Nutritional effects of fat on carbohydrate metabolism.

Obesity is commonly associated with elevated plasma levels of free fatty acids (FFAs). High levels of FFA have emerged as a major link between obesity and insulin resistance/type 2 diabetes (T2DM). Thus, acute and chronic elevations of plasma FFAs produce insulin resistance in skeletal muscle and liver. In skeletal muscle, FFA-induced insulin resistance is associated with accumulation of intramyocellular triglyceride and diacylglycerol, and with activation of protein kinase C (the beta and delta isoforms). It is suggested that FFAs interfere with insulin signalling via PKC-induced serine phosphorylation of the insulin receptor substrate-1. In the liver, FFAs cause insulin resistance by interfering with insulin suppression of glycogenolysis. In beta-cells, FFAs potentiate glucose-stimulated insulin secretion acutely and chronically. It is postulated that this prevents the development of T2DM in most (>80%) obese insulin-resistant people who have FFA-mediated insulin resistance. Elevated levels of FFA also seem to activate a pro-inflammatory and pro-atherogenic pathway (the IkappaB/NFkappaB pathway) and may be responsible, at least in part, for the increase in atherosclerotic vascular disease seen in patients with T2DM. As increased plasma levels account for up to 50% of insulin resistance in obese patients with T2DM, lowering of plasma FFAs could be a new and promising approach to the treatment of T2DM.