Journal · Metabolic
Losing Weight and Losing Muscle: What the Growth Hormone Axis Has to Do With It
The short answer
Weight loss is not fat loss. Some of what comes off is lean tissue, and the faster the loss, the more of it tends to be. The growth hormone axis is the system the body uses to protect lean mass under energy restriction, which is why it keeps coming up in this conversation.
Questions this article answers
The number on the scale hides two different things
When someone loses twenty kilograms, that is not twenty kilograms of fat. Body composition work on GLP-1-based therapy consistently shows that a meaningful share of the loss is lean mass, and a 2026 systematic review and meta-analysis in Diabetes, Obesity and Metabolism looked specifically at how body composition remodels during this treatment (reference 1). A separate 2026 review in the Journal of Clinical Medicine examined the same question (reference 2).
This is not unique to these drugs. It happens with any substantial calorie deficit, and it has been known for decades. What is different now is the scale and speed at which it is happening to a lot of people at once.
Lean mass is not vanity. It is where glucose gets disposed of, it is most of what determines strength and function later in life, and losing it lowers resting metabolic rate, which quietly makes the weight easier to regain.
Why the body gives up muscle at all
Under energy restriction the body is solving a problem: it needs amino acids and it needs to reduce its running costs. Muscle is metabolically expensive tissue, and it is also a protein reserve. From the body’s point of view, breaking some down is a sensible response to scarcity.
What normally argues against that decision is the growth hormone axis. Growth hormone and IGF-1 push in the opposite direction: they promote the release of fat from tissue for fuel while supporting the maintenance of lean tissue. Metabolic Precision (pp. 69–73) sets out what a functioning axis actually does, and preserving lean mass during fat loss is central to it.
Which raises the obvious question, and it is the question this whole category of compounds exists to answer: if that axis is what protects muscle under restriction, what happens when the axis itself is not working well?
The part the books put first
Metabolic Precision (p. 73) makes a point that cuts against the way most people approach this, and it is worth sitting with: the growth hormone axis responds poorly in a body that is insulin resistant or carrying a lot of visceral fat. That state suppresses the growth hormone response in the first place.
So the person most likely to reach for a growth hormone compound, because they are carrying significant weight and worried about muscle, is often the person whose axis is least able to respond to one. The book’s rule is to correct metabolism first and treat the growth hormone layer as something that comes after, not instead.
Protein intake and resistance training are what the lean-mass argument is built on. The growth hormone layer sits on top of that foundation rather than in place of it, which is the sequence the rest of this article follows.
Tesamorelin and ipamorelin: two different doors
These two come up constantly in the same sentence, usually without anyone explaining why. The reason is mechanical.
Growth hormone release is controlled by more than one signal. GHRH opens the tap from one direction. Ghrelin, acting through its own separate receptor, opens it from another. They are not redundant; they are two independent inputs to the same output.
- Tesamorelin is a GHRH analog. It is also the one compound in this group that is an approved medicine, for reducing excess abdominal fat in people with HIV-associated lipodystrophy, with Phase 3 data behind it. That means the GHRH route has been carried all the way through a licensing process in at least one population.
- Ipamorelin works through the ghrelin receptor. It was developed as the selective member of its class, raising growth hormone without dragging cortisol and prolactin up with it, which is what distinguishes it from the earlier secretagogues.
Because they act on different receptors, the mechanism points to them complementing each other rather than competing. Two separate inputs to one output is the whole reason the pairing gets discussed in the first place.
Metabolic Precision (pp. 240–241) does give the rule the books use when this comes up: one GHRH analog, optionally with one ghrelin-receptor compound. Two GHRH analogs together compete for the same receptor and gain nothing, which is the most common mistake in this category.
What each one has behind it
Tesamorelin has the strongest record here. Phase 3 trials in its approved population, plus a randomised controlled trial in The Lancet HIV showing reduced liver fat in people with HIV and fatty liver disease (reference 3). That is real data with controls.
