Interpreting the literature relating to the nutritional determinants of insulin resistance is complicated by the wide range of methods used to determine insulin sensitivity. Excess adiposity is unquestionably the most important determinant of insulin resistance, although the effect may be tempered by a relatively high proportion of lean body mass. Weight loss is associated with improved insulin sensitivity. Thus, diet-related factors that promote excessive energy intake may be regarded as promoters of insulin resistance. In the context of energy balance, diets characterized by high intakes of saturated fat and low intakes of dietary fiber are associated with reduced insulin sensitivity. Total fat intakes greater than the usually consumed range appear to promote insulin resistance, although the relative proportions of total fat and carbohydrate within the usual range appear unimportant. Monounsaturated fatty acids with a cis configuration and fiber-rich carbohydrate foods appear to be appropriate substitutes for saturated fatty acids and rapidly digested glycemic carbohydrates. In animal studies, n-3 unsaturated fatty acids have been shown to enhance insulin sensitivity and fructose and sucrose to increase insulin resistance. However, human data are limited. Large prospective studies currently being conducted should confirm the most appropriate macronutrient composition of diets for preventing and treating insulin resistance as well as establishing whether a range of candidate genes explains the variation in response to dietary change.