Lipids in milk and the first steps in their digestion.

Human milk contains 3.0% to 4.5% fat. The fat is contained within membrane-enclosed milk fat globules. The core of the globules consists of triglycerides (98% to 99% of total milk fat) whereas the globule membrane (which originates from the mammary secretory cell's Golgi and cell membranes) is composed mainly of phospholipids, cholesterol, and proteins. Milk fat content and composition change during lactation. Whereas the triglyceride level rises, the phospholipid and cholesterol concentrations decrease during the transition from colostrum to mature milk, resulting in an increase in the size of the milk fat globules. Digestion of milk fat depends on the consecutive action of several lipases. The first step is the partial hydrolysis of the milk fat globule core by lingual and gastric lipases in the stomach. Hydrolysis continues in the duodenum, where the bile salt-stimulated lipase of human milk and pancreatic lipase complete the process initiated in the stomach.