Type of dietary fat and insulin resistance.

Animal studies have already shown the possibility to modulate insulin action by changing not only the amount of total fat, but also the type of fat. In these studies, saturated fat significantly increased insulin resistance, long- and short-chain omega(3) fatty acids significantly improved it, whereas the effects of monounsaturated and omega(6) polyunsaturated fatty acids ranged somewhere in between the two. A recent multicenter study (the Kanwu study) on humans has shown that shifting from a diet rich in saturated fatty acids to one rich in monounsaturated fat improved insulin sensitivity in healthy people, while a moderate omega(3) supplementation did not affect it; this second finding confirms previous results in type 2 diabetic patients with hypertriglyceridemia. There are also other aspects of the metabolic syndrome that can be influenced by the different type of dietary fat, particularly blood pressure and lipid metabolism. With respect to blood pressure, the majority of studies show that omega(3) fatty acids are able to reduce blood pressure in hypertensive patients, but not in normotensive individuals; this result has been confirmed also by the Kanwu study, where no changes in blood pressure were seen after omega(3) supplementation in healthy people. On the other hand, in this study, the change from saturated to monounsaturated fatty acids was able to significantly reduce diastolic blood pressure. As to the lipid abnormalities more frequently present in the metabolic syndrome (i.e., hypertriglyceridemia and low HDL cholesterol), the main effects are related to omega(3) fatty acids, which surely reduce triglyceride levels, but at the same time increase LDL cholesterol. In conclusion, there is so far sound evidence in humans that the quality of dietary fat is able to influence insulin resistance and some of the related metabolic abnormalities.