I’ve been doing 16:8 intermittent fasting for years and recently started 48-hour fasts — dropping about three pounds each fast, gaining one or two back, and trending steadily downward. I wanted to understand what the research actually says about what I’m doing to myself, so I worked with Claude (Anthropic’s AI) to produce this series. I set the structure, chose the topics, pushed back on claims that felt hand-wavy, and guided the editorial tone. Claude did the writing and research synthesis. My curiosity driving Claude’s research and prose.
Growth Hormone and Insulin Sensitivity
Research brief — the two best-evidenced hormonal responses to extended fasting, with direct human measurements.
Growth Hormone Surge
Human growth hormone (HGH) secretion increases substantially during fasting. This is among the best-measured effects of fasting in humans — researchers have drawn blood every 5 minutes over 24-hour periods to capture the pulsatile secretion patterns.
Ho et al. 1988 — Examined 24-hour GH secretion patterns in six normal adult men during fed and fasting states (day 1 and day 5 of a 5-day fast). Found that fasting enhances GH secretion through both increased pulse frequency and amplitude. (1)
Hartman et al. 1992 — The definitive study. Nine normal men, blood sampling every 5 minutes over 24 hours. Found a *-fold increase in 24-hour endogenous GH production during a two-day fast, mediated by increased secretory burst frequency and amplitude. Notably, IGF-1 concentrations were unchanged after 56 hours of fasting. (2)
The 5-fold figure from Hartman is the basis for the commonly cited “2–5x increase” claim. The range exists because individual responses vary and different studies measure slightly different things (peak amplitude vs. 24-hour integrated production).
What this does: The GH surge during fasting supports lean tissue preservation and fat mobilization. The body is shifting from burning glucose to burning fat, and elevated GH helps protect muscle mass during this transition while directing the body to use fat stores as fuel. This is part of the metabolic switch (Part 1) — the body isn’t just passively running out of food, it’s actively reconfiguring which tissues to protect and which fuel sources to tap.
Caveats: Both studies used small samples of healthy men (6 and 9 participants respectively). The findings are consistent with each other and with the broader endocrinology literature, but most subjects were young-to-middle-aged males. The GH response to fasting in women and older adults is less thoroughly characterized, though directionally similar results have been observed.
Evidence strength: Strong. Direct human measurements, replicated findings, published in top endocrinology journals. Small sample sizes but consistent results.
Insulin Sensitivity
Fasting for 48 hours produces a dramatic drop in circulating insulin and measurable improvement in insulin sensitivity. This is one of the most robustly demonstrated effects of extended fasting in human studies.
The mechanism is straightforward: with no incoming glucose, the body needs less insulin. Insulin levels drop to baseline. Cells that have been chronically exposed to high insulin (and have down-regulated their insulin receptors in response) get a reset. When food is reintroduced, the cells respond to insulin more readily.
The de Cabo & Mattson 2019 NEJM review covers this comprehensively, describing how intermittent fasting triggers neuroendocrine responses characterized by low levels of amino acids, glucose, and insulin. Eating within a 6-hour period and fasting for 18 hours can trigger the metabolic switch, with measurable improvements in insulin sensitivity. (3)
This isn’t subtle or contested — it’s one of the clearest, most directly measured effects of fasting. It’s also one of the most practically relevant for the large portion of the population dealing with insulin resistance, prediabetes, or metabolic syndrome.
Connection to 16:8 (Part 2): Even daily time-restricted eating improves insulin sensitivity. Sutton et al. 2018 showed that early TRE improved insulin sensitivity in men with prediabetes even without weight loss — the time restriction alone was enough. (4) Extended fasts amplify this effect further.
Evidence strength: Strong. Multiple human studies, reviewed in a major journal, physiologically straightforward.
Sources
- Ho et al. 1988, J Clin Invest, 81(4):968-75 — fasting enhances GH secretion: https://pubmed.ncbi.nlm.nih.gov/3127426/ (free full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC329619/)
- Hartman et al. 1992, J Clin Endocrinol Metab, 74(4):757-65 — 5-fold GH increase during 2-day fast: https://pubmed.ncbi.nlm.nih.gov/1548337/
- de Cabo & Mattson 2019, NEJM, 381(26):2541-2551 — effects of IF on health, aging, and disease: https://pubmed.ncbi.nlm.nih.gov/31881139/ (full text paywalled: https://www.nejm.org/doi/full/10.1056/NEJMra1905136)
- Sutton et al. 2018, Cell Metabolism — early TRE improves insulin sensitivity without weight loss: https://pubmed.ncbi.nlm.nih.gov/29754952/ (full text: https://www.cell.com/cell-metabolism/fulltext/S1550-4131(18)30253-5)