Low Resting Metabolic Rate in Exercise-Associated Amenorrhea is not Due to a Reduced Proportion of Energetically Expensive Tissue Compartments

Abstract

BACKGROUND: Energy deficiency in exercising women is associated with menstrual disturbances and a concomitant reduction in resting metabolic rate (RMR) when expressed relative to body size or lean mass. It remains unknown whether this apparent RMR suppression is a consequence of a reduction in metabolically active tissue compartments during energy deficiency or due to metabolic adaptations at the tissue level. PURPOSE: To explore whether the reduced RMR in women with exercise-associated amenorrhea is explained by a lower proportion of energetically expensive tissue compartments or the result of metabolic adaptations. METHODS: RMR and metabolic tissue compartments were compared among exercising women with amenorrhea (AMEN, n=42) and eumenorrheic, ovulatory menstrual cycles (OV, n=37). RMR was measured using indirect calorimetry and predicted from metabolic tissue compartments as measured by dual-energy X-ray absorptiometry (DXA). RESULTS: Measured RMR was lower than DXA-predicted RMR in AMEN (1215±31 vs. 1327±18 kcal/d, p<0.001) but not in OV (1284±24 vs. 1252±17, p=0.16), resulting in a lower ratio of measured to DXA-predicted RMR in AMEN vs. OV (91±2% vs. 103±2%, p<0.001). Total triiodothyronine was also reduced in AMEN when compared with OV (80.1±3.4 vs. 92.4±2.1 ng/dL, p=0.003). Residual mass was greater (p<0.001) and adipose tissue was reduced (p=0.003) in AMEN when compared to OV. Brain, skeletal muscle, and bone mass were not different among groups. CONCLUSION: Our findings suggest that RMR suppression in exercise-associated amenorrhea is not the result of a reduced size of energetically expensive tissue compartments but due to metabolic adaptations at the tissue level that are indicative of energy conservation.