[Macronutrients, fat mass, fatty acid flux and insulin sensitivity]

Obesity and visceral or upper body fat distribution, have a major impact on insulin sensitivity. There is strong evidence to suggest that free fatty acids (FFA) contribute to the pathogenesis of insulin resistance and the metabolic syndrome. Increased FFA release from adipose tissue or failure of FFA using tissues to remove them normally, lead to increased triglycerides (TG) and FFA fluxes. Increased delivery of FFA to muscle reduces muscle glucose uptake and utilisation by substrate competition or direct inhibition of glucose transport. Insulin resistance has been correlated with the size of intramuscular TG store. Intracellular TG have been involved in beta cell failure the so called lipotoxicity phenomena. The rate of FFA to the liver is a major determinant of hepatic TG secretion. So the regulation of FFA distribution between FFA using tissues and the partition of FFA between storage and oxidation could be involved in the developpment of insulin resistance. The dietary macronutrients could play a role in nutrient partitioning but their role in the etiology of insulin resistance is poorly understood due to a paucity of credible intervention studies in humans. However deleterious effects of saturated fatty acids on insulin action and the beneficial effects of polyunsaturated fatty acids (PUFAs) could be suspected from animal studies, and from epidemiological or clinical studies in humans. A very high intake of sucrose or fructose could be deleterious but low glycemic index foods, and fibers could have protective effects. Weight loss can induce marked improvement in insulin resistance, but weight maintenance is also required to keep long term good metabolic results.