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The growth hormone response

Also called: GH, Somatotropin, Counter-regulatory response

Written by Reviewed Sep 2026

Growth hormone output rises several times over within two days of a fast. But IGF-1, the growth signal it normally works through, does not follow it, and that changes what the rise means.

What it is

The body guards blood sugar in a lopsided way. Only one hormone lowers it: insulin. Several raise it: glucagon, adrenaline and noradrenaline, cortisol, and growth hormone. These are the "pushback" hormones, and a fast calls on all of them, on different timelines and for different jobs.

Growth hormone comes out in pulses from the pituitary gland at the base of the brain. One brain signal (GHRH) pushes it up and another (somatostatin) holds it down. The hunger hormone ghrelin pushes it up too. Because it comes in pulses, a single blood sample tells you almost nothing. The measurements worth quoting come from sampling every few minutes for 24 hours and then working out the real output with maths. Both of the key studies in people did exactly that.

Growth hormone normally does most of its work by telling the liver to make IGF-1 (insulin-like growth factor 1). IGF-1 is what carries most of its building effect to the body's tissues. During a fast, that link is broken on purpose.

How it works

Fasting raises growth hormone by changing the pattern of its release, not by clearing it more slowly. Nine healthy men had blood drawn every five minutes for 24 hours on the second day of a fast. Their daily output rose about five-fold. Two separate changes drove that. The number of bursts per day more than doubled. And the amount released in each burst also about doubled, because the bursts were stronger, not longer. The quiet gaps between bursts got shorter. How fast the blood cleared the hormone did not change. That rules out the idea that it just builds up because removal slows. The likely cause is more frequent "go" signals from the brain, plus longer and deeper breaks in the "stop" signal.

During a fast, growth hormone then frees up fat. It breaks down stored fat, and it works against insulin's effect on blood sugar. What it does not do during a fast is drive IGF-1. That is because the liver stops listening to growth hormone. It lowers its growth hormone receptors and raises a protein (IGF binding protein 1) that ties up IGF-1. The logic holds together. Freeing fat and saving sugar is useful to a fasting body. Growth is exactly what a starving body should not spend resources on. So the system splits the two.

The muscle-saving effect works through fat, not through muscle, and that has been tested directly by blocking it. 7 healthy people fasted for 37 hours. When their fat release was blocked with a drug called acipimox, their urea output and muscle protein breakdown rose by almost 50%. Adding growth hormone while fat release was blocked did nothing to protein breakdown. Giving fat back through a drip (intralipid) brought whole-body protein breakdown back down by 10 to 15%. The authors concluded that blocking fat release takes away growth hormone's ability to limit protein loss in a fast. Freeing up fat is its main way of saving protein.

What switches it on

Falling blood sugar and falling insulin, acting through the brain. The whole set of pushback hormones comes in, in order. Glucagon comes first and fastest. Adrenaline, noradrenaline and cortisol follow, rising and falling with the time of day. Growth hormone climbs into the second day.

On the timing
This is one of the few processes on the site with a real clock in people. 24 hour sampling studies exist at 24 hours, 48 hours and 5 days. In a study of 30 healthy adults, a 24 hour water-only fast clearly raised growth hormone compared with a normal eating day. But that paper does not give the size of the rise, so no percentage from it is quoted here. By the second day, daily output is about five-fold higher. In a 72 hour fast in 33 people, growth hormone output rose from the first day to the third. But it fell from the second day to the third, so it does not just keep climbing. IGF-1 is unchanged at 56 hours and has fallen by about 41% by day 5.

What has been measured

In people

This is one of the best-mapped parts of human fasting science. It was worked out in hospital research wards between the 1960s and the 1990s, with tubes in blood vessels and very frequent blood draws. Every number below is a measurement in people.

