Are metabolic adaptations to weight changes an artefact?

Area of Science:

• Metabolism and Body Composition
• Human Physiology
• Nutritional Science

Background:

• Adaptive thermogenesis (AT) is defined as fat-free mass (FFM)-independent changes in resting energy expenditure (REE).
• The influence of FFM's anatomical and molecular composition on AT has not been previously investigated.

Purpose of the Study:

• To determine how alterations in FFM composition affect AT during caloric restriction (CR) and overfeeding (OF).

Main Methods:

• 32 healthy young men underwent controlled 21-day CR followed by 14-day OF.
• FFM's anatomical (organ/tissue level) and molecular (water, mineral, protein) composition was assessed.
• REE was measured using indirect calorimetry.

Main Results:

• Weight and REE changes during CR and OF were significant.
• FFM changes accounted for substantial portions of weight loss and gain.
• Adjustments for FFM composition altered the calculated AT, revealing significant AT during OF when composition was considered.

Conclusions:

• Accounting for FFM's anatomical and molecular composition modifies AT calculations during CR and OF.
• Adjusted AT during CR was reduced, while it became significant during OF.
• Overall adjusted AT magnitude may not be sufficient to fully counteract weight changes from CR or OF.