IGF-1 and muscle growth factors
These compounds act downstream of growth hormone, on IGF-1’s receptor or on the signals that limit muscle growth. No published study has given any of them to a person as the vials are sold; what’s known comes from cell and animal work, a related approved drug, mecasermin, and gene therapy for follistatin.
- Compounds
- 5
- Approved drug
- 0
- Human studies
- 0
- Preclinical
- 5
Members
IGF-1 variants
Changed forms of IGF-1 that escape the binding proteins that normally hold it inactive. Neither is the approved IGF-1 drug, mecasermin.
Mechano growth factor
The 24-amino-acid tail of a muscle form of IGF-1; PEG-MGF adds a polyethylene glycol chain. The key cell finding failed to replicate.
Myostatin binder
A protein that mops up myostatin and activin, two brakes on muscle growth. People have only received it as gene therapy.
| Compound | What it is | Approval | Evidence | How often | Half-life | Sport |
|---|---|---|---|---|---|---|
| Follistatin-344 | Myostatin and activin blocker | Not approved | Preclinical | Once daily | about 4 minutes, then about 2 hours, in rats Animal | Banned |
Approval, evidence, half-life and sport status restate each compound’s own page, where the sources are listed. Select a name for doses, side effects and studies.
How they work
IGF-1 is a growth factor made in the liver and other tissues when growth hormone signals them. It acts on the IGF-1 receptor, a close relative of the insulin receptor, which drives growth and helps cells take up glucose, so IGF-1 can lower blood sugar. In blood, over 80% of it is held in a complex with a binding protein. The approved IGF-1 drug, mecasermin (Increlex), treats children with severe IGF-1 deficiency; none of this group’s members is that drug. Label
IGF-1 LR3 and IGF-1 DES escape those binding proteins. LR3 adds 13 amino acids at the start and swaps the third one; DES lacks the first three. More of each reaches the receptor, so in rat, pig and cell studies they were roughly 2–10 times as potent as IGF-1, though they left rat blood faster. DES also occurs naturally in human brain tissue. Animal
MGF is the 24-amino-acid tail of IGF-1Ec, a form of IGF-1 that muscle makes after hard exercise or injury. A 2002 cell study reported that the tail alone made muscle cells multiply, but two drug companies found no effect at up to 500 ng/mL, and the free tail has never been found in any cell, fluid or tissue. PEG-MGF, the same tail with a polyethylene glycol chain meant to slow its clearance, has no published data of any kind. Animal
Follistatin works on a different system. It binds myostatin and activin, two signals that hold back muscle growth, and in mice, follistatin made in muscle produced large gains. People have received only the FS344 gene, carried into thigh muscle by a virus, in a 15-person trial in muscle diseases, where most walked further. In rats, injected follistatin protein left the blood within hours, mostly taken up by the liver. Animal
How they differ
Target
AnimalIGF-1 LR3 and IGF-1 DES act directly on the IGF-1 receptor. MGF has no established target: the original claim was a separate receptor, but later lab work found E-peptide effects need the IGF-1 receptor. Follistatin acts outside cells, binding myostatin and activin before they reach their receptors.
Size
IGF-1 DES has 67 amino acids and IGF-1 LR3 has 83; natural IGF-1 has 70. MGF is a 24-amino-acid peptide, and PEG-MGF adds a chain of unstated length. Follistatin is a far larger protein, 315 amino acids once mature, coated in sugars that vary with the cells that made it.
How long they last
AnimalOnly animal data exist. IGF-1 LR3 and DES leave rat blood faster than IGF-1, DES about 4 times faster, and injected follistatin had half-lives of about 4 minutes, then about 2 hours, in rats. The half-lives quoted online for MGF (5–7 minutes) and PEG-MGF (48–72 hours) have never been measured.
Effects beyond growth
PrecautionThe IGF-1 variants’ main added risk is low blood sugar. Follistatin also blocks activin, which can lower FSH and shift reproductive hormones; the trials used the FS344 form partly because it binds activin less tightly. PEG-MGF adds the possibility of antibodies to polyethylene glycol, seen with other pegylated drugs.
