Energy expenditure and fat-free mass in men and women

Energy expenditure was measured by direct calorimetry in 15 men and women aged 22 to 55. There were fifty-nine 24-h measurements under quiet, not basal, conditions of sedentary activity with 8 h of sleep at night, regular meals, and food intake adjusted to match individual expenditures. Fat-free mass was calculated from body density determined from underwater weight. Energy expenditure over a whole day and night varied directly with fat-free mass, with a correlation coefficient, r, of 0.95 (p less than 0.001); neither age nor sex affected the relationship. During sleep alone, energy expenditure also correlated highly with fat-free mass (r = 0.93). Energy expenditure also correlated with body surface area (r = 0.90), but men and women showed regression lines with different slopes. Metabolism from indirect calorimetry, measured simultaneously, correlated nearly as well with fat-free mass and surface area but showed more variability. The close correlation between 24-h energy expenditure and fat-free mass contrasts favorably with the imprecise prediction of basal metabolic rate according to age, sex, and surface area, and supports the idea that active tissue mass determines daily energy expenditure.