IGF-1 and Muscle Hypertrophy

IGF-1 and Athletic Performance

PI3K/Akt/mTOR signaling axis

IGF-1 drives muscle hypertrophy through the PI3K/Akt/mTOR pathway, simultaneously increasing protein synthesis and suppressing protein degradation in skeletal muscle fibers.

Multiple sources, 2020-2025

Multiple sources, 2020-2025

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Insulin-like growth factor 1 (IGF-1) is a 70-amino-acid peptide hormone that mediates most of growth hormone's anabolic effects on skeletal muscle. When GH binds its receptor on hepatocytes, the liver produces and secretes IGF-1 into the circulation, where it travels to muscle tissue and activates the IGF-1 receptor (IGF-1R), a tyrosine kinase that triggers the PI3K/Akt/mTOR signaling cascade. This cascade increases protein synthesis, activates satellite cells (muscle stem cells), and suppresses the protein degradation pathways that would otherwise break down muscle tissue. The net result is muscle hypertrophy: an increase in the size and protein content of existing muscle fibers. But IGF-1's role in muscle is more nuanced than a simple growth signal. Skeletal muscle itself produces IGF-1 locally (autocrine/paracrine signaling), and this locally produced IGF-1 has distinct splice variants with different functions than circulating liver-derived IGF-1. The relationship between GH, IGF-1, exercise, and muscle growth is the biological foundation behind the interest in growth hormone secretagogues and peptide therapies for body composition. This article covers the molecular mechanism, the evidence, and the limitations. For how GH peptides affect IGF-1 levels, see CJC-1295 and IGF-1. For whether GH peptides translate to actual performance gains, see Do GH Peptides Actually Improve Strength and Power?, Growth Hormone and Athletic Performance, and GH Secretagogues in Sports.

The IGF-1 Receptor and Signaling Cascade

IGF-1 binds the IGF-1 receptor (IGF-1R), a transmembrane tyrosine kinase receptor structurally related to the insulin receptor. Ligand binding triggers autophosphorylation of the receptor's intracellular tyrosine kinase domain, creating docking sites for insulin receptor substrate proteins (IRS-1 and IRS-2). The phosphorylated IRS proteins recruit and activate phosphatidylinositol 3-kinase (PI3K), which converts the membrane lipid PIP2 to PIP3. PIP3 recruits phosphoinositide-dependent kinase 1 (PDK1) and Akt (protein kinase B) to the membrane, where PDK1 phosphorylates and activates Akt.

Akt is the central node. It drives muscle growth through two simultaneous mechanisms:

Protein synthesis activation. Akt phosphorylates tuberous sclerosis complex 2 (TSC2), releasing the inhibition on Rheb, which activates mechanistic target of rapamycin complex 1 (mTORC1). mTORC1 then phosphorylates two key downstream targets: p70S6 kinase (S6K1), which promotes ribosomal biogenesis and translation of mRNAs encoding ribosomal proteins, and 4E-binding protein 1 (4E-BP1), which releases the translation initiation factor eIF4E to begin cap-dependent mRNA translation. The combined effect is a substantial and rapid increase in the rate of new protein synthesis within muscle fibers, detectable within 30-60 minutes of IGF-1 receptor activation.

Akt also phosphorylates glycogen synthase kinase-3-beta (GSK-3-beta), inactivating it. GSK-3-beta normally phosphorylates and inhibits the translation initiation factor eIF2B, so its inactivation by Akt removes another brake on protein synthesis. This means IGF-1 activates translation through at least three convergent mechanisms: mTORC1/S6K1, mTORC1/4E-BP1/eIF4E, and GSK-3-beta/eIF2B.

Protein degradation suppression. Akt phosphorylates the FoxO family of transcription factors (FoxO1, FoxO3a), causing their nuclear exclusion. In the nucleus, FoxO transcription factors drive expression of atrophy-related genes (atrogenes) including MuRF1 (muscle RING-finger protein 1) and atrogin-1/MAFbx, which are E3 ubiquitin ligases that tag muscle proteins for proteasomal degradation. By removing FoxO from the nucleus, IGF-1/Akt signaling suppresses the ubiquitin-proteasome pathway that dismantles muscle proteins during atrophy conditions (disuse, cachexia, aging).

This dual action, simultaneously building new protein and preventing breakdown of existing protein, makes the IGF-1/Akt axis a uniquely potent driver of net muscle protein accretion.

