Ibutamoren and growth hormone: key differences

Ibutamoren (MK-677) is often advertised as an "oral growth hormone"—a convenient alternative to injectable somatropin. In fact, they are completely different substances: one causes the pituitary gland to secrete its own hormone, the other is the hormone itself, introduced from the outside. The editors explain how they differ in mechanism, evidence base, medical status and risks.

What is ibutamoren and what is growth hormone

Growth hormone (somatotropin) is a protein hormone of 191 amino acids produced by the anterior lobe of the pituitary gland. The medicinal product — recombinant human growth hormone (somatropin) — is identical to the natural one and is administered only by injection, since the protein is destroyed in the digestive tract.

Ibutamoren (MK-677) is a small non-peptide molecule developed by Merck in the 1990s. It mimics the action of ghrelin, a stomach hormone that stimulates appetite and the release of growth hormone. Ibutamoren is active when taken orally, which made it attractive to researchers.

So, the comparison of "ibutamoren vs growth hormone" is really a comparison of two strategies. The first is to stimulate one's own growth hormone production system. The second is to introduce the hormone itself from the outside, bypassing the regulation of the pituitary gland.

Ibutamoren is sometimes mistakenly classified as a SARM because it is sold alongside them. However, it has nothing to do with the androgen receptor. Its target is the growth hormone-stimulating receptor (GHS-R1a), the same receptor that ghrelin acts on.

Mechanism: stimulation versus substitution

Natural secretion of growth hormone is impulsive: the pituitary gland secretes it in "waves", the largest of which occur during deep sleep. This process is regulated by somatoliberin (stimulates), somatostatin (inhibits) and ghrelin (enhances). Growth hormone stimulates the liver and other tissues to produce insulin-like growth factor-1 (IGF-1), through which a large part of its anabolic effects is realized.

Ibutamoren, by activating the ghrelin receptor, enhances natural growth hormone impulses. In a study by Chapman et al. (1996), daily administration of MK-677 in elderly subjects increased growth hormone secretion and IGF-1 levels while preserving the pulsed nature of secretion. Since the mechanism depends on the pituitary gland, if it is damaged, ibutamoren will not work.

Recombinant growth hormone works differently: after subcutaneous injection, a concentration peak is created, independent of the natural rhythm. With regular administration, the exogenous hormone suppresses its own secretion through feedback — IGF-1 and the growth hormone itself stimulate the production of somatostatin.

An important difference: the ghrelin system is not only responsible for growth hormone. Activation of the GHS-R1a receptor increases appetite and may also moderately affect cortisol and prolactin secretion. Therefore, ibutamoren is not a "pure" growth hormone stimulator, but a substance with a wider spectrum of effects.

Another difference concerns the duration of action. The effect of ibutamoren on IGF-1 persisted over months of daily administration in studies, meaning it did not disappear due to receptor habituation, as occurs with some other secretagogues.

What clinical studies have shown

The best-known study of ibutamoren is the randomized, placebo-controlled trial by Nass et al. (2008) in Annals of Internal Medicine. Healthy subjects aged 60–81 years received 25 mg of MK-677 per day for a year. Growth hormone and IGF-1 levels increased to values ​​typical of young adults, lean mass increased, but strength and physical function did not significantly improve.

The same study reported increased appetite, increased fasting glucose, and decreased insulin sensitivity, as well as moderate fluid retention. Previously, Murphy et al. (1998) showed that MK-677 reduced nitrogen loss under caloric deficit conditions in healthy volunteers, i.e., counteracted catabolism.

Growth hormone has been studied incomparably better, primarily in patients with its deficiency, for which clinical guidelines have been developed (Molitch et al., 2011). In healthy subjects, a systematic review by Liu et al (2008) found that growth hormone increased lean mass but did not improve strength and endurance, and that the increase in lean mass was partly due to fluid retention.

A joint observation holds true for both substances: increases in IGF-1 and fat-free mass do not equate to improvements in strength or athletic performance. These indicators in studies in healthy people often differed.

Side effects and risks

Some of the side effects of the two substances are common, because they are associated with an increase in growth hormone and IGF-1. The other part is specific. Below are the main ones described in clinical studies and reviews.

