OBJECTIVE: To investigate physiological differences between habitual high-fat (HF) and low-fat (LF) consumers, which could influence the balance between energy expenditure and energy intake, and the potential for weight gain.

SUBJECTS: 16 young, lean males (eight HF and eight LF consumers;% energy from fat 44.3 and 32.0, respectively).

MEASUREMENTS: Habitual dietary variables (from FFQ), body mass index (BMI), body fat%(measured by impe-dance), resting metabolic rate (RMR)(indirect calorimetry), substrate oxidation and basal heart rate, postprandial thermogenesis and heart rate in response to a high-fat (low carbohydrate (CHO)) and high-CHO (low fat) challenge.

RESULTS: HF and LF (selected for their intake of fat) did not differ significantly in BMI or% body fat. HF had a significantly higher RMR (1624 vs 1455 kcal/d) and basal heart rate (66 vs 57 bpm) than LF. Differences in oxygen utilisation and heart rate were maintained over a 180 min period, following the high-fat and high-CHO challenge meals. HF had a significantly lower resting respiratory quotient (RQ) than LF and the differences in average RQ were significant over the 180 min examination period. HF had a significantly lower RQ response to the high fat (low CHO) than to the high CHO (low fat) challenge; this effect was not observed in LF. HF had higher total energy intake than LF and a higher absolute (but not%) intake of protein.

CONCLUSION: Significant differences in basal energy expenditure and fat oxidation between habitual HF and LF consumers have been observed. The contributions of energy intake and protein intake (g not%) remain to be determined. In this particular group of subjects (young adult males) a high energy intake characterised by a large fat component is associated with metabolic adaptations which could offset the weight inducing properties of a high-fat diet. These physiological differences may be important when considering the relationship between dietary-fat and obesity.