Satellite Cells: The Muscle Stem Cell Connection

Muscle fibers are post-mitotic cells that cannot divide. Growth of existing fibers (hypertrophy) requires satellite cells, a population of muscle stem cells that reside between the basal lamina and sarcolemma of muscle fibers. When activated by mechanical stress, damage, or growth factors, satellite cells proliferate, differentiate into myoblasts, and fuse with existing muscle fibers, donating their nuclei. These additional nuclei increase the fiber's transcriptional capacity, supporting the increased mRNA production and protein synthesis demanded by hypertrophy. Each myonucleus controls a finite volume of cytoplasm (the myonuclear domain), so adding nuclei is necessary for fiber growth beyond a certain size threshold.

IGF-1 is a primary activator of satellite cells. It stimulates their proliferation through the PI3K/Akt/mTOR pathway and their differentiation through MAPK/ERK signaling. A GHRP-biotin conjugate study demonstrated that growth hormone-releasing peptide stimulation of IGF-1 promoted myocyte differentiation through both IGF-1 and collagen type I pathways, providing evidence that GH-derived IGF-1 elevation can drive the satellite cell differentiation required for muscle growth.^[1]

The distinction between types of IGF-1 matters for satellite cell biology. The liver produces the systemic endocrine form. But skeletal muscle produces its own IGF-1 splice variants through alternative mRNA splicing. Mechano-growth factor (MGF), also called IGF-1Ec in humans, is produced in response to mechanical loading (exercise) and is thought to primarily activate satellite cell proliferation, while the mature IGF-1Ea isoform drives differentiation and fusion. This local production circuit means that exercise stimulates muscle growth partly through autocrine IGF-1 signaling that is independent of circulating IGF-1 or GH levels.

This has a practical implication: the relationship between blood IGF-1 levels (which GH secretagogues can raise) and muscle-specific IGF-1 signaling (which exercise directly stimulates) is not straightforward. Raising circulating IGF-1 through GH peptides and raising local muscle IGF-1 through resistance exercise may activate overlapping but distinct growth pathways. For how exercise-induced GH compares to exogenous peptide stimulation, see Natural GH Release from Exercise vs Exogenous Peptides.

IGF Binding Proteins: The Regulators of Availability

Circulating IGF-1 does not float freely in the blood. Over 99% is bound to one of six IGF binding proteins (IGFBP-1 through IGFBP-6), with IGFBP-3 carrying approximately 75-80% of circulating IGF-1 in a ternary complex with the acid-labile subunit (ALS). This binding extends IGF-1's half-life from minutes (free IGF-1) to hours (bound IGF-1) and regulates its bioavailability to target tissues.

The binding proteins are not merely carriers. IGFBP-3 can both inhibit and potentiate IGF-1 action depending on the tissue context. In some situations, IGFBP-3 sequesters IGF-1 away from its receptor, reducing signaling. In others, IGFBP-3 localizes IGF-1 near its target cells and releases it slowly, creating a sustained local concentration that amplifies signaling. IGFBP-1 is regulated by insulin and nutritional status: during fasting, IGFBP-1 rises and binds more IGF-1, reducing its bioavailability. After a meal, insulin suppresses IGFBP-1, freeing IGF-1 for receptor binding.

The practical implication is that total serum IGF-1 (the most commonly measured value) does not reflect the amount of IGF-1 available to bind muscle receptors. Free IGF-1, the bioactive fraction, is a small percentage of the total and is regulated by binding protein levels, which are themselves influenced by nutritional status, insulin levels, and liver function. A person could have high total IGF-1 but low free IGF-1 due to elevated binding proteins, or vice versa. This adds uncertainty to the interpretation of IGF-1 levels as a surrogate for anabolic potential.

Exercise, mTOR, and the Independence Question

The relationship between IGF-1 and exercise-induced muscle growth is less direct than often assumed. While IGF-1 activates mTOR through the PI3K/Akt pathway, exercise also activates mTOR through pathways that do not require IGF-1 signaling at all.

Mechanical loading activates mTORC1 through phosphatidic acid (PA) production and through the diacylglycerol kinase zeta (DGKz) pathway. Eccentric contractions generate phosphatidic acid in the muscle membrane, which directly binds and activates mTOR independently of Akt. This means resistance exercise can stimulate protein synthesis even when IGF-1 receptor signaling is blocked, as demonstrated in studies using genetic models where the IGF-1 receptor was conditionally deleted from skeletal muscle, which still showed partial hypertrophic responses to overload.

