# Protein Quality and PDCAAS > Protein quality scoring in MBBS Biochemistry: PDCAAS, DIAAS, limiting amino acids, cereal-legume complementation, biological value, NPU and Indian diet pairing. - Canonical URL: https://prepelephant.com/topics/mbbs/biochemistry/protein-quality-pdcaas - Exam / course: MBBS · Subject: Biochemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Protein Quality and PDCAAS", PrepElephant, https://prepelephant.com/topics/mbbs/biochemistry/protein-quality-pdcaas ## Direct answer PDCAAS — protein digestibility corrected amino acid score — compares a food protein's limiting indispensable amino acid against reference requirements for preschool children and multiplies the result by true faecal digestibility, with the score capped at 1.0: casein, egg and soy sit at or near the ceiling while wheat gluten, limited by lysine, trails near 0.25. The cap hides genuine superiority, and faecal digestibility ignores anti-nutritional factors and ileal absorption, which is why the FAO proposed DIAAS in 2013 — scoring each amino acid by ileal digestibility without a cap. For India the practical translation predates every score: cereals (lysine-poor) and legumes (methionine-poor) complement each other — dal with rice or roti — into a protein of near-complete quality. ## What you must remember - **PDCAAS formula:** limiting-amino-acid score times true faecal digestibility, truncated at 1.0; casein 1.0, egg about 1.0, soy around 0.9, beef about 0.92, wheat about 0.4-0.5, wheat gluten near 0.25. - **Limiting amino acids by food:** cereals limited by lysine, legumes by methionine (with cysteine), maize by lysine and tryptophan; gelatin completely lacks tryptophan — the classic disqualifying example. - **Older indices:** biological value (nitrogen retained over nitrogen absorbed; whole egg scores about 93-100 by convention), net protein utilisation (retained over ingested — the digestibility-adjusted version), and protein efficiency ratio (weight gain per protein eaten, rat-based). - **NPU relationship:** net protein utilisation equals biological value times digestibility — a one-line derivation exams reward. - **DIAAS (2013):** ileal digestibility of each indispensable amino acid, reference pattern across three age groups, no truncation — scores can exceed 100, allowing genuine ranking above the old ceiling. - **Complementation logic:** mixing within the same meal is ideal, though complementation across a day's diet is practically sufficient; rice-dal, khichdi and idli (rice plus black gram, further improved by fermentation) are the Indian standards. - **Requirement anchor:** about 0.8-1.0 g per kilogram per day for adults; the ICMR-NIN 2020 recommendation for Indian adults is 0.83 g/kg/day — the number to quote nationally. - **Conditional essentials:** arginine, glutamine, cysteine and tyrosine become limiting in critical illness — the rationale behind specialised enteral formulas. ## Scoring a meal, worked through Take 100 grams of wheat protein: its lysine content supplies only about 40-50 per cent of the reference requirement, so the amino acid score is set by lysine, and after multiplying by a digestibility near 0.9, PDCAAS lands near 0.4-0.5 — wheat cannot rescue itself, because its other amino acids are plentiful but unusable without lysine to balance them. Now pair the wheat with chickpea protein, whose own score is capped by methionine near 0.6-0.7 but whose lysine is generous. The blended pattern approaches adequacy for both limits — this is complementation, and it is why a roti-dal meal scores like a far more expensive protein. Two practical caveats follow. First, mixing must roughly balance the limiting amino acids; a trace of dal cannot rescue a plate of rice. Second, processing matters: heat-damaged lysine (Maillard products) lowers the usable score even when chemical analysis looks fine — a subtle point DIAAS-era scoring tries to capture through ileal assays. ## How the exam frames it Question one is definitional: write the PDCAAS formula, name the cap, give two high and two low scorers — this is scoring by structure. Question two is the trap: "which has higher quality, casein (PDCAAS 1.0) or a hypothetical protein scoring 1.2 before truncation?" — the cap makes them equal on paper, which is precisely the criticism DIAAS answers; candidates who know why the cap was criticised demonstrate depth. Question three is Indian and practical: a lacto-vegetarian patient asks whether dal alone is adequate protein — the honest answer separates quantity (legumes provide ample protein per serving) from quality (methionine-limited, so pair with cereals), and mentioning the ICMR-NIN adult value of 0.83 g/kg/day converts physiology into counselling. The final viva favourite is pure relationship arithmetic: biological value, digestibility and net protein utilisation — expect to derive one from the others on the spot. ## Frequently asked questions ### How is PDCAAS calculated, and why is it capped? The limiting indispensable amino acid score against a preschool reference pattern is multiplied by true faecal digestibility, then truncated at 1.0 — a convention that prevents any protein from appearing "better than requirement". ### Which amino acid limits cereal and legume proteins respectively? Lysine limits cereals; methionine (with its precursor cysteine) limits legumes — the biochemical basis of cereal-legume complementation. ### Why was DIAAS introduced in 2013? PDCAAS truncation hid quality differences, and faecal digestibility overestimates absorption; DIAAS uses ileal amino-acid digestibility and no cap, permitting scores above 100. ### How do biological value and net protein utilisation relate? Biological value is nitrogen retained divided by nitrogen absorbed; net protein utilisation is nitrogen retained divided by nitrogen ingested, equal to biological value times digestibility. ### Why does dal alone not make a complete protein? Its methionine content falls short of requirement patterns, so cereals — methionine-adequate, lysine-poor — must supply the complementary deficit in the same overall dietary pattern.