Insulin resistance is associated with diabetes mellitus, ischemic heart disease, and hypertension both independently and as part of syndrome X. Environmental influences on SI are incompletely understood. Exercise has a strong beneficial effect and obesity a strong adverse effect. The balance of evidence suggests that a high-fat diet is likely to reduce insulin sensitivity but the effects of dietary carbohydrates are more controversial. Extensive studies in animals showed a detrimental effect of diets very high in fructose or sucrose, particularly in association with induction of hypertriglyceridemia. The more limited studies in humans had conflicting results, partly because of heterogeneity of design. Certain groups of subjects may be more sensitive to adverse effects of high intakes of dietary sucrose or fructose. More carefully controlled studies in humans are needed to provide evidence on which to base public health policies with respect to dietary carbohydrates and SI.