This does not diminish IGF-1's role. The IGF-1/Akt pathway contributes to the full hypertrophic response, particularly the satellite cell activation and anti-atrophy components that mechanical loading alone may not fully activate. The current understanding is that muscle hypertrophy is driven by multiple convergent signals: mechanical tension (via PA/mTOR), metabolic stress (via AMPK modulation), IGF-1/Akt signaling (from both systemic and local sources), and potentially additional pathways involving Wnt signaling and IL-6 family cytokines. IGF-1 is one essential input among several.

The practical consequence is that raising IGF-1 levels without providing a mechanical stimulus (resistance exercise) is unlikely to produce muscle hypertrophy, because the mechanically activated pathways are also required for the full response. Conversely, resistance exercise without IGF-1 signaling still produces partial hypertrophy, suggesting that exercise is the more essential of the two inputs.

The GH-IGF-1 Axis: Peptide Interventions

The GH-IGF-1 axis is the primary pathway through which growth hormone secretagogues affect muscle. GH released from the pituitary in response to GHRH, GHRPs, or ghrelin receptor agonists travels to the liver and stimulates IGF-1 production. Circulating IGF-1 then acts on muscle (and other tissues) through the IGF-1 receptor cascade described above.

CJC-1295, a long-acting GHRH analog with a drug affinity complex (DAC) that extends its half-life, activated the GH/IGF-1 axis in normal adult subjects, raising serum IGF-1 levels and producing changes in serum protein profiles that reflected increased anabolic signaling.^[2]

MK-0677 (ibutamoren), an oral non-peptide ghrelin receptor agonist, increased serum IGF-1 in hemodialysis patients in a randomized blinded study, demonstrating that the GH/IGF-1 axis can be pharmacologically activated in catabolic clinical populations where muscle wasting is a concern.^[3]

Growth hormone secretagogue treatment in hypogonadal men raised serum IGF-1 levels, with the elevation corresponding to the degree of GH stimulation.^[4] However, the relationship between IGF-1 elevation and actual muscle growth is not linear. Raising IGF-1 levels is a biomarker of GH axis activation, but whether the magnitude of IGF-1 increase predicts the magnitude of muscle hypertrophy is poorly established.

The GH-IGF-1 axis also operates in disease contexts. In HIV/AIDS, disruption of the GHRH-GH-IGF-1 axis contributes to wasting and metabolic abnormalities, and therapeutic interventions targeting this axis have been investigated for lean mass preservation.^[5]

IGF-1 Beyond Muscle: Brain and Metabolic Roles

IGF-1 is not muscle-specific. It has systemic effects that complicate the narrative of IGF-1 as purely a muscle growth factor.

In the brain, GH and IGF-1 have broad actions on neuropsychiatric function. A 2026 review found that IGF-1 modulates neurogenesis, synaptic plasticity, neuroprotection, and cognitive function, with IGF-1 deficiency associated with cognitive impairment and neuropsychiatric symptoms.^[6] IGF-1 signaling regulates neuropeptide expression in hypothalamic neurons, creating feedback loops between peripheral IGF-1 levels and central nervous system function.^[7]

IGF-1 also plays an unexpected role in metabolic homeostasis during starvation. A 2022 study found that IGF-1, rather than being simply an anabolic signal, helps maintain blood glucose during fasting, suggesting a more complex metabolic role than the straightforward "growth factor" label implies.^[8]

The IGF-1 receptor can be hijacked by pathogens. A 2025 study found that viral insulin/IGF-like peptides inhibit IGF-1 receptor signaling to enhance viral replication, demonstrating that the ubiquity and importance of IGF-1 signaling makes it a target for molecular parasitism.^[9]

The Sarcopenia Intersection: GLP-1 Drugs and Muscle Loss

A clinically relevant intersection has emerged between the IGF-1/muscle axis and GLP-1 receptor agonist therapy. GLP-1 drugs like semaglutide and liraglutide produce weight loss, but 20-40% of that weight loss comes from lean body mass rather than fat. This muscle loss raises concerns about sarcopenia, particularly in older adults.

