1. Eight young men of normal weight were maintained for 1 week on a weight-maintenance diet followed by a 1-week period of over-feeding with extra fat designed to increase energy intake by 50%. Two 36 h calorimetry sessions with low and high physical activities were included in each feeding period. Faecal and urine collections permitted checks on energy malabsorption and nitrogen excretion.
2. Over-feeding led to increases in body-weight, faecal energy and N excretion and in estimated N retention. Faecal energy outputs on the maintenance and over-feeding diets were 5 and 4.4% of the respective gross energy intakes.
3. Energy expenditure on fat over-feeding increased by 5.6% on the low-activity regimen and 6.4% on the high-activity regimen. This amounted, in terms of the extra energy intake, to 9 and 11 % on the inactive and active schedules respectively. The increase affected day- and night-time rates of energy expenditure plus the basal metabolic rate. Individuals with a low percentage body fat showed the greatest response to over-feeding.
4. Nutrient-balance studies derived from calorimetry suggested that fat over-feeding led to substantial fat deposition with no evidence of sparing of carbohydrate oxidation. The theoretical cost of depositing dietary fat was exceeded, suggesting that regulatory thermogenic mechanisms may have been stimulated to a small extent.
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