The Bioavailability Problem, With Actual Numbers

Defining the term

Bioavailability is the fraction of an administered dose that reaches systemic circulation unchanged. Intravenous administration is 100 percent by definition. Every other route is measured against it.

Subcutaneous injection for peptides is typically high — often well above half, frequently much more. Oral is where the numbers collapse.

Three barriers, multiplied

An orally administered peptide faces gastric acid and pepsin, then pancreatic and brush-border proteases in the small intestine, then the epithelial barrier itself, which is selective against large hydrophilic molecules. Anything that survives all three then passes through the liver before reaching circulation.

These are multiplicative. If a peptide survives ten percent of gastric transit, ten percent of intestinal proteolysis and crosses at one percent, the product is 0.01 percent. This is why unassisted oral peptide bioavailability is commonly cited in the range of a tenth to a few percent, and often lower.

The engineering that moves the number

Enteric coating delays release past the stomach. Protease inhibitors co-formulated with the peptide reduce enzymatic attack. Permeation enhancers such as SNAC or C10 transiently increase epithelial permeability. Structural approaches — cyclisation, non-natural amino acids, PEGylation — make the molecule a worse substrate for proteases.

These work, and the proof is that commercially approved oral peptide products exist. It is also instructive that those products require large oral doses relative to their injectable equivalents and strict dosing conditions such as fasting with a specified volume of water. The engineering raises a very small number to a merely small one.

Variability is the harder problem

A low but consistent bioavailability can be dosed around. High variability cannot. Oral peptide absorption is sensitive to gastric pH, food, transit time and inter-individual differences, which means the same dose in the same person on two different days can produce different exposure.

For a supplement positioned around general support, variability is tolerable. For anything where dose matters precisely, it is disqualifying.

Reading a label with this in mind

Comparing an oral capsule to an injectable on milligrams alone is a category error. A meaningful comparison requires the route and ideally the exposure.

The reasonable reading of a well-made oral peptide supplement is that it delivers a small, consistent amount with a real formulation strategy behind it — not that it replaces an injection.