Growth hormone during fasting, and why a muscle-preservation conclusion drawn from it is not settled.
What the evidence says
- Fasting: GH production increases 5-fold on day 2, in nine men; integrated GH concentration rises from 2.82±0.50 to 8.75±0.82 µg·min/ml by day 5 of a fast, in six men — the paper’s values; it states no fold change for the five-day comparison — and the rise had come by the first day: ”increased after 1 d of fasting (2.82±0.50 vs. 7.82±1.12, control vs. day 1; P = 0.0009) and remained elevated on day 5″[1],[2]
- GH enhancement occurs through both increased pulse frequency (2x more bursts) and amplitude (2x larger bursts)[1]
- Blocking the GH receptor during a 36-hour fast suppressed lipid mobilisation and oxidation (free fatty acids 1226 ± 83 vs. 1074 ± 65 µmol/liter, P = 0.03; ketones 3080 ± 271 vs. 2015 ± 235 µmol/liter, P ≤ 0.01) while protein metabolism was unaffected, assessed by isotope tracer[3]
- The study tested 36 hours under partial receptor blockade in 10 healthy young men; protein-sparing over longer fasts or fuller blockade is not what it measured[3]
- None of these studies measured fasting against chronic calorie restriction on muscle[2],[1],[3]