Protein Quality: PDCAAS, DIAAS and the Scores of Indian Staples

Protein quality is a measure of how well the protein in a food or a diet meets your need for the nine essential amino acids, once digestion is counted. It matters in India because ICMR-NIN's 2020 allowance of 0.83 g of protein per kg of body weight a day assumes protein of good quality, and a footnote raises the figure to 1 g per kg for people whose diet is cereal-based. Two scores are used to rate quality, PDCAAS and the newer DIAAS. This entry explains both and sits in the proteins and amino acids section.

What is protein quality, and what makes a protein complete?

Quality depends on two things: the amino acids a protein contains and how much of them you absorb. A 2018 review in the Indian Journal of Medical Research defines it as a food's capability to meet the daily amino acid requirements. The nine that count are described in the entry on essential amino acids.

A complete protein, in everyday use, is one that supplies all nine in at least the proportions the body needs; an incomplete one falls short on one or more. ICMR-NIN's Dietary Guidelines for Indians, 2024, do not divide foods that way. They name the limiting amino acids of grains, pulses and nuts, and note that a meal with both pulses and grains is of higher quality than either alone. The label is shorthand for a score, not a property a food has or lacks.

How is an amino acid score worked out?

Divide the milligrams of each essential amino acid in a gram of the food's protein by the milligrams in a reference pattern, and take the lowest answer. That line is the limiting amino acid, and its ratio is the amino acid score. The table does this for Indian staples, using the Indian Food Composition Tables 2017 and the adult pattern ICMR-NIN recommended in 2020. The scores were calculated for this entry with no allowance for digestibility, so they are not PDCAAS or DIAAS values.

Grains are held back by lysine, the cereal gap, and pulses by the sulphur amino acids covered under methionine. A score says nothing about how much protein a food has; NNWA's table of protein in Indian foods ranks foods by grams.

PDCAAS: the score corrected for digestibility

PDCAAS stands for protein digestibility corrected amino acid score. The FAO's 2013 report describes it plainly: the limiting amino acid score is multiplied by protein digestibility, to judge how well a dietary protein can match the demand for amino acids. A joint FAO and WHO consultation recommended it in 1989 and it was adopted in 1991. Digestibility here is a single figure for the whole protein, measured from what is lost in faeces, and the final value is cut off at 100.

The same report lists what critics found wrong with it. PDCAAS gives no extra credit to high-quality proteins, because of the cut-off. It overestimates foods that contain antinutritional factors. It does not adequately account for how available each amino acid is, since lysine damaged in processing still counts. And the report concludes that digestibility measured at the end of the small intestine reflects the amino acids absorbed better than a faecal figure does.

What does DIAAS change?

DIAAS, the digestible indispensable amino acid score, corrects each amino acid by its own digestibility, measured at the end of the small intestine. The FAO's expert consultation recommended in 2013 that it replace PDCAAS. The score is 100 times the lowest ratio of digestible amino acid in a gram of the food's protein to the same amino acid in a gram of the reference pattern. For a single food it may run above 100.

The report gives examples. On the pattern for young children, wheat scores a DIAAS of 40, peas 64 and whole milk powder 122. It suggests, as illustrations and not as rules, that a food might be called an excellent source of protein at 100 or more and a good source at 75 to 99. It also admits the weak point: the consultation judged the data on ileal digestibility in humans insufficient to apply the method in practice, and advised using faecal crude protein digestibility in the meantime.

Protein quality in ICMR-NIN's 2020 requirements for Indians

ICMR-NIN's 2020 report sets the average protein requirement of healthy Indian adults at 0.66 g per kg a day and the allowance at 0.83 g, which is 54 g for a 65 kg man and 45.7 g for a 55 kg woman. The committee took high-quality protein as its premise, removed the PDCAAS corrections that had been applied to safe intakes, and introduced DIAAS into the document. The 2010 allowance had been 1 g per kg.

That lower figure comes with a condition. The report improved the cereal, legume and milk composition of its recommended diet to 3 to 1 to 2.5, from 11 to 1 to 3 in 2010, and a footnote keeps the requirement at 1 g per kg a day for people on a cereal-based diet with low-quality protein. Its model diets show the correction at work: the one for a moderately active man supplies 88 g of protein, of which 71 g is digestible, a PDCAAS of 84 per cent. Use the protein calculator for your own figure.

Which scores have been measured for Indian staples?

Few, and mostly digestibility figures from which a score can be built. No table of PDCAAS or DIAAS values for cooked dal, roti or rice was found in a source that could be opened for this entry, so none is quoted. What exists is a set of studies from Bengaluru that fed labelled foods to healthy Indian adults and to children under two, and measured the true ileal digestibility of their essential amino acids.

