Although Americans have decreased the percent of energy they consume from fat, obesity and obesity-related comorbidities have progressively increased. Less attention has been paid to the role of carbohydrates, especially carbohydrate source, in these metabolic diseases. However, recent epidemiologic studies demonstrate consistently higher rates of cardiovascular disease and type II diabetes in individuals deriving a greater percentage of energy from refined grains and simple carbohydrates than from whole grains. Differences in the metabolic response to carbohydrates can be classified by glycemic index (GI), the blood glucose response to a given food compared with a standard (typically white bread or glucose). Classification of carbohydrates as "simple" or "complex" is of little use in predicting GI, because GI is influenced by starch structure (amylose versus amylopectin), fiber content, food processing, physical structure of the food, and other macronutrients in the meal. Low-GI diets have been reported to lower postprandial glucose and insulin responses, improve lipid profiles, and increase insulin sensitivity. Moreover, high-GI diets stimulate de novo lipogenesis and result in increased adipocyte size, whereas low-GI diets have been reported to inhibit these responses. Thus, the GI of dietary carbohydrates appears to play an important role in the metabolic fate of carbohydrates and, consequently, may significantly affect the risk of cardiovascular disease, diabetes, and obesity.