The protein-retaining effects of growth hormone during fasting involve inhibition of muscle-protein breakdown.
Level 3 - non-randomized controlled study
Within-subject controlled crossover study in humans; randomization not specified in abstract.
PubMed 11147801 · doi:10.2337/diabetes.50.1.96
What was done
Eight healthy subjects were examined across four conditions: 1) basal postabsorptive state, 2) 40 hours of fasting, 3) 40 hours of fasting with somatostatin-induced growth hormone (GH) suppression (with basal insulin and glucagon replacement), and 4) 40 hours of fasting with somatostatin suppression plus exogenous GH replacement. During the final 4 hours of the 28-hour hormone infusion protocols, investigators measured forearm phenylalanine kinetics (muscle protein breakdown and balance), urinary urea excretion, serum urea, free fatty acids, lipid oxidation, and IGF-I levels.
What was found
Compared with the GH-replaced fasting condition, fasting with GH suppression reduced total IGF-I by 35% and free IGF-I by 70%. Fasting with GH suppression significantly elevated 24-hour urinary urea excretion (basal: 392 ± 44, fast: 440 ± 32, fast-GH: 609 ± 76, fast+GH: 408 ± 36 mmol/24 h, P < 0.05) and serum urea (basal: 4.6 ± 0.1, fast: 6.2 ± 0.1, fast-GH: 7.0 ± 0.2, fast+GH: 4.3 ± 0.2 mmol/l, P < 0.01). Forearm negative phenylalanine balance worsened under GH suppression (basal: 9 ± 3, fast: 15 ± 6, fast-GH: 17 ± 4, fast+GH: 11 ± 5 nmol/min, P < 0.05), driven by increased muscle protein breakdown (phenylalanine rate of appearance: basal: 17 ± 4, fast: 26 ± 9, fast-GH: 33 ± 7, fast+GH: 25 ± 6 nmol/min, P < 0.05). Free fatty acid concentrations and lipid oxidation also declined without GH (P < 0.01).
Why it matters
This study provides direct mechanistic evidence that the rise in growth hormone during short-term fasting actively conserves body protein by inhibiting skeletal muscle proteolysis and maintaining circulating free IGF-I.
Limits
The sample size is very small (n = 8), and the abstract does not report participant demographics (age, sex) or whether the order of conditions was randomized. Findings reflect acute 40-hour fasting kinetics in healthy individuals and cannot be directly extrapolated to prolonged starvation, cachexia, or clinical catabolic states.
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