Are metabolic adaptations to weight changes an artefact?

Background: Adaptive thermogenesis (AT) is currently defined as the fat-free mass (FFM)-independent change in resting energy expenditure (REE) in response to caloric restriction (CR) or overfeeding (OF). So far, the impact of changes in the anatomical and molecular composition of FFM on AT has not been addressed. Objectives: To assess the impact of changes in FFM composition on AT. Methods: FFM was assessed in 32 healthy young men during controlled 21-d CR and 14 d of subsequent OF. Anatomical (i.e., the organ/tissue level) and molecular (i.e., water, mineral, and protein content and thus body density) composition of FFM were characterized. REE was measured by indirect calorimetry. Results: With CR, body weight and REE decreased by 4.2 +/- 0.9 kg and 173 +/- 107 kcal/d, respectively, with corresponding increases of 3.5 +/- 1.2 kg and 194 +/- 110 kcal/d during OF (P < 0.001 for all changes). Changes in FFM explained 56.7% and 66.7% of weight loss and weight gain, respectively. Weight changes were associated with changes in various anatomical (i.e., masses of skeletal muscle, liver, kidneys. and brain) and molecular components (total body water, protein, and bone minerals) of FFM. After adjustments for changes in FFM only, AT was 116 +/- 127 (P < 0.001) and 27 +/- 115 kcal/d (NS) with CR and OR respectively. Adjustments for FFM and its anatomical and molecular composition reduced AT in response to CR to 83 +/- 116 and 122 +/- 123 kcal/d (P < 0.05 and P < 0.001) whereas during OF, AT became significant at 87 +/- 146 kcal/d (anatomical; P < 0.05) and 86 +/- 118 kcal/d (molecular; P < 0.001). Conclusions: Adjusting changes in REE with under- and overfeeding for the corresponding changes in the anatomical and molecular composition of FFM decreased AT after CR and increased AT after OF, but overall adjusted AT was likely not large enough in magnitude to be able to prevent weight loss or resist weight gain.

Automated summary

This study evaluated the impact of changes in fat-free mass (FFM) composition on adaptive thermogenesis (AT), finding that changes in FFM composition had a significant effect on weight loss and weight gain. Adjusting for changes in anatomical and molecular composition reduced AT after caloric restriction (CR) and increased AT after overfeeding (OF), but the overall adjusted AT may not be enough to prevent weight loss or resist weight gain.