  • The largest study: a 72 hour fast in 33 healthy people, 16 men and 17 women, which set normal ranges for the whole set of pushback hormones. Blood sugar, insulin, C-peptide and proinsulin all fell. Glucagon and free fatty acids rose steadily. Adrenaline, noradrenaline and cortisol rose, with a clear daily rhythm on top. That means a single blood sample at one time of day can badly mislead. Growth hormone output rose from the first day to the third, but fell from the second day to the third. Men had higher blood sugar and glucagon, and lower free fatty acids, during the fast (American Journal of Physiology: Endocrinology and Metabolism, 2001).
  • A 24 hour fast in 30 seemingly healthy volunteers. Each person did one day of water-only fasting and one day of normal eating. Growth hormone clearly rose. Haemoglobin, red blood cell count, haematocrit, total cholesterol and HDL all rose. Triglycerides, bicarbonate and weight fell. When haemoglobin, red cell count and haematocrit all rise together, it points to the blood's fluid part shrinking, not to new red cells being made. The size of the growth hormone rise is not given in the summary of the paper, so it is not quoted here (Nutrition, Metabolism and Cardiovascular Diseases, 2013).
  • A two day fast in 9 healthy men, with blood drawn every 5 minutes for 24 hours on a normal day and on day two. Daily growth hormone output rose from 78 to 371 micrograms per litre, a five-fold rise. Bursts per day rose from 14 to 32. The amount per burst rose from 6.3 to 11. The gap between bursts shortened from 143 to 88 minutes. How long the hormone lasted in the blood did not change, at 18 against 20 minutes. Blood IGF-1 after 56 hours of fasting did not change (Journal of Clinical Endocrinology and Metabolism, 1992).
  • The matching experiment, in 8 healthy people fasting for 40 hours. Growth hormone was shut off with a drug and then given back. Insulin and glucagon were replaced the same way both times, so only growth hormone differed. Daily urea in the urine was 392 mmol at the start, 440 when fasting, 609 when fasting without growth hormone, and 408 when fasting with growth hormone given back. So shutting off growth hormone in a 40 hour fast raised urea in the urine by about 50%. That makes growth hormone a real part of how the body saves protein (Diabetes, 2001).
  • The blocking experiment, in 7 healthy people over a 37 hour fast, tested four ways. Blocking fat release with acipimox raised urea in the urine and blood, and muscle protein breakdown, by almost 50%. Adding growth hormone while fat release was blocked did not change protein breakdown. Giving fat back through a drip (intralipid) lowered urea and cut whole-body protein breakdown by 10 to 15%. Freeing up fat is growth hormone's main way of saving protein (Journal of Clinical Endocrinology and Metabolism, 2003).
  • A five day fast in 6 healthy men, with 24 hour sampling checked two different ways. Pulses per day rose from 5.8 to 9.9. The total daily amount rose from 2.82 to 8.75 (in micrograms times minutes per mL). The biggest pulse rose from 5.9 to 12.3 ng/mL. Over the same days, IGF-1 (measured as somatomedin C) fell from 1.31 to 0.77 U/mL. Blood sugar fell from 4.9 to 3.2 mmol/L, and free fatty acids rose from 0.43 to 1.55 mmol/L (Journal of Clinical Investigation, 1988).
  • What happens when the system is overridden. People with obesity were given human growth hormone during 5 to 6 weeks of starvation. Insulin rose 2 to 3 fold, then blood sugar rose 50%. Fat release kept going. Blood ketones rose 20 to 40%, to a total of 10 to 12 mmol/L, with 150 to 320 mmol a day lost in the urine. People reported nausea, vomiting, weakness and muscle aches. Urea in the urine fell by half, but total protein loss did not change, because ammonia in the urine rose 50% (Journal of Clinical Investigation, 1971).

In animals and in cells in a dish

The data from people here are good enough that this page does not lean on other species. One animal note does matter. The pulse pattern of growth hormone is quite different in rats and people. That difference is why the 5 day study was run in people in the first place.

  • In rats, growth hormone comes out in a different pulse pattern from the one in people. That is why the 24 hour sampling studies in people were designed the way they were (Journal of Clinical Investigation, 1988).
  • How growth hormone receptors are controlled, and how the brain sets the pulse rhythm, has a large body of work in rodents and in cells in a dish. No specific papers from that work are cited here, because the measurements in people are direct and enough on their own.

Why it matters

The claim that fasting raises growth hormone is true, and the published numbers behind it are big. A rise of about five-fold in daily output by the second day, with clearance unchanged, is a real and large change in the body.

What it means is the part that gets lost. Growth hormone normally works through IGF-1 from the liver. During a fast, IGF-1 does not move at 56 hours, and it falls by day 5. The liver has stopped listening to growth hormone on purpose. So the surge is not a building state. It is a fat-freeing, sugar-saving state that has been cut off from growth.

