Effects of exercise on fluid exchange and body composition in man during 14-day bed rest

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Abstract
To determine the cause of the body weight loss during bed rest (BR), fluid balance and anthropometric measurements were taken from 7 men (19-21 yr) during three 2-wk BR periods which were separated by 3-wk ambulatory recovery periods. Caloric intake was 3073 .+-. 155 (SD) kcal/day. During 2 of the 3 BR periods they performed supine isotonic exercise at 68% of .ovrhdot.VO2 max on the ergometer for 1 h/day; or supine isometric exercise at 21% of maximal leg extension force for 1 min followed by a 1 min rest for 1 h/day. No prescribed exercise was given during the other BR period. During BR, body weight decreased slightly with no exercise (-0.43 kg, NS), but decreased significantly (P < 0.05) by -0.91 kg with isometric and by -1.77 kg with isotonic exercise. About 1/3 of the weight reduction with isotonic exercise was due to fat loss (-0.69 kg) and, the remainder, to loss of lean body mass (-0.98 kg). The reduction in body weight during bed rest has 2 major components: a loss of lean body mass caused by assumption of the horizontal body position that is indepdent of the metabolic rate, and a loss of body fat content that is proportional to the metabolic rate.