What is IGF-1?

What is IGF-1?

IGF-1 is a 70-amino-acid peptide hormone that carries out most of the body's growth signals, and its structure is close enough to insulin that the two share overlapping receptors. The liver makes the bulk of the IGF-1 that circulates in your blood, and it does so mainly when the pituitary gland releases growth hormone (GH).

For that reason IGF-1 is often called the messenger of growth hormone: GH tells the liver to make IGF-1, and IGF-1 then does much of the downstream work on bone, cartilage, organs, and skeletal muscle. It is also the hormone doctors measure to gauge GH status, because IGF-1 stays steady across the day while GH comes in short pulses.

In a muscle context, IGF-1 matters because it is one of the strongest known anabolic signals: it tips the balance toward building tissue and away from breaking it down. Two facts make it more interesting than a simple blood hormone. First, muscle produces its own IGF-1 locally, which acts on the same or neighboring fibers rather than travelling through the bloodstream.

Second, mechanical loading, the kind you create by lifting, changes which form of IGF-1 the muscle expresses. Those local actions, not the amount floating in your blood, are where most of the real muscle story sits.

How it works

IGF-1 works by binding the IGF-1 receptor (IGF1R) on the surface of a muscle cell, which triggers a signaling cascade that ends in more muscle protein being built. The receptor auto-phosphorylates and recruits adaptor proteins, activating the PI3K/Akt pathway. Active Akt does two anabolic jobs at once. It switches on mTOR (specifically mTORC1), the master regulator of protein synthesis, which ramps up ribosome activity so the fiber assembles more contractile protein.

At the same time, Akt phosphorylates and shuts down the FoxO transcription factors, which would otherwise turn on the atrophy genes (atrogin-1 and MuRF-1) that break muscle down. So IGF-1 pushes both levers of net muscle balance: it raises muscle protein synthesis and lowers muscle protein breakdown. IGF-1 also supports satellite cells, the muscle stem cells that donate new nuclei to a growing or repairing fiber.

Locally, mechanically loaded muscle can splice the IGF-1 gene into a variant nicknamed mechano growth factor (MGF), released as a pulse after damaging exercise, that helps activate those satellite cells for repair. Here is the honest limit, though: while the mechanism is real and powerful in the lab, the acute rise in circulating IGF-1 you get from a hard workout has repeatedly failed to predict how much muscle you gain. The signaling matters; the transient blood spike does not appear to be the driver.

How to apply it

• Growth hormone drives production: GH released from the pituitary is the main signal telling the liver to secrete IGF-1. This GH to IGF-1 axis is why the two are measured together and why IGF-1 is treated as a stable proxy for GH status across the day.
• It activates PI3K/Akt/mTOR: Binding the IGF-1 receptor turns on Akt, which activates mTORC1 to raise muscle protein synthesis. This is the same intracellular pathway that mechanical loading and leucine-rich protein feeding converge on to build contractile tissue.
• It suppresses muscle breakdown: Akt phosphorylates the FoxO transcription factors, keeping the atrophy genes atrogin-1 and MuRF-1 switched off. By lowering protein breakdown while raising synthesis, IGF-1 shifts net protein balance toward growth from both directions.
• It activates satellite cells: IGF-1 and its MGF variant help wake up satellite cells, the muscle stem cells that fuse into damaged or growing fibers and add myonuclei. New nuclei let a fiber sustain a larger volume of protein over the long term.
• Food and insulin modulate it: Adequate calories and protein raise IGF-1, while fasting and low protein lower it, because IGF-1 partly reflects nutritional status. Insulin and IGF-1 share receptor family and downstream signaling, which is why feeding is anabolic on the same pathway.
• The systemic exercise spike is not the lever: A hard leg session briefly raises circulating IGF-1, GH, and testosterone, but multiple studies show this transient rise does not predict hypertrophy. Local signaling, mechanical tension, and total training volume drive growth, not chasing a blood-hormone bump.

Types

Worked example

The clearest way to see IGF-1's real role is to look at what happens when researchers deliberately raise or hold down the post-exercise hormone response and then measure the muscle people actually gain. Across the key studies, the hormonal spike and the growth barely line up.

The takeaway is not that IGF-1 is useless, it is a genuine anabolic signal. It is that the brief bloodstream surge you get from a workout is not the mechanism that grows muscle. Volume, mechanical tension, protein, and consistency are. Chasing an IGF-1 spike with special routines or supplements is effort spent in the wrong place.

IGF-1 vs growth hormone

GH is the upstream signal and IGF-1 is the downstream messenger that carries out much of GH's growth effect. Measuring IGF-1 is how clinicians read GH status, because IGF-1 is stable while GH is pulsatile and hard to catch on a single blood draw.

By goal

• Lifters chasing muscle: Do not train to chase an IGF-1 spike, the evidence says the acute rise does not build muscle. Focus on progressive overload, roughly 10 or more hard sets per muscle per week, and 1.6 to 2.2 g of protein per kg of bodyweight. That is what actually raises the signaling that matters.
• Older adults and sarcopenia: IGF-1 falls with age, and lower levels track with muscle loss (sarcopenia). Resistance training plus adequate protein is the proven lever to keep muscle and function, working through the same IGF-1 and mTOR signaling rather than through hormone injections.
• Clinical and medical use: Recombinant IGF-1 (mecasermin) is an approved drug for specific growth disorders, not a physique aid. Using IGF-1, GH, or peptide analogs for muscle is banned in sport, unproven for healthy adults, and carries real risks including low blood sugar and possible cancer-growth signaling.

