EP 112:29

What actually happens when you inject a GLP-1

Name: What actually happens when you inject a GLP-1
Uploaded: 2026-09-28
Duration: 2 min 29 s
Description: Follow one shot of semaglutide from a four-millimetre needle into the fat, the blood, the pancreas and the brain, and see why a fatty tail on albumin makes it last a week.

Follow one shot of semaglutide from a four-millimetre needle into the fat, the blood, the pancreas and the brain, and see why a fatty tail on albumin makes it last a week.

Transcript and sources

This is what actually happens when you inject a GLP-1 like semaglutide. The needle is just four millimetres long, and skin is only about two millimetres thick, so the tip stops in the fat underneath.
Less than a millilitre of liquid spreads out around the fat cells, not into them. From there it seeps into tiny blood vessels. Levels peak about one to three days later, and about eighty-nine percent of the dose makes it in.
Your body's own GLP-1 is gone in a minute or two. An enzyme called DPP-4 cuts it, and the kidneys clear what's left. Semaglutide lasts about a week.
Two small changes explain it. At position eight, one amino acid is swapped for one that DPP-4 can't cut. And at position twenty-six, it carries a fatty tail, eighteen carbons long.
That tail makes it grab onto albumin, the most common protein in your blood. More than ninety-nine percent of it is riding on albumin at any moment, and riding on albumin, it's too big for the kidneys to filter.
Here's the surprise: it's not the fat that makes it last. Given straight into a vein, its half-life is about the same. The week comes from albumin.
When it reaches a cell with a GLP-1 receptor, it docks, the receptor changes shape, and the cell makes a messenger called cyclic AMP.
In the pancreas, that boosts insulin, but only when blood sugar is high, and it turns glucagon down.
In animals, semaglutide didn't cross the blood-brain barrier. It reached appetite areas through a few spots where that barrier is naturally open, like the area postrema. That map comes from mice and rats, not from people.
It also slows the stomach, mostly early in treatment. In one study, people on semaglutide ate about a third less at a free-choice lunch, and reported fewer, weaker cravings.
One shot a week builds into a steady level after about four to five weeks. Stop, and the drug fades out over the following weeks. In one trial, people regained about two thirds of the weight they'd lost within a year.
Four millimetres of needle, one week of albumin, and a signal that reaches your brain. Every source is at peplexicon.com.

Sources