Influence of two dairy fats on lipid synthesis in the pig: comparative study of liver, muscle and the two backfat layers.

The development of pig adipose tissues is influenced by several factors such as localisation and dietary fat. The neck and backfat subcutaneous adipose tissues consist of two layers with different chemical compositions, and it is not yet known whether such variations are due to differences in lipogenic enzyme activities or to other factors. The aim of the present study was to compare the lipogenic activities of tissues from different sites (Longissimus dorsi muscle, liver, subcutaneous backfat) and between the outer and inner backfat layers. The effects of dietary fat from two animal sources on the lipogenic activities and fatty acid composition of these tissues were also compared. 28 Large White x Pietrain cross-bred swine, averaging 75 (initial) to 108 kg (final) live weight, were allocated into 2 groups. They were fed diets with the same energy and lipid contents but provided either by goat's milk or cow's milk. The lipid content and fatty acid composition of the tissues were determined and the following lipogenic enzymes activities measured: acetyl-CoA carboxylase (CBX); glucose-6-phosphate dehydrogenase (G6PDH) and malic enzyme (ME). Both groups showed similar average daily gain, carcass composition and tissue lipid contents. Lipogenic activities were highest in backfat (p < 0.001), intermediate in Longissimus dorsi muscle and lowest in liver. The lipogenic activities in backfat tissue were greater (p < 0.001) in the inner than in the outer layer in both groups. Animals fed on goat's milk exhibited greater (p < 0.05) CBX and ME activities in the backfat, and CBX (p < 0.05) activity in the Longissimus dorsi muscle. In backfat tissue of animals fed on goat's milk, CBX, G6PDH (p < 0.01) and ME (p < 0.05) were greater in the inner than in the outer layer, whereas in animals fed on cow's milk the inner backfat layer exhibited the slightly higher (p < 0.05) lipid content. Differences in CBX activity between the two dairy diets tended to be greater (p < 0.001) in the inner backfat than in the outer layer. This suggests that the inner layer might be more sensitive to dietary source fat. It is concluded that the layers of pig subcutaneous backfat should not be considered as a single entity, but rather as two separate tissues.