What Are Peptides? A Complete Beginner's Guide

A peptide is a chain of two to roughly fifty amino acids linked by peptide bonds. Above that length the molecule is generally called a protein. Insulin (51 amino acids) sits right at the boundary, which is why definitions vary between textbooks.

Because peptides are small and highly specific, they act like keys: each one binds a particular receptor and triggers a defined downstream signal rather than a broad metabolic shift.

Most therapeutic peptides bind G-protein coupled receptors on the cell surface. GLP-1 agonists, ghrelin mimetics, and melanocortin agonists all work this way.

A smaller group acts intracellularly. KPV suppresses NF-kB transcription; SS-31 binds cardiolipin inside the mitochondrial membrane. These require cellular uptake, which is why delivery route matters so much.

Digestive proteases cleave peptide bonds, so oral bioavailability is typically under 1%. Subcutaneous injection bypasses first-pass metabolism entirely.

Exceptions exist: some oral GLP-1 formulations use an absorption enhancer, Semax and Selank are given intranasally, and GHK-Cu works topically because its target is the skin itself.

Only a handful of peptides — tirzepatide, tesamorelin, bremelanotide — have completed large randomized trials with FDA review.

Popular repair peptides such as BPC-157 and TB-500 rest almost entirely on rodent studies. That is not proof they do not work; it means the human dose, duration, and long-term risk are genuinely unknown.