And the protein-saving effect, which is the reason people care, works by freeing fat, not by acting on muscle. Block fat release and the effect vanishes completely. That is about as direct a test as you can run in people.

A claim you will see repeated

The figures usually quoted are that growth hormone rises 1,300% in women and 2,000% in men. Those numbers are real, and they are quoted accurately. But they come from a press release put out by the Intermountain Medical Center Heart Institute in April 2011, about work shown at a heart conference. Three things get lost when they are passed around. The percentages are for a 24 hour fast, but the claim is nearly always attached to fasts of several days. It is a press release, not a published paper a reader can check. And the same release reports that LDL cholesterol rose 14% and HDL rose 6% during the fast, a detail that never travels with the growth hormone number. The researchers' own closing line was that it is not yet time to start a fasting diet. The number you can check is the five-fold rise in daily output on day two, from nine men with blood drawn every five minutes. The second myth is about how it works. The fasting growth hormone surge is widely said to protect muscle directly. The blocking experiment shows otherwise. With fat release blocked, adding growth hormone did nothing at all to protein breakdown. It saves protein by freeing up fat.

Sources

  1. Study in people1992
    Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men

    Journal of Clinical Endocrinology and Metabolism

    9 healthy men had growth hormone measured every 5 minutes for 24 hours, once on a normal eating day and once on day two of a fast. Daily growth hormone output rose five-fold. Bursts went from about 14 to 32 a day, and each burst was bigger. How long the hormone lasted in the blood did not change. IGF-1 did not change after 56 hours.

    9 normal men, serum GH every 5 minutes for 24 hours on a fed day and on the second day of a fast, multi-parameter deconvolution. 24 hour production rate 78 plus or minus 12 to 371 plus or minus 57 micrograms per litre of distribution volume, a five-fold rise, P = 0.0001. Bursts per 24 hours 14 plus or minus 2.3 to 32 plus or minus 2.4, P = 0.0006. Mass per burst 6.3 plus or minus 1.2 to 11 plus or minus 1.6, P = 0.002. Half-life unchanged, 18 plus or minus 2.2 versus 20 plus or minus 1.5 minutes, P = 0.47. Serum IGF-1 unchanged after 56 hours.

  2. Study in people1988
    Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man

    Journal of Clinical Investigation

    6 healthy men had 24 hours of blood sampling on a normal eating day and on days 1 and 5 of a 5 day fast. Growth hormone pulses rose from about 5.8 to 9.9 a day. Total growth hormone over 24 hours rose from 2.82 to 8.75, and the biggest pulses went from 5.9 to 12.3 ng/mL. Somatomedin C, also called IGF-1, fell. Blood sugar fell from 4.9 to 3.2 mmol/L, and free fats in the blood rose.

    6 normal adult men, 24 hour sampling on a control fed day and on days 1 and 5 of a 5 day fast, analysed by discrete pulse detection and Fourier time series. Pulse frequency 5.8 plus or minus 0.7 to 9.9 plus or minus 0.7 per 24 hours, P = 0.028. Integrated 24 hour concentration 2.82 plus or minus 0.50 to 8.75 plus or minus 0.82 micrograms times minute per mL, P = 0.0002. Maximal pulse amplitude 5.9 plus or minus 1.1 to 12.3 plus or minus 1.6 ng/mL, P < 0.005. Somatomedin C fell 1.31 plus or minus 0.22 to 0.77 plus or minus 0.18 U/mL, glucose 4.9 to 3.2 mmol/L, free fatty acids 0.43 to 1.55 mmol/L.

  3. Study in people2001
    Reference intervals for glucose, beta-cell polypeptides, and counterregulatory factors during prolonged fasting

    American Journal of Physiology: Endocrinology and Metabolism

    33 healthy adults, 16 men and 17 women, fasted for 72 hours. Blood sugar, insulin and related markers all fell. Glucagon and free fats in the blood rose steadily. Stress hormones rose, following the daily body clock. Growth hormone was higher on day three than day one, but lower on day three than day two. Men had higher blood sugar and glucagon. People with a higher BMI had higher insulin.

    33 healthy subjects, 16 male and 17 female, across a 72 hour fast. Glucose, insulin, C-peptide and proinsulin all decreased (P < 0.001). Glucagon and free fatty acids increased linearly. Adrenaline, noradrenaline and cortisol increased with a clear circadian rhythm superimposed. Growth hormone secretion increased from the first to the third day (P < 0.05) but decreased from the second to the third (P = 0.03). Males had higher glucose and glucagon and lower free fatty acids (P < 0.05); higher BMI produced higher insulin and C-peptide.