Closest human evidence
TrialFollistatin has the most: gene therapy in 15 people with muscle diseases, with no adverse effects reported. ACE-083, a follistatin-based drug, grew the injected muscle in healthy women without making it stronger, and its Phase 2 trials were terminated. The IGF-1 variants lean on mecasermin, a different molecule; MGF and PEG-MGF have nothing.
History
- 1978The amino acid sequence of human IGF-1 was published, showing its close structural likeness to proinsulin.
- 1986A shortened IGF-1 lacking its first three amino acids, the molecule now sold as IGF-1 DES, was identified in human fetal brain.
- 1997Mice lacking myostatin grew far more muscle, identifying myostatin as a brake on muscle growth.
- 2005FDA approved mecasermin (Increlex), recombinant human IGF-1, for children with severe primary IGF-1 deficiency.
- 2015The first follistatin gene therapy results were published: 4 of 6 patients with Becker muscular dystrophy walked further after the FS344 gene was injected into their thigh muscles.
Comparisons and guides
Questions
What is IGF-1 LR3?
An engineered 83-amino-acid form of IGF-1, the growth factor the liver makes in response to growth hormone. Its changes stop IGF binding proteins from holding it, so more stays free to act; in rat and pig studies it was 2–6 times as potent as IGF-1. It was developed as a cell-culture supplement and has never been tested in people.
Is IGF-1 LR3 the same as mecasermin (Increlex)?
No. Mecasermin is recombinant human IGF-1, approved in 2005 for children with severe primary IGF-1 deficiency, and its label warns of severe low blood sugar. IGF-1 LR3 is a different, engineered molecule with no human data. The compound pages borrow mecasermin’s warnings for the IGF-1 variants only as precautions.
Does MGF build muscle?
That hasn’t been shown. No published study has given MGF or PEG-MGF to a person. The 2002 cell finding that it made muscle cells multiply wasn’t reproduced by two drug companies, and the free peptide has never been found in the body. The same peptide made prostate and bone cancer cells grow in lab dishes.
What does follistatin do?
It is a natural protein that binds myostatin and activin, signals that hold back muscle growth; it was first identified as a protein that lowers FSH, a fertility hormone. In mice it built muscle. In people it has only been given as gene therapy into thigh muscle, in 15 patients, not as the injectable protein sold in vials.
Are IGF-1 peptides banned in sport?
Yes. WADA’s 2026 list bans IGF-1 and its analogues, which covers LR3 and DES, and names mechano growth factors under S2.3, at all times. Follistatin is banned under S4.3 as a myostatin-binding protein.
Sources
- Increlex (mecasermin) prescribing information, sections 4, 5, 6 and 12 (DailyMed, rev. 05/2026); Drugs@FDA approval, August 2005
- Rinderknecht and Humbel, amino acid sequence of human IGF-I and its homology with proinsulin (J Biol Chem 1978) PMID 632300
- Sara et al., a variant form of IGF-I identified in human fetal brain (Proc Natl Acad Sci U S A 1986) PMID 3460078
- Francis et al., IGF-I analogues show the role of binding-protein escape in potency (J Mol Endocrinol 1992) PMID 1378742
- Tomas et al., low-binding IGF-I variants lower blood sugar more than IGF-I in pigs and marmosets (J Endocrinol 1997) PMID 9415072
- McPherron et al., myostatin regulates skeletal muscle mass in mice (Nature 1997) PMID 9139826
- Fornaro et al., MGF peptide has no apparent effect on myoblasts or primary muscle stem cells (Am J Physiol Endocrinol Metab 2014) PMID 24253050
- Mendell et al., Phase 1/2a follistatin gene therapy trial in Becker muscular dystrophy (Mol Ther 2015) PMID 25322757
- Kogure et al., IV follistatin is delivered mainly to the liver in rats (Hepatology 1996) PMID 8690405
- WADA 2026 Prohibited List, sections S2.3 and S4.3
Each compound’s page lists the sources for its own doses, status and studies.