What Do Peptides Do in the Body?

What do peptides do at the most basic level?

**Short answer:** they carry messages between cells by binding receptors, not by acting as bulk building material.

A peptide is a short chain of amino acids, usually about 2 to 50, joined by peptide bonds (MedlinePlus, U.S. National Library of Medicine). What a peptide does depends on its sequence, because the order of amino acids sets the shape, and the shape is what lets it fit a receptor. When it fits, it triggers a response inside the cell.

That single idea explains most peptide function. "The widest variety of signaling molecules in animals are peptides, ranging in size from only a few to more than a hundred amino acids," and this group includes peptide hormones, neuropeptides, and a diverse array of polypeptide growth factors (Cooper, The Cell: A Molecular Approach, NCBI Bookshelf NBK9924). In the nervous system, neuropeptides form the largest group of neurotransmitters, and some act both at synapses and as neurohormones on distant cells (Basic Neurochemistry, NCBI Bookshelf NBK28247).

So the honest one-line answer to "what do peptides do" is: they signal. The rest of this page is a tour of what those signals accomplish, grouped by function. For the structural side, what peptides are and how they differ from proteins, see [what are peptides](/what-are-peptides).

What do metabolic peptides do?

**Short answer:** they change appetite and blood-sugar signaling, which is the most clinically validated thing peptides do in humans.

The GLP-1 family is the flagship here. GLP-1 is a gut peptide that signals fullness and helps control blood sugar. Drugs that mimic or extend it, and newer molecules that hit two or three receptors at once, drive body-composition and glucose outcomes in large human trials.

- Semaglutide, a GLP-1 receptor agonist, reported a mean weight change of about -15.3 kg versus -2.6 kg on placebo over 68 weeks in the STEP 1 trial of 1,961 adults, with 50.5% of participants reaching at least 15% weight loss (Wilding et al., NEJM 2021;384:989-1002; doi:10.1056/NEJMoa2032183). - Tirzepatide, a GIP plus GLP-1 dual agonist, reported up to about 22.5% mean weight reduction over 72 weeks in SURMOUNT-1 (Jastreboff et al., NEJM 2022;387:205-216; doi:10.1056/NEJMoa2206038). - Retatrutide, an investigational GLP-1, GIP, and glucagon triple agonist, reported a mean weight change up to about 24.2% at 48 weeks in a phase 2 trial of 338 adults (Jastreboff et al., NEJM 2023; doi:10.1056/NEJMoa2301972).

A second, smaller metabolic angle comes from mitochondrial peptides, covered further down.

What this class does, stated plainly: it turns down appetite and improves glucose handling. Semaglutide and tirzepatide are FDA-approved for specific uses; retatrutide is investigational. See [retatrutide](/retatrutide), [tirzepatide](/tirzepatide), and [peptides for fat loss](/peptides-for-fat-loss).

What do growth-hormone peptides do?

**Short answer:** they prompt the body to release more of its own growth hormone, rather than adding growth hormone directly.

Two mechanisms show up in this class. Growth-hormone-releasing hormone (GHRH) analogs act on the GHRH receptor on the pituitary. Ghrelin-mimetic secretagogues act on a separate receptor.

- CJC-1295, a long-acting GHRH analog, raised mean growth hormone about 2 to 10 fold (sustained about 6 days) and IGF-1 about 1.5 to 3 fold (up to about 2 weeks) after a single dose in healthy adults (Teichman et al., JCEM 2006;91(3):799-805). - Ipamorelin, a selective GH secretagogue, raises growth hormone with minimal effect on cortisol and prolactin, which is what made it stand out from earlier secretagogues (Raun et al., Eur J Endocrinol 1998;139(5):552-561; doi:10.1530/eje.0.1390552). - Tesamorelin, a GHRH analog and the one FDA-approved member here, was studied for HIV-associated visceral fat: visceral adipose tissue fell about 15.2% versus a 5.0% rise on placebo (Falutz et al., NEJM 2007;357:2359-2370; doi:10.1056/NEJMoa072375).

