The vertebrate organism ingests complex whole foods containing carbohydrates, fats or lipids, minerals, proteins, and vitamins. Digestion simplifies foods to the major components and to simpler moieties, e.g., carbohydrates to simple sugars, proteins to amino acids, and lipids to fatty acids (FAs). Digestive products are absorbed and distributed to body tissues where they are resynthesized to complex cellular components or oxidized for energy. Digestion, including that of lipids, primarily occurs in the stomach and intestine of nonruminant mammals, but in ruminants, much digestion occurs in the rumen. Absorption is concentrated in the small intestine.

Lipids are one of the major components of feeds and of vertebrate cells. Although there are exceptions, lipids are not water-soluble, i.e., they are hydrophobic. Animal lipids are generally complex structures built from FAs, e.g., phospholipids or triacylglycerol, or from the sterol nucleus (a complex set of ring structures). Plant lipids are even more complex than animal lipids.

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