Effect of reduced dietary intake on energy expenditure, protein turnover, and glucose cycling in man

The effect of a 50% reduction in food intake on energy expenditure, protein metabolism, glucose cycling, and body composition was investigated in eight moderately overweight men. The prestudy mean calorie and protein intake was determined for eight subjects. They were then maintained on this diet for 6 weeks (mean ± SEM, 3,269 ± 75 kcal/d, 20.0 ± 0.5 g N/d, period I), after which the diet was reduced uniformly in the major foodstuffs by 50% for the next 4 weeks (1,555 ± 38 kcal/d, 9.6 ± 5 g N/d, period II). At the end of each period we measured (1) body fat and fat free mass by underwater weighing, (2) 24-hour energy expenditure by indirect calorimetry in a calorimeter, (3) whole body protein synthesis and breakdown rates with ¹⁵N glycine, and (4) glucose cycling between glucose and glucose-6-phosphate and fructose cycling between fructose-6-phosphate and fructose-1,6 bisphosphate with 6,6-D₂- and 2-D₁-labeled glucose. The results were subjects lost 4.0 ± 0.1 kg fat (by underwater weighing) during the 4 weeks on the reduced-energy regimen. Protein turnover and glucose cycling were reduced by 20% and 15%, respectively. Twenty-four-hour energy expenditure was 2,553 ± 166 kcal/d for period I and 2,369 ± 69 kcal/d for period II, giving a difference of 184 ± 34 kcal/d between the two periods. In conclusion, (1) although energy intake was reduced by 50%, the decrease in energy expenditure was small due to the buffering effect of body fat. (2) The reductions in protein turnover and glucose cycling could account for approximately 28% and 7% of the 184-kcal/d difference in energy expenditure between the two periods.