Hormonal and physiological correlates of energy expenditure and substrate oxidation in middle-aged, premenopausal women

An understanding of the hormonal and physiological correlates of energy exp enditure and substrate oxidation in middle-aged women will increase our kno wledge of factors that promote changes in energy balance and adiposity. We measured resting and postprandial energy expenditure and substrate oxidatio n in 59 middle-aged, premenopausal women (mean +/- SD age, 47 +/- 2 yr) to examine the hormonal and physiological correlates of energy and substrate m etabolism. Energy expenditure and substrate oxidation were measured at rest using indirect calorimetry and urinary nitrogen excretion and for 180 min after the ingestion of a liquid meal (10 kcal/kg fat-free mass; 410 +/- 44 Gal). Fasting hormone levels were measured by RIA, glucose tolerance was de termined by a 75-g oral glucose tolerance test, body composition was measur ed by dual energy x-ray absorptiometry, and peak aerobic capacity was deter mined by a treadmill test. Using stepwise regression analysis, we found tha t resting energy expenditure was predicted by fat-free mass and serum lepti n concentration (r(2) = 66%; P < 0.01), fat oxidation was predicted by rest ing energy expenditure (r(2) = 17%; P < 0.01), and carbohydrate oxidation w as predicted by serum leptin and appendicular skeletal muscle mass (r(2) = 218; P < 0.01). No variables were related to postprandial energy expenditur e or substrate oxidation. We conclude that in middle-aged, premenopausal wo men, variation in resting energy expenditure and substrate oxidation is pri marily explained by fat-free mass and serum leptin levels. Thus, changes in metabolically active tissue mass or leptin concentration may partially con tribute to changes in resting energy expenditure or substrate oxidation in middle-aged women.