Common misconceptions

• "The right workout spikes IGF-1 and that spike builds muscle." The transient rise in circulating IGF-1 after training does not predict hypertrophy. West and Phillips found no correlation between the IGF-1 spike and 12-week gains in 56 men, and later reviews confirmed the acute hormone surge is not a major driver of growth.
• "More circulating IGF-1 is always better." IGF-1 has a U-shaped health curve. Very low levels track with frailty and muscle loss, but chronically high levels are linked in epidemiology to greater risk of some cancers, because IGF-1 is a general pro-growth signal. More is not simply better.
• "IGF-1 or MGF injections and peptides are a safe shortcut to muscle." Injectable IGF-1, LR3, and MGF are unapproved for physique goals, banned in sport, and poorly studied for safety in healthy adults. Risks include hypoglycemia, joint and organ overgrowth, and theoretical cancer promotion. Training and protein remain the proven route.
• "GH injections build big muscle by raising IGF-1." GH raises IGF-1 and lean-mass readings, but much of that is fluid retention, not contractile muscle. Controlled trials show GH adds little real strength or muscle-fiber size in healthy trained adults, despite the hormonal changes it produces.

Related terms

IGF-1 FAQ

What is IGF-1 in simple terms?

IGF-1 is a hormone that carries out the body's growth signals. Your liver makes most of it when growth hormone tells it to, and it acts like insulin's cousin. In muscle, it switches on the machinery that builds new protein and slows the breakdown of existing tissue.

Does IGF-1 actually build muscle?

IGF-1 is a genuine anabolic signal in the lab, activating the mTOR pathway that builds muscle protein. But the brief rise in blood IGF-1 you get from a workout does not predict real-world gains. Training volume, mechanical tension, and protein drive muscle growth, not the hormone spike.

Does exercise increase IGF-1?

Yes, resistance exercise briefly raises circulating IGF-1, growth hormone, and testosterone for a short window afterward. Muscle also expresses local IGF-1 in response to loading. However, studies show the size of the circulating spike does not correlate with how much muscle or strength you gain.

What is the difference between IGF-1 and growth hormone?

Growth hormone is the upstream signal released in pulses by the pituitary gland. IGF-1 is the downstream messenger the liver makes in response, and it carries out much of growth hormone's effect on tissue. Doctors measure IGF-1 because it stays steady while growth hormone comes in bursts.

How do I increase IGF-1 naturally?

Adequate calories, protein of about 1.6 to 2.2 g per kg of bodyweight, resistance training, and good sleep all support healthy IGF-1, since it partly reflects nutritional and growth-hormone status. Fasting and low protein lower it. Chasing higher IGF-1 for muscle, though, is not a proven strategy.

What is mechano growth factor (MGF)?

MGF is a splice variant of IGF-1 that mechanically loaded or damaged muscle expresses as a short pulse. It is thought to help activate satellite cells for repair and growth. Its role is real in animal models but still debated in humans and often oversold by supplement sellers.

Is high IGF-1 dangerous?

IGF-1 follows a U-shaped curve. Very low levels track with frailty and muscle loss, but chronically high levels are linked in population studies to higher risk of some cancers, because IGF-1 promotes cell growth broadly. Extremely high levels also occur in the disease acromegaly.

What is a normal IGF-1 level?

In healthy adults, serum IGF-1 usually sits roughly between 100 and 300 ng/mL, but the reference range depends heavily on age and sex. Levels peak in adolescence and decline steadily with age. A blood test is interpreted against an age-matched range, not one fixed number.

Do IGF-1 or IGF-1 LR3 injections work for muscle growth?

Injectable IGF-1 and the longer-acting LR3 version are not approved for building muscle, are banned in sport, and lack good safety data in healthy adults. Risks include dangerously low blood sugar, tissue overgrowth, and possible cancer promotion. Training and nutrition remain the proven route.

What foods or supplements raise IGF-1?

Higher protein and calorie intake, especially dairy and total animal protein, tends to raise IGF-1, since it reflects nutritional status. No legal supplement reliably raises IGF-1 in a way shown to build extra muscle. Deliberately pushing IGF-1 higher for physique goals is not supported by evidence.

References

1. Yoshida T, Delafontaine P. Mechanisms of IGF-1-Mediated Regulation of Skeletal Muscle Hypertrophy and Atrophy. Cells, 2020. PMC7564605
2. Van Every DW, D'Souza AC, Phillips SM. Hormones, Hypertrophy, and Hype: An Evidence-Guided Primer on Endogenous Endocrine Influences on Exercise-Induced Muscle Hypertrophy. Exercise and Sport Sciences Reviews, 2024. PMC11460760
3. Morton RW, et al. Neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men. J Appl Physiol, 2016. PMC4967245
4. 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. PubMed 22105707
5. Physiology, Growth Hormone. StatPearls, NCBI Bookshelf
6. Insulin-like growth factor 1. Wikipedia