Hormonal
Massive weight loss induces a metabolic adaptation (adaptive thermogenesis) that reduces resting metabolic rate (RMR) significantly more than can be explained by the loss of fat-free mass (FFM) and fat mass alone, even when FFM is preserved through vigorous exercise.
If you lose a lot of weight, your body will fight back by lowering your resting calorie burn, even if you keep your muscle. This is a normal survival mechanism, not a failure. To maintain your new weight, you likely need to sustain higher levels of physical activity or adjust your calorie intake, rather than expecting your metabolism to return to its pre-weight-loss state.
Despite relative preservation of FFM, exercise did not prevent dramatic slowing of resting metabolism out of proportion to weight loss. This metabolic adaptation may persist during weight maintenance and predispose to weight regain unless high levels of physical activity or caloric restriction are maintained.
Why this rating
Observational study of a specific competition; lacks a control group, but uses rigorous metabolic measurements (indirect calorimetry, DLW).
Source
Metabolic Slowing with Massive Weight Loss despite Preservation of Fat-Free Mass
Darcy L. Johannsen et al. · The Journal of Clinical Endocrinology & Metabolism · 2012
DOI 10.1210/jc.2012-1444
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