What this class does: it raises growth-hormone output and downstream IGF-1. That is not free of tradeoffs. A long-term study of an oral ghrelin mimetic reported that fasting glucose rose and insulin sensitivity fell at 25 mg/day, even as fat-free mass rose about 1.1 kg over 12 months (Nass et al., Ann Intern Med 2008;149(9):601-611). See [CJC-1295 and ipamorelin](/cjc-ipamorelin), [ipamorelin](/ipamorelin), and [tesamorelin](/tesamorelin).

What do healing and repair peptides do?

**Short answer:** in animal models they signal for new blood vessels and faster tissue repair; strong human data are still thin.

- BPC-157, a synthetic pentadecapeptide, has been reported to promote angiogenesis and tendon and muscle healing through pathways including VEGFR2 and the nitric-oxide (Akt-eNOS) route in animal models. Human data remain very limited, from a few pilot studies (Sikiric et al., review, PMC7096228). - TB-500 is a fragment tied to thymosin beta-4. The seven-amino-acid actin-binding motif of thymosin beta-4 was shown to be necessary and sufficient to promote angiogenesis in that work (Philp et al., FASEB J 2003;17(14):2103-2105; PMID 14500546).

The pattern in this class is angiogenesis plus cell migration, which are core steps in wound repair. The caution is just as important: most of this evidence is preclinical, so an animal result is not a human outcome. See [BPC-157](/bpc157), [TB-500](/tb-500), and [peptides for recovery](/peptides-for-recovery).

What do cosmetic and copper peptides do?

**Short answer:** copper peptides signal skin cells to remodel the matrix, increasing collagen and related proteins in studies.

GHK is a natural tripeptide that binds copper. The GHK-Cu complex has been reported to increase collagen, elastin, and glycosaminoglycan synthesis and to support wound healing in animal studies, and the same review describes GHK modulating the expression of roughly 4,000 human genes, many tied to tissue repair (Pickart and Margolina, Int J Mol Sci 2018;19(7):1987; PMID 29986520). The review also notes plasma GHK declines with age, from roughly 200 ng/mL near age 20 to about 80 ng/mL by age 60.

What this class does: it acts as a repair and remodeling signal in skin. See [GHK-Cu](/ghkcu) and [copper peptides](/copper-peptides).

What do mitochondrial peptides do?

**Short answer:** they signal from the mitochondria to the rest of the cell to shift energy use, largely through AMPK.

MOTS-c is a 16-amino-acid peptide encoded within mitochondrial DNA. It activates AMPK, a master energy sensor, and in mice it prevented high-fat-diet weight gain and insulin resistance (Lee et al., Cell Metab 2015;21(3):443-454). Later work reported MOTS-c is exercise-induced and moves into the nucleus in response to metabolic stress, linking it to muscle homeostasis with age (Reynolds et al., Nat Commun 2021; PMC7817689).

What this class does: it acts as a metabolic stress signal that nudges cells toward glucose uptake and fat oxidation. Human evidence is early, and exercise-induced does not mean an injection substitutes for exercise. See [MOTS-c](/mots-c).

What do vitality and sexual-health peptides do?

**Short answer:** at least one acts on brain melanocortin signaling to affect sexual desire, and it is FDA-approved.

Bremelanotide (PT-141) is an MC4R agonist. In the RECONNECT phase 3 trials of about 1,267 premenopausal women with hypoactive sexual desire disorder, 1.75 mg given as needed met its co-primary desire and distress endpoints; the most common adverse event was nausea, in about 40% of participants (Kingsberg et al., Obstet Gynecol 2019;134:899-908). It was FDA-approved in June 2019.

What this class does: it acts through central melanocortin pathways rather than through blood-flow mechanics, which is what separates it from the more familiar options in this space. See [PT-141](/pt-141).

