Growth Hormone, IGF-1 and How Muscle Actually Grows
Here is the uncomfortable truth the supplement industry would rather you didn’t know: the brief growth-hormone spikes you get from training, deep sleep or fasting do very little to build muscle on their own. Muscle grows mainly because of mechanical tension applied repeatedly through progressive overload, fed by adequate protein. The natural GH and IGF-1 surges are real, and a healthy axis matters for tissue repair, but the controlled human trials are clear that chasing the spike is not the same as growing the muscle. This guide separates the established science from the gym folklore.
The GH/IGF-1 axis in plain English
Growth hormone (GH) is released in pulses by the pituitary gland, controlled by two opposing hypothalamic signals: GH-releasing hormone (GHRH) turns it on, and somatostatin turns it off. Most circulating GH does not act on muscle directly. Instead, it travels to the liver and triggers the production of insulin-like growth factor 1 (IGF-1), which is the main carrier of GH’s growth-promoting effects around the body.1 IGF-1 binds its receptor on muscle and activates the PI3K/Akt/mTOR pathway, which switches on protein synthesis and dampens protein breakdown.2
So far, so anabolic. But there is a crucial wrinkle that destroys the simple “more GH equals more muscle” story. Muscle also makes its own IGF-1 locally in response to loading, and a large body of work shows that mechanical-load-induced muscle growth can proceed even when IGF-1 receptor signalling is blocked.3 In other words, the IGF-1 your liver pumps out after a GH pulse is not the lever that turns a hard set into a bigger muscle. The mechanical signal is upstream of, and largely independent from, the systemic hormone.
Evidence note. The point that mTOR activation after mechanical loading is independent of IGF-1 receptor signalling comes from rodent genetic models, which is strong mechanistic evidence but not the same as a human trial. The human evidence on GH spikes (below) is what really settles the practical question.
How muscle actually grows
Hypertrophy is simply a sustained surplus of muscle protein synthesis (MPS) over muscle protein breakdown, repeated across weeks until fibres thicken. Four things drive it, and none of them is “a hormone spike.”
- Mechanical tension. This is the primary stimulus. Muscle fibres contain proteins that sense force and convert it into a chemical signal (mechanotransduction), which activates mTOR and ramps up MPS. Tension applied through a challenging range, taken close to failure, is the signal that matters most.2
- Muscle protein synthesis and the leucine threshold. Training raises MPS for roughly 24 to 48 hours; protein, and specifically the amino acid leucine, is what feeds it. Studies suggest you need roughly 2.5 to 3 g of leucine, equivalent to about 25 to 40 g of high-quality protein, to maximally trigger the MPS response in a single meal.4 Older adults need more because of “anabolic resistance.”
- Satellite cells. These are muscle stem cells that can fuse into fibres and donate new nuclei to support growth. They clearly matter for long-term and large-scale growth and for repair, though rodent work shows the early phase of overload hypertrophy can occur even when satellite cells are depleted, because existing nuclei ramp up their output.5 Their role is supporting, not the on-switch.
- Progressive overload. The dose that ties it together. Because muscle adapts, the tension has to keep increasing over time, through more weight, more reps or more quality sets, or growth stalls.
A neat illustration of how indirect this all is: a 2016 study by Damas and colleagues tracked novice trainees and found that the MPS spike after the very first sessions correlated with muscle damage, not growth. Only once the damage settled, around three weeks in, did the MPS response start tracking actual hypertrophy.6 The body is messy, and acute markers are easy to misread, which is exactly the trap the GH-spike story falls into.
Key facts
- Most GH acts indirectly, by raising liver IGF-1; muscle also makes IGF-1 locally when loaded.
- Mechanical tension, not a hormone spike, is the primary driver of hypertrophy.
- Around 70% of daily GH output occurs during deep (slow-wave) sleep.7
- Exercise-induced GH, testosterone and IGF-1 spikes do not predict or enhance muscle growth in trials.8
- Injected GH adds lean “mass” that is largely fluid and connective tissue, with no strength benefit.9
The myth: does the GH spike build muscle?