A 2025 review examined the emerging role of myostatin inhibitors in managing GLP-1-associated sarcopenia and metabolic disorders. Myostatin is a negative regulator of muscle growth that acts through the activin receptor, opposing the IGF-1/Akt/mTOR hypertrophy pathway. Inhibiting myostatin could preserve muscle mass during GLP-1-induced weight loss.^[10]

Liraglutide treatment in animal models affected the myostatin-activin-follistatin-IGF-1 axis, with 18-day treatment altering metabolic parameters and regional body composition including changes in the balance of anabolic (IGF-1, follistatin) and catabolic (myostatin, activin) muscle regulators.^[11] Extended 35-day treatment confirmed effects on total and regional fat-free, lean, and bone mass alongside changes in the same regulatory axis.^[12]

The potential of SARMs and antimyostatin agents to address lean body mass loss from GLP-1 agonists was reviewed in 2025, with the authors noting that combination approaches targeting both the GLP-1 receptor (for metabolic benefit) and the IGF-1/myostatin axis (for muscle preservation) represent a logical therapeutic strategy.^[13]

This intersection is directly relevant to the peptide therapeutics field. GH secretagogues that raise IGF-1 levels could theoretically counteract GLP-1-associated muscle loss, though no clinical trials have tested this combination. The biology provides a rationale: if GLP-1 agonists shift the myostatin-IGF-1 balance toward catabolism, restoring IGF-1 signaling through GH peptides could rebalance the equation. But the practical challenge is substantial. Combining a GLP-1 agonist (which reduces appetite and food intake) with a GH secretagogue (which requires adequate protein intake for anabolic effects) creates competing demands. Reduced food intake limits the amino acid substrate available for muscle protein synthesis, potentially negating the IGF-1-mediated anabolic signal. Adequate protein intake during GLP-1 therapy may be a prerequisite for any IGF-1-based strategy to preserve lean mass.

The broader context is that aging, weight loss interventions, and catabolic disease states all involve dysregulation of the GH-IGF-1-muscle axis. Cancer cachexia, chronic kidney disease, chronic obstructive pulmonary disease, and age-related sarcopenia each involve altered IGF-1 signaling, elevated myostatin, or impaired mTOR activation. Understanding the IGF-1 signaling cascade in muscle is foundational to rational therapeutic approaches for all of these conditions, not just the athletic performance applications that dominate popular discussion of IGF-1. For more on GLP-1 and sarcopenia, see GLP-1 Weight Loss and Sarcopenia: The Hidden Risk in Older Adults. For the hexarelin article covering GH releasing peptides, see Hexarelin: The Most Potent GHRP.

Limitations of the IGF-1/Muscle Evidence

Several important caveats frame the IGF-1 muscle hypertrophy evidence.

Correlation is not causation. Exercise increases both muscle size and IGF-1 levels, but whether IGF-1 is a driver or a marker of hypertrophy is debated. Some studies show that mTORC1 activation in early exercise is independent of the IGF-1/PI3K/Akt pathway, suggesting that mechanical loading activates protein synthesis through parallel mechanisms that do not require IGF-1. This does not mean IGF-1 is irrelevant, but it means IGF-1 is one of several inputs into the hypertrophy response rather than the sole switch.

Circulating versus local IGF-1. Blood IGF-1 levels (which peptide therapies can raise) reflect liver production. Muscle-specific IGF-1 splice variants (which exercise stimulates) are produced locally and may have different effects on satellite cells and protein synthesis. The assumption that raising serum IGF-1 through GH peptides will produce the same muscle response as exercise-induced local IGF-1 production is not well-supported.

Age and responsiveness. The IGF-1/Akt/mTOR pathway becomes less responsive with aging (anabolic resistance), which is one reason why older adults gain muscle less efficiently despite adequate protein intake and training stimulus. Whether pharmacological IGF-1 elevation can overcome anabolic resistance is an open question with limited clinical data.

Cancer risk. IGF-1 is a growth factor that promotes cell proliferation and survival. Epidemiological data associates higher circulating IGF-1 with increased risk of certain cancers (prostate, breast, colorectal). This creates a tension in the risk-benefit calculation of any intervention that raises IGF-1 levels, particularly for chronic use. The relationship is dose-dependent and context-dependent: physiological IGF-1 levels are necessary for normal tissue maintenance, and only supraphysiological or chronically elevated levels appear to carry increased risk.

Insulin resistance. IGF-1 receptor signaling shares components with insulin receptor signaling (IRS-1, PI3K, Akt), and chronically elevated IGF-1 can affect insulin sensitivity. GH itself is diabetogenic (it promotes insulin resistance), while IGF-1 has insulin-sensitizing properties. The net metabolic effect of GH secretagogue-induced GH/IGF-1 elevation depends on the balance between the direct effects of GH (insulin resistance) and the effects of IGF-1 (insulin sensitization), which varies between individuals and with dose.

