In man there is evidence that the ability to adjust fat oxidation to fat intake is less effective than the ability to adjust carbohydrate and protein oxidation to carbohydrate and protein intake. The short-term (3-day) effects of a low-fat (LF), mixed (M), and high-fat (HF) diet on human substrate balances were studied using a respiration chamber. Subjects were 14 young female students classified by means of their scores on psychometric questionnaires as “restrained” or “unrestrained” eaters. Subjects were in energy balance, ie, the mean difference between energy intake (EI) and energy expenditure (EE) was 86 ± 85 kJ/d. The fat content of the food significantly influenced the 24-hour respiratory quotient (RQ) and nonprotein respiratory quotient (NPRQ). For both the LF and M diets, the 24-hour RQ was significantly lower than the food quotient (FQ), whereas the RQ on the HF diet was not different from the FQ. Oxidation of fat and carbohydrate significantly increased with, respectively, an increasing fat and carbohydrate content of the diet for both restrained- and unrestrained-eating subjects. Restrained-eating subjects showed a decreased fat oxidation compared with unrestrained eaters in response to a HF diet, resulting in a positive fat balance for restrained-eating subjects. On a LF diet, fat balance was negative for both groups of subjects, indicating net endogenous fat oxidation. In conclusion, restrained-eating subjects have more difficulty in the handling of a HF diet, possibly explaining their higher susceptibility to becoming obese.