Tissue losses and metabolic adaptations both contribute to the reduction in resting metabolic rate following weight loss

Objective: To characterize how losses of energy-expending tissues and metabolic adaptations contribute to reductions in resting metabolic rate (RMR) following weight loss. Methods: Secondary analysis of the CALERIE trial assessed changes in RMR, body composition, and metabolic hormones over 12 months of caloric restriction in 109 adults. Tissue-related contributions to RMR change were estimated by weighting changes in organ/tissue masses (skeletal muscle, adipose tissue, bone, brain, inner organs, residual mass) derived from DXA with tissue-specific metabolic rates; the remaining RMR reduction was attributed to metabolic adaptations. Results: RMR decreased by 101 ± 12 kcal/d as participants lost 7.3 ± 0.2 kg by 6 months, with no further RMR decline to 12 months. On average, 60% of the total RMR reduction was explained by losses of energy-expending tissues, and 40% by metabolic adaptations. Skeletal muscle loss (1.0 ± 0.7 kg) was not significantly related to RMR change (r=0.14, p=0.16), whereas adipose tissue loss (7.2 ± 3.0 kg) correlated with both reduction in RMR (r=0.42, p<0.001) and metabolic adaptations (r=0.31, p<0.001). Metabolic adaptations were associated with declines in leptin (r=0.27, p<0.01), triiodothyronine (r=0.19, p<0.05), and insulin (r=0.25, p<0.05). Conclusions: Both tissue loss and metabolic adaptations contribute to RMR reduction during weight loss, with adipose tissue loss—rather than skeletal muscle—more closely linked to RMR reductions and adaptive responses. Personalized strategies addressing the predominant cause of RMR reduction may aid weight maintenance.

Alexandra Martin, Darius Fox, Chaise A. Murphy, Hande Hofmann, Karsten Koehler