Fiber type specificity. The IGF-1/mTOR pathway preferentially drives hypertrophy in type II (fast-twitch) muscle fibers over type I (slow-twitch) fibers, at least in preclinical models. This has implications for the expected effects of IGF-1 elevation: individuals with a higher proportion of type II fibers may respond more to IGF-1-mediated growth signals than those with predominant type I fiber composition. Endurance athletes (more type I) may experience different responses than strength athletes (more type II) to the same IGF-1 elevation.

Individual variation. IGF-1 receptor density, binding protein levels, GH pulse amplitude, and downstream signaling efficiency all vary between individuals due to genetics, age, sex, and hormonal status. Two people with identical serum IGF-1 levels may have vastly different muscle responses because of differences in the intracellular machinery that converts IGF-1 receptor activation into protein synthesis. This variation makes it difficult to predict individual responses to GH peptide therapies based on population-level IGF-1 data.

The Bottom Line

IGF-1 drives skeletal muscle hypertrophy through the PI3K/Akt/mTOR signaling cascade, simultaneously increasing protein synthesis (via mTORC1, S6K1, eIF4E) and suppressing protein degradation (via FoxO phosphorylation and atrogene suppression). Locally produced muscle IGF-1 splice variants activate satellite cells for fiber repair and growth. GH secretagogues and ghrelin receptor agonists raise circulating IGF-1 through the GH/IGF-1 axis, but the translation from elevated blood IGF-1 to actual muscle hypertrophy depends on training stimulus, age, nutritional status, and the contribution of local versus systemic IGF-1 signaling. The emerging intersection with GLP-1-associated muscle loss has created interest in combining anabolic peptide strategies with metabolic therapies.

Frequently Asked Questions

How does IGF-1 cause muscle growth?

IGF-1 binds the IGF-1 receptor on muscle fibers, activating the PI3K/Akt/mTOR signaling cascade. This increases protein synthesis through mTORC1 activation of ribosomal machinery and simultaneously suppresses protein breakdown by phosphorylating FoxO transcription factors that would otherwise upregulate muscle degradation genes. IGF-1 also activates satellite cells, the muscle stem cells that donate nuclei to growing fibers.

Does raising IGF-1 levels build muscle?

Raising circulating IGF-1 through GH secretagogues or ghrelin receptor agonists activates the GH/IGF-1 axis, but the relationship between blood IGF-1 levels and actual muscle hypertrophy is not straightforward. Exercise-induced local IGF-1 production in muscle (including the MGF splice variant) may be more relevant to hypertrophy than systemically circulating IGF-1. Raising IGF-1 without concurrent resistance training is unlikely to produce meaningful muscle growth.

What is mechano-growth factor (MGF)?

MGF (IGF-1Ec in humans) is a splice variant of IGF-1 produced locally by skeletal muscle in response to mechanical loading during exercise. Unlike liver-derived circulating IGF-1, MGF primarily activates satellite cell proliferation rather than differentiation. This local production circuit means exercise stimulates muscle growth through autocrine IGF-1 signaling that is partly independent of blood GH or IGF-1 levels.

Can peptides increase IGF-1 levels?

Yes. CJC-1295 (a GHRH analog) raised serum IGF-1 in healthy adults (Sackmann-Sala et al., 2009). MK-0677 (ibutamoren) increased IGF-1 in hemodialysis patients in a randomized trial (Campbell et al., 2018). GH secretagogues in hypogonadal men raised IGF-1 corresponding to GH stimulation (Sigalos et al., 2017). These elevations confirm GH/IGF-1 axis activation but do not directly prove muscle growth results from the elevation.

Does GLP-1 weight loss affect IGF-1 and muscle?

GLP-1 receptor agonists produce weight loss that includes 20-40% lean body mass. Liraglutide treatment altered the myostatin-activin-follistatin-IGF-1 axis in animal studies, shifting the balance of anabolic and catabolic muscle regulators (Gutierrez et al., 2026). Myostatin inhibitors are being investigated as countermeasures to preserve muscle during GLP-1 therapy (Baik et al., 2025).

Is high IGF-1 dangerous?

Epidemiological data associates higher circulating IGF-1 with increased risk of certain cancers including prostate, breast, and colorectal. IGF-1 promotes cell proliferation and survival, which benefits muscle growth but could also promote tumor growth. This creates a tension in the risk-benefit calculation of interventions that chronically elevate IGF-1 levels. The clinical relevance of this association for individuals using GH peptides is not well-established.