sider the most common dietary n-6 and n-3 fatty acids, linoleate and α-linolenate, as truly essential since these fatty acids must be ingested. Additional long chain polyunsaturated fatty acids can be constructed from these fatty acids, but ingestion of fatty acids with the n-3 and n-6 double bonds is a true requirement.

In response to entry of fat into the intestine during digestion of a meal, the liver secretes bile into the gut; with the help of intestinal peristalsis, food fats are emulsified. Simultaneously, the pancreas secretes digestive enzymes, including lipases and esterases, into the small intestine. Fat digestion begins at the surface of emulsion particles with pancreatic lipase-catalyzed hydrolysis of triacylglycerol molecules into two fatty acids and a monoacylglyceride. These products of initial lipolytic activity and bile salts are active in further breakdown of emulsion particles and, with phospholipid and cholesterol bile micelles, form the micellar phase from which lipid absorption is maximal. In most laboratory studies of nonhuman primates, fat digestion and absorption were essentially quantitative, with less than 5% of dietary fat lost in the feces; this was true in a wide array of types and amounts (up to 40% of ME) of fat ingested (L. Rudel and P. Huth, unpublished). The processes of fat digestion and absorption also facilitate the absorption of cholesterol and fat-soluble vitamins from the intestine. These molecules are incorporated into emulsion particles and micelles, from which they pass into the enterocyte. However, only about 50% of cholesterol is absorbed from the intestine (Rudel et al., 1994; Wilson and Rudel, 1994), though some of the molecular processes involved are different for sterols and fatty acids.

Once inside the intestinal enterocyte, fatty acids and glycerides are reassembled into triacylglycerol molecules and incorporated into newly forming chylomicrons that include a protein, apolipoprotein B48. Nonhuman primates and humans share the characteristic presence of only apolipoprotein B48 in the intestine for transport of TAGs in chylomicrons, in contrast with the liver, where only apolipoprotein B100 is used for TAG secretion in very-low-density lipoproteins (VLDLs) (Klein and Rudel, 1983). Newly absorbed cholesterol is also esterified and incorporated into chylomicrons, although it makes up only about one percent (by mass) of these particles.

Capture of high-energy fatty acids from chylomicrons is efficient. The chylomicron particles are secreted by the enterocytes into basolateral spaces, where they cross into the lymphatic lacteals and enter the body via the thoracic lymph duct. This pathway of entry into the bloodstream directs fats first to the peripheral tissues, where interactions with lipoprotein lipase (LPL), attached to the endothelial cells of most tissues, can occur. Removal of TAG molecules from chylomicrons then proceeds with the LPL-catalyzed hydrolysis of TAGs into two fatty acids and a monoacylglyceride. In this form, the molecules pass across cell membranes and enter cells. In adipose tissue, TAG molecules are reassembled and stored for later use. In most other

Front Matter (R1-R20)

Overview (1-4)

1 Feeding Ecology, Digestive Strategies, and Implications for Feeding Programs in Captivity (5-40)

2 Energy (41-57)

3 Carbohydrates and Fiber (58-74)

4 Protein (75-86)

5 Fats and Fatty Acids (87-93)

6 Minerals (94-112)

7 Vitamins (113-149)

8 Water (150-158)

9 Pathophysiologic and Life-Stage Considerations (159-181)

10 Diet Formulation, Effects of Processing, Factors Affecting Intake, and Dietary Husbandry (182-190)

11 Nutrient Requirements (191-194)

12 Composition of Foods and Feed Ingredients (195-258)

13 Food as a Component of Environmental Enhancement (259-265)

Appendix (266-268)

About the Authors (269-272)

Index (273-286)