  4. Study in people2003
    The decisive role of free fatty acids for protein conservation during fasting in humans with and without growth hormone

    Journal of Clinical Endocrinology and Metabolism

    7 healthy people fasted 37 hours, four times, under different conditions. Blocking fat breakdown with a drug raised signs of protein loss, including muscle protein breakdown, by almost 50%. Giving growth hormone did not stop that loss while fat breakdown was blocked. Putting fat back into the blood through a drip cut whole-body protein breakdown by 10 to 15%. So growth hormone can only protect protein during a fast when the body can break down fat.

    7 normal subjects, 37 hour fast, four conditions including GH replacement, GH plus antilipolysis with acipimox, no GH plus antilipolysis, and GH plus antilipolysis plus intralipid. Blocking lipolysis raised urea excretion, serum urea and muscle protein breakdown by almost 50%. Adding GH during fasting with antilipolysis did not influence indices of protein degradation. Restoring free fatty acids with intralipid cut whole-body protein degradation by 10 to 15%. The authors conclude that inhibition of lipolysis eliminates the ability of GH to restrict fasting protein loss.

  5. Study in people2001
    The protein-retaining effects of growth hormone during fasting involve inhibition of muscle-protein breakdown

    Diabetes

    8 healthy people were tested four ways: after an overnight fast, after a 40 hour fast, after a 40 hour fast with growth hormone blocked, and after the same with growth hormone given back. Protein waste in urine was 392 mmol a day at baseline, 440 fasted, 609 with growth hormone blocked, and 408 with it given back. Muscle breakdown in the forearm moved the same way.

    8 normal subjects, four conditions: basal postabsorptive, 40 hour fast, 40 hour fast with somatostatin suppression of GH, and 40 hour fast with GH suppressed plus exogenous GH replacement, with insulin and glucagon replaced identically in the last two arms. Urinary urea excretion in mmol per 24 hours: 392 plus or minus 44 basal, 440 plus or minus 32 fasted, 609 plus or minus 76 fasted without GH, 408 plus or minus 36 with GH replaced (P < 0.05). Forearm muscle protein breakdown moved the same way.

  6. Study in people1971
    Metabolic response to human growth hormone during prolonged starvation

    Journal of Clinical Investigation

    Obese people 5 to 6 weeks into a total fast were given growth hormone. Insulin rose 2 to 3 fold, then blood sugar rose 50%. Ketones rose 20 to 40%, to 10 to 12 mmol/L in total. They lost more potassium in urine and had nausea, vomiting, weakness and muscle aches. One sign of protein loss fell 50%, but another rose 50%, so total protein loss did not change.

    Human growth hormone administered to obese subjects during 5 to 6 weeks of starvation. Serum insulin rose 2 to 3 fold, preceding a 50% rise in blood glucose. Lipolysis persisted. Blood beta-hydroxybutyrate and acetoacetate rose 20 to 40%, giving total blood ketoacid concentrations of 10 to 12 mmol/L and ketonuria of 150 to 320 mmol per day, with increased urinary potassium loss and symptoms of nausea, vomiting, weakness and myalgias. Despite a 50% reduction in urea excretion, total nitrogen loss was unchanged because urinary ammonia rose 50%.

  7. Study in people2013
    Randomized cross-over trial of short-term water-only fasting: metabolic and cardiovascular consequences

    Nutrition, Metabolism and Cardiovascular Diseases

    30 healthy volunteers tried one day of drinking only water and one day of normal eating. Growth hormone rose on the fasting day, but the summary does not say by how much. Red blood cell counts, total cholesterol and HDL rose. Blood fats (triglycerides), bicarbonate and weight fell. In those who fasted first, most markers were back to normal by 48 hours.

    30 apparently healthy volunteers, randomised crossover of one day of water-only fasting against a day of usual eating. Growth hormone rose (p = 1.1 x 10 to the minus 4). Haemoglobin, red cell count, haematocrit, total cholesterol and HDL rose; triglycerides, bicarbonate and weight fell. In those randomised to fast first, most markers including growth hormone and cholesterol had returned to baseline by 48 hours. The magnitude of the growth hormone change is not stated in the abstract.