Endogenous vs Administered Peptides

Two different situations

An endogenous peptide is one the body makes itself, in the amount, place and moment its own systems decide. An administered peptide is one that enters the body from outside, as a medicine, a food ingredient, a cosmetic or another product. The molecule can be the same in both cases, but the situation is very different.

Many peptides occur naturally in the body, and they often serve as signaling molecules for physiological functions. [1] Examples from the literature include GnRH, a 10-amino-acid hormone made by neurons in the hypothalamus, and GLP-1, a 37-amino-acid peptide that regulates insulin production and secretion. [2] GHK, a three-amino-acid peptide, has been reported in human plasma, saliva and urine, with levels that decline with age. [3]

Why the body's own version is not the same as a product

Body-made peptides are often short-lived. GLP-1 has a very short half-life in the body, [2] and unmodified insulin has a reported half-life of less than 5 minutes. [4] Natural peptides can be easily broken down by enzymes. [2]

An administered peptide changes several things at once:

• Timing and amount. A product can raise levels at times and to degrees the body would not.


• Route. Many peptides rely on injection because they are degraded by digestive enzymes and cross membranes poorly. [4]


• Form. The product may be the natural sequence or an altered version. See natural, synthetic and modified peptides.


• Purity and formulation. Impurities, aggregation and other ingredients can affect how the body responds. The FDA states that compounded GHRP-2 for injection or nasal use may pose a risk of immune reactions because of aggregation and peptide-related impurities. [5] This applies to that compound and does not describe all peptides.

Medicines designed from the body's own peptides

Drug development has long drawn on endogenous peptides. A review notes that peptide drug discovery has diversified beyond its traditional focus on endogenous human peptides to include structures found in other natural sources or designed through medicinal chemistry. [6]

Because natural peptides are unstable, developers often alter them to improve absorption, half-life and selectivity. [1] Some examples:

• Leuprolide has the same biological activity as GnRH by activating GnRH receptors and is used as a GnRH receptor agonist. [2]


• Liraglutide is a chemically synthesized analogue of human GLP-1. [2]

Those are approved medicines, with a defined indication and a regulator's review. They are not evidence that other products that mimic or borrow a body molecule's name have been tested or are safe. See approved medicines vs research peptides.

"It occurs naturally, so it must be safe" is not a valid inference

Finding a molecule in the body shows that the body makes it. It does not show what happens when a person takes a product containing it, at what amount or by what route, or for how long. Evidence for a product has to come from studies of that product in the people it would be used by. This library marks claims that rest on body levels or lab findings alone as preclinical or marketing claim, depending on the case. For more, see claim check: peptides are natural so safe.

A related point: a hormone level that moves in a lab test is not the same as a health benefit. See biomarker changes vs meaningful health outcomes.