What happens to growth hormone levels during fasting, and why does this matter for muscle preservation?
The Short Answer
Growth hormone (GH) increases significantly during fasting; the measured figures are below. It is often credited with sparing muscle during a fast, but the one direct test did not show that, and whether fasting preserves muscle better than continuous restriction is a difference Trepanowski’s 2018 trial did not find. So the rise itself is real and well measured; what it does for muscle is not shown.
Something to Sit With
Does knowing what growth hormone is argued to do for muscle, and how little of it has been measured, change how you think about skipping meals?
What fear about muscle loss might have been holding you back from trying fasting?
Learn More
What Growth Hormone Does.
Growth hormone serves multiple functions beyond its name suggests:
Protein sparing, often claimed. GH is often credited with protecting muscle during a fast; the one direct test, below, did not show it.
Fat mobilization. GH increases lipolysis, the release of stored fat for energy.
Blood sugar regulation. GH helps maintain stable blood glucose during fasting by promoting gluconeogenesis (creating glucose from non-carbohydrate sources) and reducing glucose uptake by tissues, preserving it for the brain.
Tissue repair. GH promotes cellular repair and regeneration.
The Fasting Effect.
Two studies give the figures, and they are different studies with different fasts. Ho and colleagues (1988) measured integrated GH concentration rising from 2.82 to 8.75 µg·min/ml in six men across a five-day fast, a little over three-fold on those numbers, and the rise had already occurred by the first fasted day. Hartman and colleagues (1992) measured the 24-hour production rate rising five-fold during a two-day fast in normal men (78 to 371 µg/Lv).[1] The rise comes through both more frequent bursts and larger ones.[2]
The pattern matters:
- Most of the rise is already there on the first fasted day, and levels stay up from there
- The increase is pulsatile: GH is released in bursts, especially during sleep
- Eating immediately suppresses GH release
Whether chronic calorie restriction gives the same boost is not something these studies measured: they tested fasting alone.
Why This Matters for Muscle.
A common fear about fasting is muscle loss. “Won’t my body eat my muscles?”
The growth hormone response is the reason usually given for why this fear is overstated, and it is weaker than it sounds:
The direct test. Blocking the hormone’s receptor across a 36-hour fast suppressed fat mobilization, while protein metabolism was unaffected. The blockade was partial by design, over 36 hours in 10 healthy young men, so the test does not demonstrate protein sparing at that duration, which is not the same as showing there is none.[3]
Calorie restriction was not compared. The fasting rise in growth hormone is measured; none of the studies behind it compared fasting with chronic calorie restriction.[2] So they measured no difference in mechanism, and, on the trial below, none in outcome.
Practical Implications.
Neither approach beats the other on muscle. In a 24-week randomized trial of 100 overweight or obese adults, the fat-free-mass difference between groups was not significant: alternate-day fasting and daily calorie restriction came out level.[4] The trial says they tie. What you can take from this page is that fasting cost no more muscle than daily dieting did. Not that either one preserved it.
Feeling smaller isn’t the measure. The sensation of being lighter or smaller during a fast is not a reading of your muscle, in either direction.
Eating breaks the response. When you eat, GH drops. This is normal and fine: you get the benefit during the fast, then eat and recover. The rhythm matters.
Exercise is the part that actually builds. Resistance training during fed periods combines the GH elevation of the fast with a muscle stimulus. That combination’s advantage over the alternatives is not established; muscles being maintained is not the same as muscles being built.
Further reading:
- Ho KY et al., “Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man”[5] — Journal of Clinical Investigation, 1988
- Hartman ML et al., “Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men”[6] — Journal of Clinical Endocrinology & Metabolism, 1992
- Cahill GF Jr, “Fuel metabolism in starvation”[7] — Annual Review of Nutrition, 2006
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