Protein quantity plant-based diets is shown not to be an issue. Inadequate amino acid supply is not an issue with most cereal-based diets.
When used to score plant-based diets in India, no marked deficiencies are identified. All regions score > 1 for adults, whilst for children scores range from > 1, (Tamil Nadhu) from 6 months of age to 0.78 (West Bengal), rising to 0.9 in the 2-5 year old, consistent with reports that high-lysine maize supports similar weight and height growth to that of casein.
Digestibility is identified as a problem for some cereals (millet (Panicum miliaceum) and sorghum (Sorghum sp.)) and generally is poorly understood.
A new maintenance requirement pattern is developed, with higher values than those of Food and Agriculture Organization / World Health Organization / United Nations University (1985) but lower values than the Massachusetts Institute of Technology pattern (Young et al. 1989).
Calculations of age-related amino acid requirements are based on most recent estimates of human growth and maintenance protein requirements, a tissue amino acid pattern and the new maintenance amino acid pattern. These values appear valid when used to score plant proteins, since they indicate values similar to or less than the biological value measured directly in young children.
The protein digestibility-corrected amino acid score (PDCAAS) has been adopted by FAO/WHO as the preferred method for the measurement of the protein value in human nutrition.
PDCAAS = Amino Acid Score x Digestibility
The method is based on comparison of the concentration of the first limiting essential amino acid in the test protein with the concentration of that amino acid in a reference (scoring) pattern. This scoring pattern is derived from the essential amino acid requirements of the preschool-age child.
Although the principle of the PDCAAS method has been widely accepted, critical questions have been raised in the scientific community:
- the validity of the preschool-age child amino acid requirement values (more than 4 times greater than the EAA requirement for an adult),
- the validity of correction for fecal instead of ileal digestibility,
- the truncation of PDCAAS values to 100%.
The reference scoring pattern was based on studies performed more than 25 years ago on a limited number of 2-year-old children recovering from malnutrition.
According to the current official recommendations, a 2-year old child needs ~ 3x higher essential-to-non-essential amino acid ratio, and needs essential amino acids in different proportions than adult. Methionine/cysteine is the limiting essential amino acids for adults, and for children it is lysine or tryptophan.
The use of fecal digestibility overestimates the nutritional value of a protein because amino acid nitrogen entering the colon is lost for protein synthesis in the body and is, at least in part, excreted in urine as ammonia.