Energy expenditure and physical activity in relation to bone mineral density in women with anorexia nervosa

Abstract

Objectives: To assess sleeping metabolic rate (SMR), average daily metabolic rate (ADMR), and total bone mineral density (TBMD) in women with anorexia nervosa, and to evaluate the effect of daily physical activity on TBMD.

Design: We compared women with anorexia nervosa and controls using measurements on body composition, and energy expenditure. Relations between these measurements were investigated.

Setting: Daily living environments in The Netherlands, and body composition and energy expenditure laboratory of the Department of Human Biology.

Subjects: Twelve adult, non-hospitalized women with anorexia nervosa, and sixteen adult normal weight women.

Interventions: Average daily metabolic rate was measured with the doubly labeled water method and sleeping metabolic rate in a respiration chamber. TBMD was measured by dual energy X-ray absorptiometry, and percentage body fat was calculated combining the results from underwater weighing and deuterium dilution.

Results: TBMD was significantly lower in anorexia than in controls (0.989±0.081 vs 1.144±0.054 g/cm²). Also ADMR and SMR were reduced in anorexia. The physical activity index (PAI=ADMR/SMR) was not significantly different from PAI in controls. In anorexia, TBMD was related to the PAI (R²=0.35, P< 0.05). Finally, stepwise multiple regression revealed that PAI together with the study groups as dummy variables could explain 69% of the variation in TBMD.

Conclusion: These findings show that in anorexia TBMD is reduced, but that nonetheless physical activity has a significant positive effect on bone density.