TABLE 5-1 Common names, scientific names, and short-form designations of fatty acids

Common Name

Scientific Name

Short-Form Designation

Butyric acid

butanoic acid

C4:0

Caproic acid

hexanoic acid

C6:0

Caprylic acid

octanoic acid

C8:0

Capric acid

decanoic acid

C10:0

Lauric acid

dodecanoic acid

C12:0

Myristic acid

tetradecanoic acid

C14:0

Palmitic acid

hexadecanoic acid

C16:0

Stearic acid

octadecanoic acid

C18:0

Palmitoleic acid

9-hexadecaenoic acid

C16:1 n-7 cis

Oleic acid

9-octadecaenoic acid

C18:1 n-9 cis

Elaidic acid

9-octadecaenoic acid

C18:1 n-9 trans

Linoleic acid

9,12-octadecadienoic acid

C18:2 n-6,9 all cis

α-Linolenic acid

9,12,15-octadecatrienoic acid

C18:3 n-3,6,9 all cis

γ-Linolenic acid

6,9,12-octadecatrienoic acid

C18:3 n-6,9,12 all cis

Arachidic acid

eicosanoic acid

C20:0

Behenic acid

docosanoic acid

C22:0

Eicosenoic acid

11-eicosenoic acid

C20:1 n-9 cis

Erucic acid

13-docosaenoic acid

C22:1 n-9 cis

Brassidic acid

13-docosaenoic acid

C22:1 n-9 trans

Nervonic acid

15-tetracosaenoic acid

C24:1 n-9 cis

Dihomo-γ-linolenic acid

8,11,14-eicosatrienoic acid

C20:3 n-6,9,12 all cis

Arachidonic acid

5,8,11,14-eicosatetraenoic acid

C20:4 n-6,9,12,15 all cis

Timnodonic acid

5,8,11,14,17-eicosapentaenoic acid

C20:5 n-3,6,9,12,15 all cis

Clupanodonic acid

7,10,13,16,19-docosapentaenoic acid

C22:5 n-3,6,9,12,15 all cis

Docosahexaenoic acid

4,7,10,13,16,19-docosahexaenoic acid

C22:6 n-3,6,9,12,15,18 all cis

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.

FAT ABSORPTION

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



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