Responses of endurance-trained subjects to caloric deficits induced by diet or exercise

The purpose of this study was to examine the effect of 7 d of caloric deficit (1,000 kcal X d-1) induced by diet or exercise on weight loss and exercise performance of six endurance-trained males. The diet of each subject was controlled during the weeks preceding the dietary restriction and exercise to normalize nitrogen balance. Weight and blood chemistries were monitored daily. Submaximal and maximal exercise responses were tested at the end of each week. Weight loss during the exercise week was 0.76 kg, significantly less (P less than 0.05) than in the dietary restriction week (-2.16 kg). Seven d of cumulative nitrogen loss was greater (P less than 0.05) during the diet week (-24.5 g) than the exercise week (-11.1 g). Resting hematocrit, hemoglobin, total plasma protein, and albumin were significantly reduced during the exercise week compared to all other weeks. Maximal exercise capacity, measured as VO2 (61 ml X kg-1 X min-1) or duration of exercise, was not affected by either method of caloric deficit, but R values and lactates were lower than controls for both dieting and exercise deficits. The results suggest that endurance-trained individuals lose weight more slowly and conserve more protein when using exercise to induce a caloric deficit compared to dietary restriction. The results also suggest the possibility of the sequestering of blood proteins to supplement muscle protein synthesis during periods of exercise-induced caloric deficits.