The degradation of a compound in the gut and liver before it reaches the bloodstream — the main reason you can’t just swallow most peptides.After oral absorption, compounds travel through the portal vein to the liver before entering systemic circulation. Peptidases in the GI epithelium and hepatic enzymes degrade most peptides before they can be absorbed or distributed. The result is near-zero oral bioavailability for most therapeutic peptides. This is why injectable administration is standard for peptide therapeutics. Companies selling oral peptide supplements are claiming a delivery mechanism that basic biochemistry says should not work for most peptides — claims that require extraordinary evidence.