This thesis focuses on the replacement of animal fat by vegetable oil in the diet for growing-finishing pigs. Generally, but not exclusively, fats of animal origin contain higher proportions of saturated fatty acids (SFA) than vegetable oils that are commonly rich in polyunsaturated fatty acids (PUFA). There is concern among the consumers of pork meat as to the use of fats of animal origin in the diet of pigs. High intake of animal fats by pigs leads to pork meat rich in SFA, which in turn increases the risk of cardiovascular disease in man. Consequently, consumers tend to prefer animal products derived from animals fed on diets containing oils of plant origin. The PUFA concentrations in pork meat can be raised by feeding pigs diets rich in PUFA, but the impact on physical meat quality was not known. From biochemical studies it follows that PUFAs are preferentially oxidised when compared to SFA. Thus, it could be suggested that the inclusion of vegetable oils into the diets for pigs would alter the metabolism of fatty acids and energy. The main question that was addressed in this thesis is as follows: What is the effect of feeding diets containing either sunflower oil (SO) or beef tallow (BT) on meat quality and fatty acid and energy metabolism? Variables measured were the incorporation of fatty acids into adipose tissue, meat and the whole body, the oxidation and de novo fatty acid synthesis of fatty acids and energy expenditure. The feeding of diets with SO instead of BT did not affect growth performance, but markedly increased the level of linoleic acid (LA) in adipose tissues and pork meat without any adverse effect on the physical aspects of pork quality. The digestibility of crude fat and fatty acids was higher for the SO diets, except for the digestibility of stearic acid. The SO diets raised the oxidation of LA and the de novo synthesis of SFA synthesis. To study fatty acid metabolism at the level of the whole body, the relatively small, Thai indigenous pigs were used as a model. In these pigs the metabolism of LA and ALA was compared after feeding diets with a low or high LA:ALA ratio. Adipose tissue and pork of the pigs fed on the high-ALA diet were enriched with ALA. The ratios of deposition: digestible intake for LA and ALA were lower than 1, indicating preferential oxidation of these fatty acids, which was associated with increased de novo synthesis of SFA synthesis in keeping with the studies in commercial pigs. For both the high-ALA and high-LA diet, the rate of ALA incorporation into the whole body was lesser than for LA, which is in line with preferential oxidation of ALA over LA. In a subsequent study, ALA metabolism was studied in pigs fed ALA-rich diets with identical levels of either SO or BT as fat background. It was found that the digestion and deposition of LA, de novo synthesis of SFA synthesis and ALA oxidation were higher, whereas ALA deposition was lower for the SO diet. It is concluded that the fat background of the diet can affect metabolism of supplemental fatty acids. In order to assess the digestibility of fatty acids more accurately, cannulated pigs were used to determine apparent ileal digestibility instead of faecal digestibility of fatty acids. The apparent ileal digestibility of fatty acids was found to be lower than apparent feacal digestibility, except for that of stearic acid. The aberrant outcome for stearic acid indicates that C18 unsaturated fatty acids are escaping from ileal digestion and are hydrogenated and transformed into C18 saturated fatty acids by microbes in the large intestine. Energy expenditure and proximate body composition was not influenced by the type of fat in the diet
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