Fat is not acted upon by the secretions of the mouth nor to any great extent by those of the stomach. The gastric juice contains an enzyme, called gastric lipase, which has the power of acting on emulsified fats, such as in cream or yolk of eggs. It plays a more important part in the digestion of infants than of adults.

The presence of fat in the stomach retards the secretion of gastric juice. Hence a certain excess of fat in the stomach is sufficient to close the pylorus for a longer or shorter time, due to the failure of the appearance of free hydrochloric acid. This doubtless accounts for the fact that fat in the diet often seems to retard and otherwise disturb digestion.

Digestion of fat takes place mainly in the small intestines. The pancreatic juice contains an enzyme called steapsin, which has the power of emulsifying fats and also of splitting them into fatty acids and glycerine.

The bile contains no such enzymes, but it is nevertheless an important factor in the digestion of fat. It has the power of increasing greatly the activity of the pancreatic lipase (steapsin), and of holding in solution the fatty acids formed by its action, so that they are more perfectly absorbed. When bile is lacking, much of the fat fails of absorption and is excreted through the intestines. Emulsification is an important aid in the splitting of fat into fatty acids and glycerine, the forms in which fat is chiefly absorbed.