We examined body composition changes in 55 normal young men during an 8-wk Army combat leadership training course involving strenuous exercise and low energy intake, with an estimated energy deficit of 5.0 +/- 2.0 MJ/day and a resultant 15.7 +/- 3.1% weight loss. Percent body fat (BF) measured by dual-energy X-ray absorptiometry (DEXA) averaged 14.3% (range 6-26%) and 5.8 +/- 1.8% (range 4-11%) at the beginning and end of the course, respectively. Men who achieved a minimum percent BF (4-6%) by 6 wk demonstrated only small additional total and subcutaneous fat losses in the final 2 wk and sacrificed increasingly larger proportions of fat-free mass. Percent BF estimated from skinfold thicknesses reflected relative changes in fat mass, although actual percent BF was overestimated. Instead of reaching a plateau after fat stores were substantially depleted, abdominal, hip, and thigh girths continued to decline with body weight loss. Final percent BF for the leanest men was similar to that observed after a 25% body weight reduction in the 1950 Minnesota study (5.2% by underwater weighting), and height-corrected final fat mass was the same (1.0 +/- 0.2 vs. 0.9 +/- 0.7 kg fat/m2), suggesting that these values represent a minimal body fat content in healthy men and that weight loss subsequent to achieving this level is contributed from the fat-free mass. Our results suggest that 4-6% BF or approximately 2.5 kg fat represents the lower limit for healthy men, as assessed by DEXA or by underwater weighing.