High fat, low carbohydrate diets are popularly advocated for weight loss and improvement in metabolic Syndrome X, a constellation of risk factors for type 2 diabetes mellitus and cardiovascular disease. The effects of an energy-restricted (to prevent weight gain in excess of normal growth) high fat (60% of energy), low carbohydrate (15%) diet were assessed in both lean rats and in rats previously rendered obese through ad libitum consumption of the same high fat diet. In obese rats, restriction of intake failed to improve impaired glucose tolerance, hyperinsulinemia, and hypertriglyceridemia, although it lowered visceral fat mass, liver lipid content and in vitro insulin hypersecretion compared with rats continuing to consume the high fat diet ad libitum. In lean rats, restricted intake of the high fat diet impaired glucose tolerance and increased visceral fat mass and liver lipid content. These findings support the conclusion that, in the absence of weight loss, a high fat, low carbohydrate diet not only may be ineffective in decreasing risk factors for cardiovascular disease and type 2 diabetes but may promote the development of disease in previously lower risk, nonobese individuals.