Chronic consumption of high-fat or -carbohydrate diets is associated with the development of obesity; however, it is not well established whether dietary protein plays a role in the development of abnormalities of lipid metabolism that occur during obesity. To determine the effect of different types of protein during diet-induced obesity on hepatic and adipocyte lipid metabolism, rats were fed casein (CAS) or soy (SOY) protein diets with 5% fat or high-fat diets with 25% fat (HF-CAS and HF-SOY) for 180 d. Rats fed soy diets had lower hepatic sterol regulatory element binding protein-1 (SREBP-1) expression and higher SREBP-2 expression than those fed casein diets, leading to less hepatic lipid deposition. On the other hand, long-term HF-SOY consumption prevented hyperleptinemia in comparison with rats fed HF-CAS. Rats fed soy protein diet showed higher adipocyte perilipin mRNA expression and smaller adipocyte area than those fed casein diets, which was associated with a lower body fat content. Furthermore, the lipid droplet area in brown adipose tissue was significantly lower in rats fed soy diets than in those fed casein diets and it was associated with higher uncoupling protein-1 (UCP-1) expression. As a result, rats fed the soy diets gained less weight than those fed the casein diets, in part due to an increase in the thermogenic capacity mediated by UCP-1. These results suggest that the type of protein consumed and the presence of fat in the diet modulate lipid metabolism in adipose tissue and liver.

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The molecular mechanisms implicated in pronounced weight loss and metabolic benefits after bariatric surgery are still unknown. Adipocyte phenotype and metabolism have not been entirely explored. However, some features of adipocyte function have been studied, such as adipocyte size and inflammation, which are both reduced after bariatric surgery. Adipocyte fat metabolism, which is partly regulated by leptin, is likely modified, since adipocyte area is decreased. Here, we show that leptin receptor expression is increased, while adipocyte size is decreased 8 months after Roux-en-Y gastric bypass. Thus, adipocyte function is possibly modified by improved leptin signaling after bariatric surgery.