Reference
Ileal Digestibility
**Ileal digestibility** is the proportion of a dietary protein's amino acids absorbed by the time digesta reaches the end of the ileum — the final segment of the small intestine — rather than measured from the feces.
Ileal versus fecal measurement
Older protein-quality methods estimated digestibility from fecal nitrogen, comparing what was eaten with what was excreted. The problem is the large intestine: when undigested protein reaches the colon, bacteria ferment it, altering the nitrogen that ultimately appears in feces. That fermentation produces short-chain and branched-chain fatty acids, ammonia, phenolic and indolic compounds, biogenic amines, hydrogen sulfide, and nitric oxide, so fecal nitrogen no longer reflects true amino acid uptake (Am J Physiol Gastrointest Liver Physiol, 2018, PMID:29597354).
Measuring at the terminal ileum sidesteps colonic fermentation entirely. The result is a per-amino-acid figure describing what was absorbed where absorption happens — in the small intestine.
Why it underlies DIAAS
Ileal digestibility is the measurement built into the Digestible Indispensable Amino Acid Score (DIAAS). The FAO proposed DIAAS in 2013 to replace the Protein Digestibility Corrected Amino Acid Score (PDCAAS), which relied on fecal true digestibility and truncated values at 1.00 (FAO Food and Nutrition Paper 92, 2013). DIAAS instead uses ileal digestibility of each individual indispensable amino acid and does not cap the score, so a protein supplying a surplus of digestible amino acids can score above 100.
The distinction is practical, not academic. In a controlled human study, the real ileal digestibility of pea protein averaged 93.6% ± 2.9% versus 96.8% ± 1.0% for casein — a difference that was not statistically significant (P = 0.22) — yet pea protein isolate had a DIAAS of 1.00 against casein’s 1.45 in that trial (higher than the ~1.1–1.2 casein DIAAS reported in most datasets), because leucine, valine, lysine and phenylalanine were each significantly less digestible in pea than in casein (Am J Clin Nutr, 2021, PMID:34665230). Scoring at the level of single amino acids captures gaps that a whole-protein average hides.
Relevance to potato protein
Ileal-based scoring is favorable to potato protein. The DIAAS for potato protein isolates has been reported as high as 100 (Food Science & Nutrition, Herreman et al., 2020, PMID:33133540), placing it among plant proteins that match high-quality animal references. For more background on the ingredient itself, see what potato protein is and how it is made.
Related terms