive compounds and more on bioaccessibility, microbial conversion, host utilization and reproducible effects in the final animal-derived food [].