The dual stable isotope tracer technique quantifies dietary protein digestibility in humans with minimal invasiveness and serves as a practical alternative to naso-ileal sampling, especially for clinical and elderly populations.

When you eat protein, your gut breaks it down into amino acids, which enter your bloodstream. Some amino acids come from the protein you ate, and some are given to you as free amino acids in a test meal. By tracking how much of each type shows up in your blood, scientists can tell how well your body broke down the protein. If the protein was poorly digested, fewer amino acids from it appear in the blood compared to the free ones. This method avoids needing a tube in the nose because it only requires a blood sample.