• Violation of carbohydrate metabolism: both substances reduce sensitivity to insulin; for ibutamoren, an increase in fasting glucose was recorded in the Nass study.
• Fluid retention and edema: characteristic of growth hormone; for ibutamoren are described moderately.
• Joint and muscle pain, carpal tunnel syndrome: well described for growth hormone, especially at high doses.
• Increased appetite: a specific effect of ibutamoren through ghrelin receptors.
• Acromegaly-like changes: possible with long-term growth hormone excess.

The cardiovascular safety of ibutamoren in elderly and debilitated people deserves special attention. One study in elderly patients after hip fracture was stopped early because of a signal of an increased incidence of heart failure. This does not prove causation, but demonstrates that the safety of the substance cannot be considered established.

Regarding cancer risks: IGF-1 is a cell growth factor, and in epidemiological studies, high IGF-1 has been associated with the risk of certain cancers. Growth hormone is contraindicated for people with active cancer. There are no long-term data on ibutamoren in this regard.

For drugs purchased illegally, the risks of counterfeiting are added: other peptides or solutions without the active substance can be sold under the name "growth hormone", and no one controls the quality of "MK-677" powders. Injectable fakes carry the risk of infection.

Medical and legal status

Recombinant growth hormone is a prescription drug. It is prescribed to children with growth hormone deficiency, Turner and Prader-Willi syndromes, chronic renal failure, adults with confirmed growth hormone deficiency, and certain other conditions. Diagnosis of deficiency in adults requires stimulation tests, and treatment requires regular monitoring of IGF-1 and metabolic parameters.

Ibutamoren is not registered as a medicinal product for any indication. It has been studied for sarcopenia, post-fracture, growth hormone deficiency and other conditions, but no studies have led to approval. Selling ibutamoren as a "supplement" is not legal in most jurisdictions.

In sport, both substances are banned by WADA in category S2 Peptide hormones, growth factors, related substances and mimetics - at any time. Special tests have been developed for growth hormone, in particular for the ratio of isoforms and biomarkers; ibutamoren is detected by the molecule itself and its metabolites. A review by Holt and Ho (2019) details the history of HGH abuse in sport and methods of detecting it.

Thus, for a person without a medical indication, none of the substances is a legal option to "improve" the body or results. For a person with suspicion of growth hormone deficiency, there is only one way - examination by an endocrinologist.

Editorial conclusion

Ibutamoren and growth hormone lead to a similar result — an increase in growth hormone and IGF-1 in the blood — but by different pathways. Ibutamoren stimulates the pituitary gland through ghrelin receptors and preserves impulse secretion, growth hormone replaces its own hormone and inhibits its production.

In clinical trials, both substances increased lean mass but did not improve strength in healthy subjects. Both reduce insulin sensitivity, and ibutamoren also increases appetite.

Growth hormone is a registered drug with clear indications, ibutamoren is an unregistered compound with limited safety data. Both options are prohibited for athletes.

We recommend that you also read our materials on IGF-1 as a laboratory marker, on growth hormone-stimulating peptides, and on insulin resistance in people who exercise.

References

1. Nass R, Pezzoli SS, Oliveri MC, et al. Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults: a randomized trial. Ann Intern Med. 2008;149(9):601–611.
2. Chapman IM, Bach MA, Van Cauter E, et al. Stimulation of the growth hormone (GH)-insulin-like growth factor I axis by daily oral administration of a GH secretogogue (MK-677) in healthy elderly subjects. J Clin Endocrinol Metab. 1996;81(12):4249–4257.
3. Murphy MG, Plunkett LM, Gertz BJ, et al. MK-677, an orally active growth hormone secretagogue, reverses diet-induced catabolism. J Clin Endocrinol Metab. 1998;83(2):320–325.
4. Liu H, Bravata DM, Olkin I, et al. Systematic review: the effects of growth hormone on athletic performance. Ann Intern Med. 2008;148(10):747–758.
5. Molitch ME, Clemmons DR, Malozowski S, et al. Evaluation and treatment of adult growth hormone deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(6):1587–1609.
6. Holt RIG, Ho KKY. The use and abuse of growth hormone in sport. Endocr Rev. 2019;40(4):1163–1185.
7. World Anti-Doping Agency. The World Anti-Doping Code: International Standard — Prohibited List. Montreal: WADA; current edition.