Human insulin resistance is increasing worldwide and is associated with common diseases in modern societies. Scientists in the United States recently investigated a variety of new information on the mechanisms that mediate insulin's biological actions and, in particular, examined the effects of dietary carbohydrates on insulin sensitivity. The researchers summarised some of the information available on the effects of simple sugars, complex carbohydrates including fibre, slowly digested starch and the general concept of glycemic index. According to the scientists available data supports the idea that consumption of diets high in total carbohydrate does not adversely affect insulin sensitivity compared with high fat diets. Animal data suggests that simple sugars, in particular fructose, have adverse effects on insulin action, but adverse effects have not been shown conclusively in humans. Increased intake of dietary fibre appears to improve insulin action and may protect against the development of diabetes. The effects of diets with high or low glycemic index on insulin action are controversial at this time. The scientists suggest that for firm conclusions to be reached, future studies must be of reasonable duration, be in defined populations and compare the effects of relevant doses of nutrients on specific endpoints of insulin action.