I tested two hypotheses: The first hypothesis was that a mixed diet containing lower GI foods would lower blood glucose and insulin levels and increase fat oxidation (FOX) compared with a diet consisting of higher GI foods. Secondly, I tested the hypothesis that variability in resting FOX would predict FOX during moderate exercise after consuming the lower and higher GI diets. Lower and higher GI diets with similar macronutrient contents were constructed using low and high GI versions of cereal, bread, fruits, snacks and vegetables. Diets were fed to 12 normal, untrained, young men for 4 d using a crossover design with a 3-day washout. The lower GI diet decreased postprandial blood glucose and insulin concentrations (P<0.05), increased plasma fatty acids (P<0.05), but did not affect plasma triglycerides or fasting plasma glucose compared with the higher GI diet. Macronutrient oxidation at rest and during the first 40 min of moderate exercise was not affected by diet. The lower GI diet decreased FOX by 20 % at 60 min of exercise compared with either the habitual or the higher GI diets. Body weight decreased slightly (-0.7 kg, P<0.05) after the lower GI diet, but did not change after the higher GI diet. Subjects were separated according to their mean resting FOX into higher and lower fat oxidizers. Both groups responded similarly to the diets in all measures. Higher fat oxidizers consumed more fat habitually, had lower postprandial glucose and insulin levels on both diets, and had higher pre-meal fatty acid concentrations regardless of the diet (P < 0.05). I concluded that inclusion of low GI foods in a balanced diet modestly elevated FOX after at least 60 min of moderate exercise in normal young men. Subject having higher resting FOX rates had a muted glycemic response and insulin response to an evening meal and had the higher FOX rates the next day during exercise. These data may be helpful in developing better diets for weight control and assist in identifying individuals at rest of weight gain.