The authors of the legume study called their figures lower than expected, and put the gain from dehulling, the step that turns whole moong into moong dal, at about 8 per cent. The article on protein in soya chunks discusses the plant protein usually rated near the top.

How do cereals and pulses raise each other's score?

Each supplies what the other lacks, so the mixture scores higher than either part. ICMR-NIN's 2024 guidelines advise cereals and pulses in a ratio of 3 to 1 by raw weight, with 250 ml of milk a day adding more. Their vegetarian My Plate for the Day supplies 49.0 mg of lysine and 28.4 mg of methionine with cysteine in each gram of protein, against a pattern of 45 and 22.

The arithmetic depends on the grain. From IFCT 2017, 150 g of raw rice with 50 g of toor dal gives protein with 48.8 mg of lysine per gram, just above the pattern. The same ratio made with atta gives 39.4 mg, a score of about 87 per cent, because wheat protein has less lysine than rice protein. Add 250 g of cow's milk and the wheat meal rises to 50.3 mg. The site's vegetable khichdi is the rice and dal version in one pot.

Low-quality protein: who is at risk in India?

People whose protein comes almost entirely from cereals. A review of national diet surveys in the British Journal of Nutrition found that Indian diets take almost 60 per cent of their protein from cereals, and that once PDCAAS was applied with lysine as the first limiting amino acid, all the populations surveyed, particularly rural and tribal ones, appeared to have inadequate quality. The 2018 review cites an analysis putting the share at risk of inadequate quality protein at 4 to 26 per cent across age groups in rural and urban India.

What does the evidence say?

First, the two scores disagree most where it matters most. A study in growing rats, in the Journal of Nutrition in 2015, compared them across 14 protein sources. DIAAS ran from 0.01 for a corn-based breakfast cereal to 1.18 for milk protein concentrate, and untruncated PDCAAS values were generally higher than DIAAS values, especially for the poorer proteins. This is animal evidence.

Second, quality tracks child growth in Indian data, without proving cause. The study of complementary foods found an inverse correlation between the DIAAS of young children's diets and stunting in a national survey, and calculated that adding egg or milk would lift the score from 80 to 100. The 2018 review cautions against attributing stunting to protein quality alone.

A worked example: scoring a dal and rice meal

This is an illustrative case, not a real person. A 34-year-old vegetarian bank employee in Coimbatore weighs 60 kg. Her lunch and dinner together use 150 g of raw rice and 50 g of toor dal. From IFCT 2017 the rice gives 11.85 g of protein and the dal 10.85 g, 22.7 g in all, carrying about 1,107 mg of lysine: 48.8 mg per gram of protein, 108 per cent of the pattern. Lysine is still the lowest of the nine lines, so the amino acid score is 108, cut off at 100.

For a PDCAAS, assume a digestibility of 80 per cent, a round figure inside ICMR-NIN's range of 70 to 85 for vegetarian foods and not a measured value for this meal: 100 times 0.80 gives 80 per cent. Rice eaten alone would score 82 times 0.80, about 66 per cent. The decision that follows is about grams. If her whole diet has this quality or better, with curd added, her allowance is 0.83 g per kg, 49.8 g a day. If it slides back to mostly cereal, ICMR-NIN's footnote applies and the figure is 60 g.

Common mistakes and myths about protein quality

The commonest mistake is to compare scores that were made differently: a DIAAS of 40 for wheat on the young child's pattern and a PDCAAS for a diet on the adult pattern are not on the same scale. The second is to score foods one at a time when nobody eats them that way; the FAO report calculates a mixture of wheat, peas and milk powder at 82 per cent for adults. The third is to let quality stand in for quantity. The article on a high-protein Indian diet deals with the grams.

Safety: high protein intakes and when to see a doctor

Chasing quality with supplements carries its own cautions. ICMR-NIN's 2020 report caps protein at under 40 per cent of energy for adults. Its 2024 guidelines say that high levels of protein as supplement powders are not advisable, and that high protein intakes may worsen kidney function in people who already have kidney disease. The article on whether you need protein powder goes through the choice.

See a doctor if a child is not growing as expected, if you are losing weight or muscle without trying, or if your feet or face swell. A protein score is a planning tool for healthy people. It is no reason to stop or change a prescribed medicine or a diet your doctor has set.

Studying protein quality at NNWA

Protein requirements, amino acid scoring and the planning of vegetarian diets are taught at NNWA, a private skill academy in Kolkata that holds its classes online in English and Hindi. Its courses, the Certification in Nutrition and Sports Nutrition among them, prepare people to coach healthy clients and to refer medical cases to a doctor. None of them is a degree or a dietitian qualification.