Hormonal
In healthy lean men, a significant drop in leptin levels during short-term fasting is both necessary and sufficient to trigger physiological adaptations in the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-thyroid (HPT) axes, as maintaining leptin levels via replacement-dose recombinant leptin prevents these hormonal changes.
If you are lean and healthy, short-term fasting will cause your sex hormones (testosterone) and thyroid signaling to drop. This is a normal, protective response driven by low leptin levels, not a sign of permanent damage. It is reversible once you resume normal eating.
Replacement-dose r-metHuLeptin administered during fasting prevents the starvation-induced changes in the hypothalamic-pituitary-gonadal axis and, in part, the hypothalamic-pituitary-thyroid axis... Thus, in normal men, the fall in leptin with fasting may be both necessary and sufficient for the physiologic adaptations of these axes, which require leptin levels above a certain threshold for activation.
Why this rating
Randomized, double-blinded, placebo-controlled clinical trial in healthy men, though with a small sample size (n=6 evaluable).
Source
The role of falling leptin levels in the neuroendocrine and metabolic adaptation to short-term starvation in healthy men
Jean L. Chan et al. · Journal of Clinical Investigation · 2003
DOI 10.1172/jci17490
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