Protein Digestibility Corrected
Amino Acid Score
Indispensable amino acids -
The present internationally accepted categorization of food protein quality, PDCAAS
(Protein Digestibility Corrected
Amino Acid Score), measures protein quality as a ratio of its
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,
Amino Acid Score (AAS) is given by (Limiting EAA/Reference EAA),
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
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
(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
more than 4 times greater than
the EAA requirement for an adult.
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
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).
limiting amino acid
in egg protein, which is for this age group Tryptophan, puts the egg protein Amino Acid Score (AAS) at
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
this still leaves the child with 1500 calories energy deficit
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
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
satisfy most of their protein needs from so called
"lower quality protein" sources,
with less concentrated protein content. But the "high-quality"/"low-quality" (or "value",
"potency", etc.) labels make it look like inferior choice.
The fact is that what is - officially and causally - presented as
protein "quality" is merely a quantitative indicator. From the
standpoint of a balanced, healthy diet, it is exactly the opposite. It
is the foods with lower protein "quality" ("potency", "value", etc.) - or,
foods with lower protein content relative to other basic nutrients -
that are nutritionally superior as a
principal protein source.
Aside from this blatant neglect of the broader nutritional context
when determining what is actually a quality protein food, another major
PDCAAS drawback is a complete
neglect of the optimum
amino acids ratio
in a protein.
Animal proteins, having in general higher relative essential amino acid
content satisfy the need for them sooner than the need for non-essential
amino acids, especially in adults. This makes foods of animal origin
less efficient protein source than indicated by their PDCAAS rating, in
comparison with plant foods, which have generally lower
essential-to-non-essential amino acid profile, closer to the official
old vs. adult protein needs
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
As a consequence,
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
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
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
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
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.