For decades the bodybuilding world assumed that the post-workout surge in GH, testosterone and IGF-1 was a key cause of muscle growth, and that training to maximise the “hormonal response” was the route to size. This idea has been tested directly, and it does not hold up.
The decisive experiments came from Stuart Phillips’ lab. In one design, young men trained their arms under two conditions: a low-hormone condition (arm exercise alone) and a high-hormone condition (the same arm exercise immediately followed by a large bout of leg exercise that floods the blood with GH and testosterone). After weeks of training, there was no difference in arm muscle growth or strength between the two arms, despite hugely different hormone exposure.8 A companion cohort study in 56 men found no correlation between the size of the post-exercise GH, free testosterone or IGF-1 response and the eventual gains in lean mass or strength.10
The conclusion across this literature, summarised in a 2024 evidence review pointedly titled “Hormones, Hypertrophy, and Hype,” is that transient, physiological elevations of these hormones are neither necessary for nor enhancers of muscle growth in healthy young people.11 What rises locally inside the loaded muscle, and the tension itself, is what counts. The systemic spike is largely a bystander.
| Claim | Folklore says | Evidence says |
|---|---|---|
| Post-workout GH spike | Builds muscle; train to maximise it | No effect on hypertrophy or strength in controlled trials8 |
| Mechanical tension / overload | One factor among many | The primary driver of hypertrophy2 |
| Protein dose | “The more the better” | Benefit plateaus near 1.6 g/kg/day12 |
| Deep sleep / fasting GH surge | An anabolic shortcut | Good for health and the axis; little direct muscle effect7 |
| Injected GH for muscle | Adds quality muscle and strength | Adds mostly fluid; no strength gain; real risks9 |
What genuinely raises GH naturally
Several everyday behaviours do raise GH, sometimes dramatically. They are worth doing, but for the right reasons: sleep, body composition and metabolic health, not as a hypertrophy hack.
Deep sleep
The largest GH pulse of the day fires shortly after you fall asleep, locked to the first episode of slow-wave (deep) sleep. In healthy adults, roughly 70% of daily GH output occurs during sleep, and the size of the pulse tracks the amount of deep sleep you get.7 Chronic sleep restriction blunts this. Protecting deep sleep is the single most reliable way to keep your natural GH rhythm intact, and it benefits recovery and training quality far more through rest itself than through the hormone.
Training
Hard resistance and high-intensity exercise produce a sharp, short-lived GH rise, larger with shorter rest periods and higher metabolic stress. As covered above, this acute spike is not what builds the muscle; the loading is. The spike is a marker of effort, not a mechanism of growth.
Fasting
Fasting is a potent GH stimulus. A classic study by Ho, Hartman and colleagues found that a five-day fast raised 24-hour GH secretion several-fold, by suppressing somatostatin and amplifying GH pulses.13 Importantly, this rise in GH during a fast is thought to help preserve lean tissue and mobilise fat for fuel, not to build new muscle, and it occurs alongside a fall in IGF-1. Fasting has metabolic merits, but it is not a muscle-building strategy, and prolonged fasting while trying to gain muscle is counterproductive.
If your goal is muscle, the boring fundamentals beat any GH manoeuvre: progressive resistance training near failure, around 1.6 g/kg/day of protein spread across meals, and enough sleep. You can sanity-check a supplement plan against that hierarchy with our stack builder.
Injected GH: the risks and the reality
If physiological spikes do little, what about pharmacological doses of injected recombinant GH, as used (illegally, in sport) to build muscle? Here the evidence is both clearer and more sobering.
