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The Fat Loss Mechanism — Direct vs Mediated Effects
Growth hormone does not oxidise fat. It signals adipocytes to release it. Once GH binds to receptors on fat cells, it activates hormone-sensitive lipase (HSL), the rate-limiting enzyme that hydrolyses stored triglycerides into free fatty acids (FFAs) and glyce
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- Growth hormone does not oxidise fat. It signals adipocytes to release it. Once GH binds to receptors on fat cells, it activates hormone-sensitive lipase (HSL), the rate-limiting enzyme that hydrolyses stored triglycerides into free fatty acids (FFAs) and glycerol. Those FFAs enter the bloodstream and become available for beta-oxidation in mitochondria. But availability doesn't guarantee oxidation. If energy demand is low (sedentary state, caloric surplus), the FFAs re-esterify back into triglycerides and return to storage. The entire cascade depends on the metabolic context surrounding GH elevation.
- Preclinical models demonstrate this clearly. A 2009 study in Journal of Endocrinology administered ipamorelin to diet-induced obese rats at 300 mcg/kg twice daily for six weeks. GH levels rose significantly, but fat mass reduction was only observed in the group pair-fed a 20% caloric deficit. The ad libitum-fed group showed no change in adiposity despite identical GH elevation. The peptide enabled lipolysis, but thermodynamic deficit drove fat oxidation. This is not a failure of the peptide. It's confirmation that GH is permissive, not causative.
- Human data is more limited but directionally consistent. A small 2015 trial published in Clinical Endocrinology evaluated ipamorelin (200 mcg subcutaneously before bed) in 24 adults with abdominal obesity over 12 weeks. Mean GH AUC increased 8.4-fold, IGF-1 rose by 42%, and DEXA-measured visceral fat decreased by 6.2%. But only in participants who maintained structured caloric restriction (−500 kcal/day). The control group receiving ipamorelin without dietary intervention showed no significant body composition change. The peptide amplified the fat loss environment; it didn't create one independently.