Fasting Increases Growth Hormone Secretion Up to 5-Fold in Men: What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

Researchers wanted to understand how prolonged fasting changes the way the body secretes growth hormone (GH) — specifically whether fasting increases GH, and whether that increase comes from more frequent pulses, larger pulses, or both. Growth hormone regulates fat mobilisation, muscle preservation, and cell repair, making it central to understanding how fasting affects body composition and recovery. This study provided the first detailed characterisation of GH secretion patterns during a 5-day fast in healthy men, using high-frequency blood sampling taken every 20 minutes over a full 24-hour period. You can read more about the metabolic changes that occur during fasting in our article on what happens to your body hour by hour when you fast.

Who Was Studied

Participant profile: 9 healthy adult men; no obesity, no metabolic disease, no medications affecting GH axis.

How the fasting protocol worked in this study: Participants underwent a complete 5-day water fast with no caloric intake. Blood was drawn via indwelling catheter every 20 minutes over a continuous 24-hour sampling period at the end of the fast, and separately during a fed reference period. This allowed researchers to plot the full pattern of GH pulses — not just a single snapshot measurement.

What the Researchers Found

Growth Hormone Secretion

• The single most striking finding: mean 24-hour GH concentration increased approximately 5-fold during the fast. This was driven by both more frequent pulses and dramatically larger pulse amplitude.
• All 9 participants showed the same directional response — the effect was consistent, not driven by outliers.
• GH secretion in the fed state was characterised by a few modest pulses, primarily at night. In the fasted state, pulses occurred more frequently throughout the day and night, and each pulse was substantially larger.

IGF-1 (Insulin-like Growth Factor 1)

• IGF-1 (somatomedin-C) fell significantly during the fast, despite the dramatic rise in GH.
• This apparent contradiction — high GH, low IGF-1 — is explained by a state of "GH resistance" or reduced hepatic sensitivity to GH during fasting. The liver, under caloric deprivation, does not respond to GH by producing IGF-1 in the usual way.
• This dissociation is now understood as a physiologically adaptive mechanism: GH drives fat mobilisation directly during fasting, while the IGF-1 axis (which promotes growth and cell division) is appropriately downregulated during energy scarcity.

Insulin and Metabolic Markers

• Fasting insulin levels fell significantly, as expected during prolonged caloric restriction.
• Free fatty acids increased — consistent with GH-driven fat mobilisation.
• No adverse cardiac or metabolic events were reported during the 5-day fast.

What Did Not Change

• GH pulse duration remained relatively stable — the increases were in frequency and amplitude, not in how long each pulse lasted.
• No adverse effects on muscle protein breakdown markers beyond what would be expected from the fast's length.

What the Researchers Concluded

The authors concluded that fasting powerfully augments GH secretion in healthy men by increasing both the frequency and amplitude of secretory bursts, and that this occurs despite falling IGF-1 — demonstrating that the two are regulated independently during energy restriction. They proposed that the GH elevation during fasting serves to protect lean tissue and mobilise fat stores as an alternative energy source.

What This Means If You Fast

• Growth hormone is fasting's anabolic bodyguard. The rise in GH during fasting is one of the key mechanisms through which the body preserves muscle mass while burning fat. This directly addresses the common concern that fasting will cause muscle loss. For more on this, see our article on does intermittent fasting destroy muscle?.
• You don't need a 5-day fast to see GH benefits. While this study used a 5-day water fast, later research has shown that shorter fasting periods (16–24 hours) also elevate GH, particularly through the overnight pulse that occurs during sleep. The 5-day model established the physiological principle; intermittent fasting leverages a milder version of the same mechanism daily.
• Low insulin is a prerequisite for GH to work. GH and insulin have a reciprocal relationship — when insulin is high (as it is in a high-carbohydrate, frequent-eating pattern), GH secretion is suppressed. Fasting drops insulin, which creates the hormonal environment for GH to rise. This is one reason why eliminating snacking and reducing carbohydrates amplifies fasting results.
• The IGF-1 drop makes sense. Lower IGF-1 during fasting is not harmful — it's appropriate. IGF-1 drives cellular growth and proliferation, which you want after eating to rebuild tissue. During fasting, lower IGF-1 shifts the body toward repair and autophagy rather than growth. This is a feature, not a bug.
• Sleep amplifies the GH response. GH is secreted in pulses, with the largest pulse normally occurring shortly after deep sleep onset. Fasting amplifies both the size and frequency of these pulses. Going to sleep in a fasted state — as occurs naturally in time-restricted eating protocols with earlier eating windows — may enhance this effect.
• This is not a performance enhancer finding. The GH elevation during fasting is a physiological adaptation to energy restriction. It is not comparable to exogenous GH supplementation and does not produce the same effects. The context matters: fasting GH serves survival and metabolic function, not supraphysiological muscle building.

