After determining the dietary fat intake in 40 subjects based on a 7-day recording period, the effects of a high-fat breakfast (52% of metabolizable energy from fat) on postprandial fat and carbohydrate metabolism as well as on subsequent lunch intake were investigated in 28 lean, male subjects with habitual dietary fat intakes between 21 and 44% (of daily energy intake). Correlational analysis and comparisons between a low-fat group (LF; fat intake < or =35%, n = 10) and a high-fat group (HF; fat intake > or =40%, n = 11) with a similar body mass index (LF: 22.7, HF: 22.4) and a low body fat content (LF and HF: 10.7) demonstrated that the fat level of the habitual diet did not affect the baseline values and the postprandial changes in the respiratory quotient and in the plasma levels of glucose, insulin, lactate, free fatty acids, beta-hydroxybutyrate, and triglycerides induced by the high-fat breakfast. Only the area under the curve for insulin was higher, and the lactate/insulin ratio was lower in the HF group than in the LF group. Moreover, hunger and satiety ratings and lunch intake (amount, duration, microstructure of eating) after the high-fat breakfast were similar in all subjects. Thus, the habitual level of dietary fat did not alter the acute effects of a fat-rich breakfast on whole-body and hepatic fatty acid oxidation and eating behavior at lunch under the present test conditions. Yet, a long-term high fat intake appears to have subtle effects on postprandial metabolism which are consistent with early signs of a developing insulin resistance.