Dietary energy sources affect the partition of body lipids and the hierarchy of energy metabolic pathways in growing pigs differing in feed efficiency

The use and partition of feed energy are key elements in productive efficiency of pigs. This study aimed to determine whether dietary energy sources affect the partition of body lipids and tissue biochemical pathways of energy use between pigs differing in feed efficiency. Forty-eight barrows (pure Large White) from two divergent lines selected for residual feed intake (RFI), a measure of feed efficiency, were compared. From 74 d to 132 ± 0.5 d of age, pigs (n = 12 by line and by diet) were offered diets with equal protein and ME contents. A low fat, low fiber diet (LF) based on cereals and a high fat, high fiber diet (HF) where vegetal oils and wheat straw were used to partially substitute cereals, were compared. Irrespective of diet, gain to feed was 10% better (P < 0.001), and carcass yield was greater (+2.3%; P < 0.001) in the low RFI compared with the high RFI line; the most-efficient line was also leaner (+3.2% for loin proportion in the carcass, P < 0.001). In both lines, daily feed intake and average daily gain were lower when pigs were fed the HF diet (-12.3% and -15%, respectively, relatively to LF diet; P < 0.001). Feeding the HF diet reduced the perirenal fat weight and backfat proportion in the carcass to the same extent in both lines (-27% on average; P < 0.05). Lipid contents in backfat and Longissimus muscle (LM) also declined (-5% and -19%, respectively; P < 0.05) in pigs offered the HF diet. The proportion of saturated fatty acids (FA) was lower but the percentage of polyunsaturated FA, especially the essential FA C18:2 and C18:3, was greater (P < 0.001) in backfat of HF pigs. In both lines, these changes were associated with a marked decrease (P < 0.001) in the activities of two lipogenic enzymes, the fatty acid synthase and the malic enzyme, in backfat. For the high RFI line, the hepatic lipid content was greater (P < 0.05) in pigs fed the HF diet than in pigs fed the LF diet, despite a reduced fatty acid synthase activity (-32%; P < 0.001). In both lines, the HF diet also led to lower glycogen content (-70%) and lower glucokinase activity (-15%; P < 0.05) in the liver. These results show that dietary energy sources modified the partition of energy between liver, adipose tissue and muscle in a way that was partly dependent of the genetics for feed efficiency, and changed the activity levels of biochemical pathways involved in lipid and glucose storage in tissues.