Study Limitations

• Very small sample size (n=9): Results are directionally consistent but the sample is insufficient to draw definitive conclusions about population-level effects. Larger studies are needed to quantify the magnitude of effect with precision.
• All male participants: No female data. Women's GH secretion differs significantly from men's due to estrogen and the menstrual cycle, so these findings cannot be directly extrapolated to women.
• 5-day fast is not representative of common IF protocols: Most people practice 16:8, 18:6, or 5:2 — not 5-day water fasts. The degree to which daily intermittent fasting produces similar GH changes has been studied less rigorously.
• No food composition control during refed periods: The fed state was normal ad libitum eating, not a controlled diet, making it a less precise baseline.
• 1988 methodology: While the blood sampling protocol was technically rigorous for its era, some assay sensitivities and analytical methods have since improved significantly.
• No long-term follow-up: The study captured GH during the fast but did not track body composition outcomes or long-term effects.

Source

Ho, K.Y., Veldhuis, J.D., Johnson, M.L., Furlanetto, R., Evans, W.S., Alberti, K.G., & Thorner, M.O. (1988). Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. Journal of Clinical Investigation, 81(4), 968–975. PMID: 3127426

Frequently Asked Questions

How much does fasting increase growth hormone?

This study found approximately a 5-fold increase in mean 24-hour GH concentration after a 5-day fast in healthy men. Both the frequency and amplitude of GH secretory pulses increased. Shorter fasting periods (16–24 hours) produce measurable but generally smaller increases, predominantly through the amplified overnight GH pulse.

Does intermittent fasting increase growth hormone?

Yes. While this study used a 5-day fast, the physiological mechanism — lower insulin enabling higher GH secretion — operates during any fasting period. Daily intermittent fasting (16:8 or 18:6) creates a recurring window of low insulin that allows GH to rise, particularly during the overnight fast while you sleep.

Why does fasting increase growth hormone?

Fasting lowers insulin levels. Insulin and growth hormone have a reciprocal relationship — high insulin suppresses GH release, while low insulin allows GH to rise. During fasting, the body also has a metabolic need to mobilise stored fat as an alternative fuel, and GH is a key driver of that process.

Does fasting increase GH more in men than women?

This study only involved men, so no direct comparison is possible from this research. Women naturally secrete more GH than men at baseline due to estrogen's stimulatory effect on GH. Whether fasting amplifies GH to the same degree in women is not well established in the literature.

Why did IGF-1 fall while GH rose during fasting?

During fasting, the liver becomes relatively resistant to GH and reduces its production of IGF-1. This is an adaptive response: IGF-1 promotes cell growth and proliferation, which would be metabolically expensive during energy scarcity. The body conserves IGF-1 for the refeeding period while using elevated GH primarily for fat mobilisation.

Related Research and Articles

• Does fasting boost human growth hormone (HGH)?
• Does intermittent fasting destroy muscle? myth vs. fact
• How intermittent fasting promotes autophagy
• What happens to your body hour by hour when you fast
• Does intermittent fasting slow your metabolism?
• Can you build muscle while intermittent fasting?
• Intermittent fasting benefits: the complete science-backed guide

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