Possible adaptive mechanisms that may defend against weight gain during periods of excessive energy intake were investigated by overfeeding six lean and three overweight young men by 50% above baseline requirements with a mixed diet for 42 d [6.2 +/- 1.9 MJ/d (mean +/- SD), or a total of 265 +/- 45 MJ]. Mean weight gain was 7.6 +/- 1.6 kg (58 +/- 18% fat). The energy cost of tissue deposition (28.7 +/- 4.4 MJ/kg) matched the theoretical cost (26.0 MJ/kg). Basal metabolic rate (BMR) increased by 0.9 +/- 0.4 MJ/d and daily energy expenditure assessed by whole-body calorimetry (CAL EE) increased by 1.8 +/- 0.5 MJ/d. Total free-living energy expenditure (TEE) measured by doubly labeled water increased by 1.4 +/- 2.0 MJ/d. Activity and thermogenesis (computed as CAL EE--BMR and TEE--BMR) increased by only 0.9 +/- 0.4 and 0.9 +/- 2.1 MJ/d, respectively. All outcomes were consistent with theoretical changes due to the increased fat-free mass, body weight, and energy intake. There was no evidence of any active energy-dissipating mechanisms.

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Many chronically ill mental patients show rapid and severe weight loss leading to severe nutritional debilitation. Excessive energy requirements secondary to hypermetabolism or hyperactivity have been proposed as the cause. This hypothesis was tested using the new doubly-labelled water (2H218O) technique to obtain accurate estimates of total energy expenditure (TEE) in 14 such patients. Mean TEE was very low (6.1 +/- 1.3 MJ/day). None of the subjects was in significant negative energy balance when studied. Mean resting metabolic rate (RMR) was lower than predicted from standard equations based on healthy elderly subjects. The energy cost of physical activity plus thermogenesis was also low (1.7 +/- 0.9 MJ/day) in all except one subject. The data refute the initial hypothesis and suggest that negative energy balance may be episodic, perhaps during periods of infection and subsequent recovery.

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OBJECTIVES: To investigate the in¯uences of inactivity and dietary macronutrient composition on energy and fat balance and to look for interactions between them. DESIGN: Two-day measurements of energy expenditure and substrate oxidation on ®ve occasions; ad libitum food intake from diets of 35% and 60% energy as fat, with and without imposed activity, and a ®xed overfeeding at 35% fat with free activity. SUBJECTS: Eight normal-weight male volunteers. MEASUREMENTS: Energy expenditure and substrate oxidation by indirect whole-body calorimetry, and macronutrient intakes from food consumption on ad libitum regimens. RESULTS: Subjects consumed the same energy, mean 11.6 MJad, regardless of activity level, on the 35% diet. Subjects consumed more energy on the 60% than the 35% diet, mean 14 vs 11.6 MJad. Inactivity induced a strong positive energy balance: 5.1 (60% diet), and 2.6 MJad (35% diet). Energy balance with activity was not signi®cantly different between diets, nor signi®cantly different from zero: 1.1 MJad (60% diet), and 7 0.2 MJad (35% diet). When intentionally overfed, subjects failed to compensate by raising voluntary activity. CONCLUSION: Energy intake was not regulated over a 2-day period in response to either imposition of inactivity or a high-fat diet. Activity proved essential to the avoidance of signi®cant positive energy balance.