Effect of dietary lipid on insulin action. Clinical studies.

Evidence from the best studies available in the literature suggest that the isocaloric substitution of dietary fat for dietary carbohydrate does not cause insulin resistance. Although many studies show several differences between the diets (for example, changes in dietary fiber or fat subtype), the diets were primarily designed to compare the effects of the fat/carbohydrate content, and a true important effect of dietary fat on insulin action should have been detected. Some studies found other changes in carbohydrate metabolism such as improvements in insulin secretion and glucose effectiveness (Sg) that could contribute to the conflicting results. For example, glucose uptake as measured by the euglycemic clamp includes both insulin-dependent and non-insulin-dependent (SG) factors, so that the M value could be affected by changes in SG as well as in insulin action. All of these studies have used isocaloric substitution so that body weight is maintained. This constraint, while important for determining and direct effect of dietary fat, may lead to false conclusions about its overall effects on insulin action. One of the most common and potent causes of insulin resistance is obesity, and dietary fat is of singular importance in the genesis of obesity. Inasmuch as an ad libitum reduced fat diet causes weight loss, the overall effect of dietary fat reduction may be an improvement in insulin action because of weight loss even in the absence of any direct effects. Long-term studies of the effects of fat content within ad libitum diets are needed to place these largely negative results from isocaloric studies in context.