The effect of 6 weeks of overfeeding on the body weight, body composition, and energy metabolism of young men

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Abstract
The effect of overfeeding on the body weight, body fat, water content, energy expenditure, and digestibility of energy and nitrogen was investigated over 42 days in six young men. The metabolic rates in standard situations of work and rest were determined. Energy intakes were apparently increased by 6.2 MJ/day and energy expenditure fell slightly by 0.3 MJ/day during overfeeding. Fecal and urinary losses of energy were a similar proportion of the gross energy intake in control and overfeeding periods (8%). Metabolizable energy intakes calculated from food tables agreed well with values derived from digestibility measurements in the control period (mean difference = +2%) but not in the overfeeding period (+8%). The implications of this are discussed. Mean body weight gain was 6.0 kg, 10% of initial weight; mean fat gain was 3.7 kg and water gain 1.8 liter. There were considerable interindividual differences in the weight and fat gain for a given excess energy intake. Metabolic rates in standard tasks were 10% higher at the end of overfeeding but expressed as kilojoules per kilogram per minute were similar to control values. Mean energy gain (144 MJ = fat gain × 39 kJ/g) was less than excess energy intake even allowing for overestimation of intakes using food tables and increases in metabolic rate. Such a discrepancy is unlikely to be due to unmeasured increases in metabolic rate but could have arisen from errors in the calculation of the variables involved. In this study where moderate weight gains were achieved by overfeeding mainly fat, increases in metabolic rate appear to be associated with increased body size and tissue gain rather than a luxuskonsumption mechanism.