Kevin Yarasheski’s trials in the early 1990s gave GH alongside resistance training to younger and older men. Fat-free mass and total body water rose more in the GH groups, but strength gains were no greater than training alone, and the extra “lean mass” appeared to be fluid retention and non-contractile (connective) tissue rather than working muscle.9 A 2008 systematic review in the Annals of Internal Medicine by Liu and colleagues, pooling 27 studies in fit young people, reached the same verdict: GH increased lean body mass (again, largely water), but did not improve strength or exercise capacity, and it raised the rate of side effects.14
Safety and legal status. In the UK, somatropin (recombinant GH) is a prescription-only medicine licensed for genuine GH deficiency and a small number of growth disorders, not for bodybuilding or anti-ageing. Using it without a prescription, or supplying it for sport, is unlawful, and it is a banned substance under anti-doping rules. Reported adverse effects include insulin resistance and type 2 diabetes, fluid retention and swelling, joint and muscle pain, and carpal tunnel syndrome.1415 Sustained excess GH causes acromegaly, in which roughly one third of patients develop diabetes and mortality is raised when untreated.16 “GH-releasing peptides” and secretagogues sold online are unlicensed and carry their own unquantified risks.
The honest summary: pharmacological GH changes how the scales and a body-composition scan read, mostly through water, while delivering no measurable strength advantage and a meaningful list of harms. It is one of the clearest examples in performance science of a hormone that “does something” without doing the thing people actually want. For the bigger picture on why hormones rarely behave like simple growth dials, see our health library and the broader insights on separating mechanism from marketing.
- I have symptoms of possible GH deficiency in adulthood (persistent fatigue, central weight gain, low mood): is testing appropriate, and what does it involve?
- Could a sleep problem (such as sleep apnoea) be disrupting my deep sleep and recovery?
- I have been offered “GH therapy” or peptides privately for muscle or anti-ageing: is this licensed and safe, and what are the risks for me?
- I have signs that worry me (new carpal tunnel symptoms, swelling, joint pain, changes in hands, feet or facial features): should I be assessed for a hormone problem?
References
- Ranke MB, Wit JM. Growth hormone: past, present and future. Nat Rev Endocrinol, 2018. PMID 29977921
- Schoenfeld BJ. The mechanisms of muscle hypertrophy and their application to resistance training. J Strength Cond Res, 2010. PMID 20847704
- Hamilton DL, Philp A, et al. Mechanical loading-induced mTOR signalling and muscle growth is independent of IGF-1 receptor signalling. Review and primary data. J Cell Sci / related work, 2010. Journal of Cell Science
- Zaromskyte G, et al. Evaluating the leucine trigger hypothesis to explain the postprandial regulation of muscle protein synthesis: a systematic review. Front Nutr, 2021. PMC8295465
- Murach KA, et al. Differential requirement for satellite cells during overload-induced muscle hypertrophy in growing versus mature mice. Skelet Muscle, 2017. PMC5504676
- Damas F, Phillips SM, et al. Resistance training-induced changes in integrated myofibrillar protein synthesis are related to hypertrophy only after attenuation of muscle damage. J Physiol, 2016. J Physiol 594:5209
- Van Cauter E, Plat L. Physiology of growth hormone secretion during sleep. J Pediatr, 1996; and Interrelations between sleep and the somatotropic axis. Sleep, 1998. Oxford Academic (PDF)
- West DWD, Phillips SM, et al. Elevations in ostensibly anabolic hormones with resistance exercise enhance neither training-induced muscle hypertrophy nor strength of the elbow flexors. J Appl Physiol, 2010. J Appl Physiol
- Yarasheski KE, et al. Effect of growth hormone and resistance exercise on muscle growth and strength in older men. Am J Physiol, 1995. Am J Physiol Endocrinol Metab
- West DWD, Phillips SM. Associations of exercise-induced hormone profiles and gains in strength and hypertrophy in a large cohort after weight training. Eur J Appl Physiol, 2012. Eur J Appl Physiol
- Morton RW, et al. Hormones, hypertrophy, and hype: an evidence-guided primer. Exerc Sport Sci Rev, 2024. Exerc Sport Sci Rev
- Morton RW, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med, 2018. PMC5867436
- Ho KY, Hartman ML, et al. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest, 1988. PMC329619
- Liu H, et al. Systematic review: the effects of growth hormone on athletic performance. Ann Intern Med, 2008. Ann Intern Med 148:747
- Human growth hormone (HGH): uses and side effects. Cleveland Clinic, 2022. Cleveland Clinic
- Insulin resistance in patients with acromegaly. Front Endocrinol / review, 2019. PMC6683662
This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.