but these enzymes have not been well
characterized in farm animals.
Esterification of fatty acids in adipose
tissue increases with increasing energy
intake in meat animals (Rule, 1995) and is
lower in times of dietary energy deficit,
such as during early lactation in dairy
cows (McNamara, 1991).
Fatty acid composition of milk,
muscle and body fat
A variety of fatty acids are found in
complex lipids of animal tissues. These
fatty acids range primarily from 14 to 20
carbons in length, with varying degrees of
unsaturation. Characteristic profiles of fatty
acids are found in individual tissues and
among species of animals. Sample profiles
of muscle and adipose tissue of beef cattle,
sheep and pigs are shown in Table 5.1.
Adipose tissue lipids from ruminants gener-
ally are more highly saturated than lipids
from non-ruminants such as pigs because
of ruminal biohydrogenation of dietary
unsaturated fatty acids. Experimental post-
ruminal infusions of unsaturated oils and
feeding formaldehyde-protected oils to
sheep and cattle results in increasing
unsaturation of adipose tissue lipids (Rule
et al., 1995). In pigs and chickens, increas-
ing amounts of dietary fat will result in
adipose tissue lipids reflecting the fatty
acid composition of the dietary fat. Body
fat generally becomes softer in these
species with supplementation of fats and
oils, because the relative amounts of de
novo synthesized palmitic acid decrease
and those of 18-carbon unsaturated fatty
acids increase (Rule et al., 1995).
Bovine milk fat contains considerable
amounts of fatty acids shorter than 14
carbons that are synthesized within the
mammary gland (Table 5.2). The fatty acid
composition of milk fat can be altered
markedly by supplementation of the diet
with fat (Palmquist et al., 1993). Dietary
long-chain fatty acids suppress de novo
synthesis of short- and medium-chain fattyLipid Metabolism 107Fig. 5.3.Major pathways of esterification of fatty acids to glycerolipids in farm animals. The key enzymes
involved are: (1) glycerophosphate acyltransferase; (2) lysophosphatidate acyltransferase; (3) phosphatidate
phosphohydrolase; (4) diacylglycerol acyltransferase; and (5) monoacylglycerol acyltransferase. Pi, inorganic
phosphate. Adapted from Rule (1995).