What a peptide actually is

Every protein in your body is a chain of amino acids folded into a shape. A peptide is the same thing, only short — the usual cut-off is about fifty amino acids, though the line is a convention rather than a law of nature. Insulin is 51 and gets called both.

The body uses peptides as messengers. GLP-1 is released by the gut after a meal and tells the pancreas and the brain what to do about it. Oxytocin, growth-hormone-releasing hormone, the endorphins — all peptides, all made in one place to carry a signal to another.

That is also why peptide drugs exist. If a hormone does something useful, a chemist can make a modified version that survives longer in the blood. Semaglutide is GLP-1 with a few changes so that it lasts a week instead of minutes. Tirzepatide is built on a different gut hormone and modified to hit two receptors.

Two practical consequences follow from being a chain of amino acids. First, stomach acid and digestive enzymes take peptides apart, which is why most are injected rather than swallowed, and why an oral version is a hard engineering problem. Second, peptides in water slowly break down, so they are shipped as a freeze-dried powder — lyophilized — and dissolved before use. That word is where LYO's name comes from.

One more thing the word does not tell you: nothing about whether a given peptide works. "Peptide" describes a shape, the way "molecule" does. Semaglutide and BPC-157 are both peptides. One has thirty randomised trials; the other has none. The category tells you how it is built, not what it has been shown to do.

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