What Peptides are, and why most of them are injected

Peptides have gone from a word most people last heard in chemistry class to something that comes up at the gym, on podcasts and at the doctor’s office. A big part of the reason is the success of semaglutide and tirzepatide, which are both peptides. Yet few people could say what a peptide actually is, or why so many peptide treatments come as injections rather than pills.

The answers are more practical than they sound, and they explain a lot about how these medicines are designed and prescribed.

A short chain of amino acids

Amino acids are the building blocks your body uses to make everything from muscle to enzymes. Link a handful of them together and you have a peptide. Keep adding to the chain and it eventually folds into a protein. Nature does not draw a sharp line between the two, so for regulatory purposes the FDA draws one at 40 amino acids, and anything longer counts as a protein.

That line produces some odd results. Insulin is often called a peptide hormone, but it has 51 amino acids across two chains, so the FDA classifies it as a protein. Glutathione sits at the other end of the scale, with just three.

Why the body uses peptides as signals

Many of the body’s hormones are peptides. GLP-1, released by the gut after a meal, prompts the pancreas to release insulin and tells the brain you have eaten. Growth hormone-releasing hormone, made in the brain, tells the pituitary gland when to release growth hormone. Each of these fits a specific receptor on the surface of particular cells, which is why peptide signals can be so precise.

That precision is also what makes peptides attractive as medicines. Researchers describe them as highly selective and, on the whole, well tolerated, and the body eventually breaks them down into amino acids, the same material it gets from food.

Why most peptides are injected

Your digestive system is built to take proteins and peptides apart. Stomach acid and digestive enzymes cut them down into single amino acids, and whatever survives is too large to pass easily through the gut wall into the blood. A peptide swallowed as an ordinary pill would mostly be digested like a meal.

Injecting a peptide just under the skin sidesteps the problem, which is why most peptide medicines, from insulin to weekly GLP-1 treatments, are given this way.

There are exceptions, and the best known is oral semaglutide. The tablet combines semaglutide with a compound called SNAC, which shields it from digestive enzymes and helps it cross the stomach lining. Even then, only a small fraction of each dose reaches the bloodstream, which is why the tablet has to be taken on an empty stomach with careful timing.

Making a peptide last

Natural peptide signals are built to be brief. The GLP-1 your gut releases after a meal is broken down within minutes, which suits a signal meant for a single meal but would be useless in a medicine you only want to take now and then.

Semaglutide shows how this was solved. It is almost identical to human GLP-1, with small changes to its amino acid sequence, one of which protects it from the enzyme that normally breaks GLP-1 down. It also carries a fatty acid chain that lets it attach to albumin, a protein that stays in the blood for weeks. Together, these changes extend its half-life in people to about a week, which is what makes a once-weekly injection possible. Tirzepatide is also designed for weekly dosing, and acts on two hormone receptors, GLP-1 and GIP, rather than one.

Sermorelin takes the opposite approach. It is a copy of the active part of the body’s own growth hormone-releasing hormone and stays short-acting, so it prompts the pituitary to release growth hormone in pulses, much like the natural signal does.

Peptides at SaltMD

At SaltMD, peptides are prescribed as part of a plan built from your lab results, including semaglutide and tirzepatide for weight management and sermorelin, which supports the body’s own growth hormone release. Treatment starts with a 30-minute consultation, in person in Atlanta or by video, and a lab panel drawn at a lab near you or at our office, or a review of blood work you have had in the past 90 days. Your prescription is filled by a partner pharmacy and shipped to you.

Learn more about peptide therapy at SaltMD, semaglutide, or what the research shows about sermorelin.

Sources

• US Food and Drug Administration. Definition of the term “biological product”. Final rule. Federal Register, February 21, 2020 (FR Doc. 2020-03505).
• Fosgerau K, Hoffmann T. Peptide therapeutics: current status and future directions. Drug Discov Today. 2015;20(1):122-128. doi:10.1016/j.drudis.2014.10.003
• Lau J, Bloch P, Schäffer L, et al. Discovery of the once-weekly glucagon-like peptide-1 (GLP-1) analogue semaglutide. J Med Chem. 2015;58(18):7370-7380. doi:10.1021/acs.jmedchem.5b00726
• Buckley ST, Bækdal TA, Vegge A, et al. Transcellular stomach absorption of a derivatized glucagon-like peptide-1 receptor agonist. Sci Transl Med. 2018;10(467):eaar7047. doi:10.1126/scitranslmed.aar7047