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

Abstract

To determine the cause of the body weight loss during bed rest (BR), fluid balance and anthropometric measurements were taken from seven men (19-21 yr) during three 2-wk BR periods which were separated by 3-wk ambulatory recovery periods. Caloric intake was 3,073 +/- 155 (SD) kcal/day. During two of the three BR periods they performed supine isotonic exercise at 68% of VO2max 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 less than 0.05) by -0.91 kg with isometric and by -1.77 kg with isotonic exercise. About one-third 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). It is concluded that the reduction in body weight during bed rest has two major components: First, a loss of lean body mass caused by assumption of the horizontal body position that is independent of the metabolic rate. Second, a loss of body fat content that is proportional to the metabolic rate.

Human Body Composition

“…These revealed a total nitrogen loss of 54 g, a calcium loss of 14 g, (160-380 mg/day), and a phosphorus loss of 8.6 g. Assuming an initial body content of 1800 g N, 1100 g Ca, and 550 g P, and taking into account unmeasured N losses, these subjects lost roughly 4% of their body N, 1.3% of body Ca, and 1.6% of body P. Metabolic balance determinations were continued during a 3-6-week recovery period, during which the subjects retained 36 g N, and so had not yet made good this loss. Greenleaf et al (1977) assayed body density in male subjects before and after a 14-day period of absolute bed rest. The diet provided 3100 kcal, 3.8 g Na, and 150 g protein.…”

Human Water Needs

“…22,23 For adults, the daily water needs of men approach 2.5 L if sedentary, 3,24 and increase to about 3.2 L if performing modest physical activity, 19,25 while more active adults living in a warm environment have daily water needs of about 6 L. 26 Examination of water turnover studies indicates that daily water turnover is 3.3 L and 4.5 L for sedentary and active men, respectively. 8,18,[27][28][29][30][31][32][33] For more active populations, even higher values (Ͼ6 L) have been reported.…”

Cardiovascular Adjustments to Gravitational Stress

“…The data from several studies with bed-rest periods of 6 h to 30 days (59, 126, 127, 170, 287, 314, 316, 338, 340-342) indicate significant plasma volume decreases within 6 h. The plasma volume contraction is progressive during the first 3 days, but then a plateau is reached at an average loss level of 350 ml or about 12%. Greenleaf et al (126) performed a similar analysis of the literature and derived a nonlinear regression equation indicating a progressive loss through 60-80 days. Their estimate was heavily influenced by data from a single long-term study (77) with very large losses, 600-1,000 ml at a duration of CHAPTER 28: RESPONSE TO STRESS: GRAVITY 1043 days.…”