What do peptides NOT do? Common misconceptions

**Short answer:** peptides are not a single wonder drug, they are not all proven in humans, and most are not oral.

- They are not interchangeable. A peptide is a specific message, so a change of a few amino acids can change what it does entirely. There is no generic "peptide effect." - They are not all human-proven. The strongest human data sit with the GLP-1 metabolic class and a handful of approved agents (tesamorelin, bremelanotide). Many healing, cosmetic, and mitochondrial peptides rest on animal models or small pilots. - They are not usually pills. Many peptides are given by injection in research settings, because the digestive tract breaks them down before they reach the blood. Insulin is the classic example of that limit. - More signal is not automatically better. Raising a growth pathway can carry metabolic tradeoffs, as the rise in fasting glucose with an oral ghrelin mimetic showed (Nass et al., Ann Intern Med 2008).

What does research report about peptide dosing?

**Short answer:** dosing is peptide-specific and comes from studies, not from a one-size number, and this section reports ranges rather than advising a dose.

The ranges below reflect what published studies and commonly studied research protocols report. This is educational, not a prescription or a personal recommendation.

These are trial figures for specific populations and endpoints, not targets for any individual. For unit math, converting a milligram amount to marks on an insulin syringe depends on how a vial is reconstituted, so it is worked as a generic reference on the [reconstitution](/reconstitution) page, not as an instruction here.

Where to go next

Start with the structural companion, [what are peptides](/what-are-peptides), then move into the function-specific hubs: [peptides for fat loss](/peptides-for-fat-loss), [peptides for recovery](/peptides-for-recovery), and [peptides for muscle growth](/peptides-for-muscle-growth). For safety, see [are peptides safe](/are-peptides-safe) and [peptide side effects](/peptide-side-effects).

FAQ

**What do peptides do in the body?** Most peptides act as signaling molecules. They bind receptors on or inside cells and trigger a response, such as releasing a hormone, changing metabolism, or starting repair. Peptides are the widest variety of signaling molecules in animals (Cooper, The Cell, NCBI Bookshelf NBK9924).

**Do peptides build muscle directly?** Not directly as building material. Growth-hormone peptides raise the body's own growth hormone and IGF-1, which are involved in growth. For example, CJC-1295 raised mean growth hormone about 2 to 10 fold and IGF-1 about 1.5 to 3 fold in healthy adults (Teichman et al., JCEM 2006). That is a signal, not a bulk protein you add.

**What do peptides do for weight?** Metabolic peptides in the GLP-1 family reduce appetite and improve blood-sugar signaling. Semaglutide reported about -15.3 kg versus -2.6 kg on placebo over 68 weeks (Wilding et al., NEJM 2021), and tirzepatide reported up to about 22.5% mean reduction over 72 weeks (Jastreboff et al., NEJM 2022).

**What do peptides do for skin?** Copper peptides act as repair signals. The GHK-Cu complex has been reported to increase collagen, elastin, and glycosaminoglycan synthesis and to modulate roughly 4,000 human genes tied to tissue repair (Pickart and Margolina, Int J Mol Sci 2018).

**What do healing peptides do?** In animal models they signal for new blood vessels and faster tissue repair. BPC-157 has been reported to promote angiogenesis and tendon and muscle healing (Sikiric et al., review, PMC7096228), and the actin-binding motif of thymosin beta-4 promotes angiogenesis (Philp et al., FASEB J 2003). Human data are limited.

**Are all peptides proven in humans?** No. The strongest human data are in the metabolic GLP-1 class and a few approved agents like tesamorelin and bremelanotide. Many healing, cosmetic, and mitochondrial peptides rest largely on animal studies or small pilots, so evidence depth is uneven across the field.

**Why are most peptides injected instead of swallowed?** The digestive tract breaks many peptides down before they reach the blood, so injection is common in research settings. Insulin is the classic example of a peptide that does not survive oral use well.

**Do peptides have side effects?** Yes, and they depend on the specific peptide. Bremelanotide's most common adverse event in trials was nausea, in about 40% of participants (Kingsberg et al., Obstet Gynecol 2019), and an oral ghrelin mimetic raised fasting glucose and lowered insulin sensitivity (Nass et al., Ann Intern Med 2008).