Peptides in fitness: what they are, and what the evidence actually says

What are peptides?

Peptides are short chains of amino acids — the same building blocks as proteins, just fewer of them. Your body makes and uses thousands of them as signalling molecules. The compounds people ask about in fitness are synthetic versions designed to mimic or trigger some of that signalling: to nudge growth-hormone release, to influence tissue repair, or to act on appetite or pigmentation pathways.

A few terms get used loosely. A peptide hormone is a signalling peptide like insulin or growth hormone. A secretagogue is something that makes your body release more of its own hormone. And some compounds sold alongside peptides — MK-677 is the common example — are not peptides at all, a distinction we flag on the individual pages.

The “research chemical” and legality problem

This is the part the marketing skips. Most of these compounds are not approved medicinesfor the uses people want them for. They are typically sold as “research chemicals” — made outside pharmaceutical quality control, where the purity, the actual dose in the vial, and the absence of contamination cannot be assumed. Several are prohibited in sport under the WADA code. Buying and injecting an unlicensed substance of unknown composition is a genuinely different proposition from taking an approved medicine.

Claimed vs shown — how we tier the evidence

Every page labels the evidence behind a compound’s fitness claims using three tiers, and flags whether the evidence is from humans, animals, or the lab bench:

• Well-supported — real, replicated human trials (usually because the compound went through drug development).
• Preliminary — some evidence, but limited, early, or animal-dominated; promising is not the same as proven.
• Marketing claim— heavily promoted, but the human evidence is weak, absent, or actively didn’t pan out.

A useful thing happens when you line them up: the compounds with the strongest evidence (tesamorelin, the GLP-1 medicines, bremelanotide) are precisely the ones that were developed and approved as medicines for specific conditions— not as fitness aids. They show what real evidence looks like, and how far most “research peptides” fall short of it.

What this section will not do

We do not give doses, protocols, routes, cycles, stacks, or sourcing information for any of these compounds — deliberately. This is a reference for understanding what they are and what the evidence says, not a how-to. If you are considering any of them, that is a conversation for a qualified clinician who knows your history.