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The present internationally accepted categorization of food protein quality, PDCAAS (Protein Digestibility Corrected Amino Acid Score), measures protein quality as a ratio of its limiting essential amino acid level vs. level of this same essential amino acid (EAA) in the reference "ideal" protein, corrected for digestibility. The formula is simple:

PDCAAS=Amino Acid Score x Digestibility,

where the Amino Acid Score (AAS) is given by (Limiting EAA/Reference EAA), with the limiting and reference essential amino acid are given in mg/g of protein, and digestibility as a ratio number from 0 to 1. Protein quality is mainly dependant on the AAS value, with digestibility for most foods being in the 0.85-0.95 range.

The reference essential amino acid profile recommended by FAO/WHO for all age groups is that considered optimal

for pre-school child, 2-5 years of age.

Why is that flawed as a protein standard for adults? For one, because the needs for protein are very different for a small child vs. adult. Daily total essential amino acid requirement alone (mg/kg/day) of a 2-year old child, at 342mg/kg per day, is about equal to the total protein requirement of an adult. In other words, it is

Since the recommended daily protein intake for this age group is 1.1g/kg (nearly 50% over the corresponding adult requirement), the ideal protein here would need to have 31% of essential amino acids in the protein total. That compares to the ideal (officially) 11% essential amino acid in total amino acids for adults.

In other words, high AAS and E/T (essential-to-total amino acids) ratio proteins - in general those of animal origin - would be closer to that ideal protein for children, while lower AAS and E/T ratio proteins - in general those of plant origin -

would be closer to the ideal protein for adults.

But how good a food is an "ideal" protein? Consider this example: a
4-year old child of 17kg would require about 19 grams of food protein a day. Probably the closest to its ideal food protein is egg protein (also beef or tuna, but animal flesh shouldn't be primary protein source at this age, because the enzymes needed for meat metabolism - such as carnosine and sulfotransferases - are not fully active, which can result in taurine deficiency, or disturbances in sulfur metabolism).

Since the limiting amino acid in egg protein, which is for this age group Tryptophan, puts the egg protein Amino Acid Score (AAS) at 0.71 (for
a 4-year old; for adults, the AAS for egg protein is 0.63, the limiting amino acid being also Tryptophan), and the egg protein digestibility is 0.97, its ratio of utilization is 0.69. This is about 15% higher utilization level than the average 0.6 ratio used to obtain the minimum safe intake level. Hence a child would satisfy its minimum daily protein requirement with about 19/1.15=16.5 grams of egg protein, or about two and a half large eggs.

That would also provide some 13g of fats (including some essential fatty acids, if the hen was consuming natural foods), as well as about 1.8g of carbohydrates.

In all, protein needs of the child would be satisfied with a caloric intake of some 185 calories, assuming that most of the protein is used for tissue building, and little or none is burned for energy. But

this still leaves the child with 1500 calories energy deficit

(from the recommended 100cal/kg energy intake for this age). Even if this caloric deficit is compensated by consuming the lowest-protein natural foods - fruit - the protein intake would nearly double (for near-balanced mix of apples, bananas, grapes and oranges). However, that would be far from healthful, with excessively high fructose intake (about 2kg, or 4.5 pounds of fruit a day), and seriously deficient in complex carbohydrates and fats.

More realistic diet, that would include grains and vegetables, would have the total protein intake up to several times the requirement - an unhealthy excess of its own.

The conclusion is that even for children, and especially for adults, it is

Applying the preschool child protein standards to adults is another major PDCAAS inconsistency. According to the current official recommendations, a 2-year old child not only needs nearly three times higher essential-to-non-essential amino acid ratio, it also needs essential amino acids in significantly different proportions than adult. The discrepancy is greatest for methionine/cystine which, according to the current official recommendation, adults need in twice greater proportion.

Needed proportions of the essential amino acids (WHO)

As a consequence, methionine/cysteine is the limiting essential amino acids for adults in nearly all foods, except grains, where lysine can be somewhat lower. For children, the limiting amino acid is most often either lysine in grains, or tryptophan in animal proteins. Obviously, according to these very criteria, optimum protein for pre-school child can not be the same as that for an adult, if these recommendations are to have any resemblance of validity.

For instance, when measured against the perfect EAA pattern for
2-year old child, egg protein has amino acid score of 0.71 (limiting EAA Tryptophan). And when measured against the perfect EAA pattern for adults, its score is 0.63 (limiting EAA again Tryptophan). Tofu AAS is 0.84 on the preschool scale (limiting EAA lysine), but only half as much - 0.42, limiting EAA being methionine/cystine - on the adult scale. When it comes to measuring protein quality, it is all of a sudden 0.84 for both.

As mentioned previously, this confusing, self-contradicting concept is at least in part motivated by more recent experimental results in the area of human protein needs, indicating significantly higher requirements for essential amino acids for adults than the current official recommendation, based on the older studies. These newly proposed adult amino acid protein needs are more similar to that for a pre-school child, especially for methionine/cystine.

But the "solution" of keeping the old adult recommendation official, while at the same time applying the preschool child protein standard to adults when determining protein quality is directly contradictory - a

logical nonsense.

The official pre-school child protein requirements are still significantly higher than what the newer research indicates for adults, and differences in the optimum amino acid profile are not negligible.

Throwing children and adults in the same category with respect to protein "quality" is even more inappropriate considering that the pre-school child protein requirement is based on a single, somewhat obscured study at the Institute of Nutrition of Central America and Panama, and also don't agree with later experimental results, indicating lower needed intake. 

In all, when it comes to the official protein intake standards and recommendations, the situation is still very much unsettled and unresolved. One would expect better from the world experts. Is having  at least the basic concept set straight too much to expect?

One could say it doesn't really matter, since the PDCASS' protein quality categorization is, from the standpoint of a healthy diet, already turned up side down. And we have more important things to worry about. At the average daily intake of proteins in excess of 100g, average American consumes nearly double the recommended safe protein intake - at least nominally. This implies that the average risk of protein deficiency is very low. On the individual level, it is still possible, and shouldn't be ruled out in assessing causes and treatment options for health conditions.

CONTINUES: Conclusion