The three main forms of fat found in food are glycerides (principally triacylglycerol [triglyceride], the form in which fat is stored for fuel), the phospholipids, and the sterols (principally cholesterol). Triacylglycerol, or other lipoproteins, is not taken up directly by any tissue, but must be hydrolyzed outside the cell to fatty acids and glycerol, which can then enter the cell.

Fats are split into fatty acids and glycerol by the enzyme. They are absorbed in the small intestine and then recombined into fats. It is stored in the body and is used when energy is needed. When tissues have less sugar/glucose during starvation or in diabetes, oxidation of fat is incomplete and products of metabolism acetones and ketones are passed into the blood and urine.