The effects of prolonged caloric restriction on protein kinetics in lean subjects has not been previously investigated. PURPOSE: To test the hypotheses that 21 days of caloric restriction (CR) in lean subjects would a) result in significant losses of lean mass despite a suppression in leucine turnover and oxidation, and b) negatively impact exercise performance. METHODS: Nine young, normal weight men (23±5 y, 78.6±5.7 kg, VO₂peak: 45.2±7.3 ml^.kg^-1.min^-1,mean±SD) were underfed by 40% of the calories required to maintain body weight (BW) for 21 days and lost 3.8±0.3 kg BW and 2.0±0.4 kg lean mass. Protein intake was kept at 1.2 g^.kg^-1.day^-1. Leucine kinetics were measured using KIC reciprocal pool model in the post-absorptive state during rest and 50 minutes of exercise (EX) at 50% of VO₂peak. Body composition, basal metabolic rate (BMR) and exercise performance were measured throughout the intervention. RESULTS: At rest, leucine flux (~131 µmol^.kg^-1.hr^-1) and oxidation (Rox; ~19 µmol^.kg^-1.hr^-1) did not differ pre- and post- CR. During EX, leucine flux (129±6 vs. 121±6) and Rox (54±6 vs. 46±8)were lower following CR than pre-CR. Nitrogen balance was negative throughout the intervention (~3.0gN^.d^-1) and BMR declined from 1898±262 kcal^.d^-1 to 1670±203. Aerobic performance (VO₂peak, endurance cycling) was not impacted by CR, but arm flexion endurance decreased by 20%. CONCLUSIONS: Three weeks of caloric restriction reduced leucine flux and oxidation during exercise in normal weight young men. However, despite negative nitrogen balance and loss of lean mass, whole body exercise performance was well maintained in response to CR.