Ipamorelin’s record runs along a different line. The founding study established the selective release that the compound is named for (reference 4), and its best-controlled human work sits in a different clinical area: a randomised, placebo-controlled study in post-operative ileus, where the question was gut motility rather than body composition.
So the mechanism is the well-mapped part, and body composition is the outcome still being measured directly. How much of it shows up in a given person tracks their own metabolic environment, which is the argument for measuring rather than assuming in either direction.
The growth hormone axis moves real physiology, which is both why it is worth engaging and why it is worth watching. Blood sugar first, because growth hormone reduces insulin sensitivity; then fluid balance; and IGF-1 itself for anyone with a cancer history. Those are the markers that tell you what the axis is actually doing.
Putting it in order
If you are losing weight and want to hold onto muscle, the order of operations is the useful part. Protein and resistance training first, because they do the heavy lifting and cost nothing. Then whether your metabolic environment is one the growth hormone axis can respond in, because that determines whether anything aimed at the axis has a chance of landing. Then, and only then, the question of a compound and which door it comes through.
That sequence is worth working through with a clinician who can measure each step rather than guess at it, which is the kind of conversation these books were written to make possible.
Questions people ask
Do GLP-1 drugs cause muscle loss?
Some lean mass is lost alongside fat, as with any substantial calorie deficit. Body composition reviews in 2026 examined how this remodelling happens during GLP-1-based therapy (references 1, 2). Protein intake and resistance training are the interventions with the best evidence for limiting it.
Can tesamorelin prevent muscle loss?
The lean-mass reasoning runs through growth hormone physiology: a working axis promotes the release of fat for fuel while supporting the maintenance of lean tissue. Tesamorelin’s own approval covers reducing excess abdominal fat in HIV-associated lipodystrophy, and that is where its Phase 3 data sits.
Why are tesamorelin and ipamorelin discussed together?
They act on different receptors. Tesamorelin is a GHRH analog; ipamorelin works through the ghrelin receptor. On mechanism the two signals add rather than compete, which is why the combination comes up. Metabolic Precision (pp. 240–241) gives the pairing rule the books use.
Why is ipamorelin described as selective?
Because it raises growth hormone through the ghrelin receptor without the rise in cortisol and prolactin that came with earlier compounds in its class (reference 4). Its compound profile covers the rest, including regulatory status.
What is the most effective way to keep muscle while losing fat?
Adequate protein and resistance training, by a wide margin. Metabolic Precision (p. 73) then puts correcting metabolism ahead of adding any growth hormone compound, because the axis responds poorly in an insulin-resistant state.
Does the growth hormone axis work the same in everyone?
No, and that is the point the books make first. An insulin-resistant state with significant visceral fat suppresses the growth hormone response, so the people most interested in this class are often the least able to respond to it until that is addressed.
Where this comes from in our books
This article is the plain-language version. The book sections below go further, with full mechanism detail, evidence tiers and how each idea fits a complete protocol. Page numbers refer to the print editions.
Metabolic Precision
The pairing rule for GHRH and ghrelin-receptor compounds · pp. 240–241
Read next
Put the framework to work
The free Protocol Builder applies the same category-first logic as the books, built from the same compound database.
Open the Protocol BuilderSee the book seriesReferences
- Lopes Menezes ML, Güntzel Chiappa ME, et al. Body composition remodelling during GLP-1-based therapy: a systematic review and meta-analysis. Diabetes Obes Metab. 2026. View source
- Woźniak M, Tarcz Z, et al. The impact of glucagon-like peptide-1 (GLP-1) receptor agonists on body composition. J Clin Med. 2026;15(17). View source
- Stanley TL, Fourman LT, Feldpausch MN, et al. Effects of tesamorelin on non-alcoholic fatty liver disease in HIV: a randomised, double-blind, multicentre trial. Lancet HIV. 2019;6(12):e821–e830. View source
- Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552–561. View source
How this article is maintained. Research and regulatory status change. We review our articles against current sources and update them as new information becomes available; the date at the top shows the last review.
Educational information only, not medical advice. Nothing in this article is a recommendation to use any compound or a dosing guide. Talk to a qualified clinician before starting, stopping or combining any compound.