Protein quality

Body protein - Daily requirement - Indispensable amino acids - Protein quality - PDCAAS - Optimum intake

Are we being mislead with the use of term protein quality? Perfect protein food would contain just the right amount of protein, which itself would contain just the right proportion of non-essential and essential amino acids. While we can easily determine what foods would have just the right amount of protein (for adults, with 10%-15% of the calories coming from protein, such ideal food is not protein-concentrated), it gets more complicated when it comes to the question of protein quality.

Both, current concepts of protein quality and the terminology used are

flawed and misleading.

Here's why.
 

The four most commonly used indicators of protein quality are:    

The first indicator, so called "protein efficiency ratio" is based on the weight gain of growing rats fed with specific foods. The next two protein "quality indicators" express proportion of nitrogen/protein retention versus intake (the former for protein maintenance, the latter for growth). The higher retention, the more effective use of the protein.

The AAS is a nominal indicator, based on fairly confused official ideas of what are the ideal essential amino acid proportions in a protein. It expresses the expected effective essential amino acid content of a protein, based on the lowest in actual proportion essential amino acid, so called limiting amino acid.

It is rather obvious that these indicators are

expressing quantitative,
rather than qualitative protein attributes.

Higher AAS doesn't mean you get better amino acids, only more of them. It doesn't mean that plant proteins, on average supplying less amino acids per gram than animal proteins, are of lower quality - even if they are often referred to as such, according to these erroneous criteria. Any particular

amino acid is chemically identical, and to the body makes no difference whether it comes from an animal or plant protein.

Somehow - and for no justifiable reason - the amount of amino acids obtained from a given quantity of protein has been proclaimed to be a measure of its quality.

For illustration, here's the raw protein content (% of total food calories) of some common foods, unadjusted for the effective reduction due to their (always) imperfect amino acid profiles:

FRUIT: apple 1.5%, apricot 12%, banana 4.5%, cantaloupe 10%, grapes 4%, orange 8.5%, watermelon 8%

MEAT and DAIRY: beef (pan fried) 33%, cheddar cheese 25%, chicken (light and dark, with skin, fried) 31%, boiled egg 32%, pork (rib, pan fried) 22%, tuna (canned, in water) 88% (59% in oil), whole cow milk 20%

BREADS, PASTA and RICE: cornbread 8.5%, white bread (wheat) 12.5%, whole wheat bread 16%, pizza crust 14.5%, white flour pasta (wheat) 14%, whole wheat pasta 17%, brown rice (long grain, cooked) 9%, white rice (long grain, cooked) 8%

NUTS: almonds 14.5%, filberts 13.5%, peanut (dry roasted) 16%, pumpkin seed (dried) 18%, sunflower seed (dried) 12.5%

BEANS: green beans (boiled) 22%, kidney beans (boiled) 27%, soybeans (roasted) 35%, tofu (raw) 42%

VEGETABLES: broccoli (raw) 44%, green cabbage (raw) 20%, kale (boiled) 23%, carrot (raw) 9%, cauliflower (raw) 33%, corn (yellow, boiled) 12%, eggplant (boiled) 13%, lettuce (butterhead) 40%, mushrooms (boiled) 32%, potato (boiled w/o skin) 8%, French fries 5%, tomato (raw) 15%

Recalling that the range of adequate protein intake for the average level of caloric intake is 10%-15% of the total caloric intake (thus higher for lower than average caloric intake, and vice versa), foods that fit nearly perfectly in this respect are breads/grains and nuts, with most other foods being protein excessive, from vegetables in the low 20%, to beans in the high 20%, and meats exceeding 30%.

Those with clearly insufficient raw protein content are found only among fruits, with not a few fruits being close to, or even exceeding the 8-10% minimum nominal protein requirement.

An interesting question is why our current official protein requirements are

below the average protein content of most foods

(other than fruit) both, foods of plant origin and even more so foods of animal origin.

As for the former, if we include fruits, the combined average does come quite close to our current 10%-15% range. As for the latter, it is possible - even probable - that surviving under extreme conditions that our predecessors had to endure for hundreds of thousands of years did require foods of higher protein content.

Meat may have been providing bulk of that extra protein needed. Chances are, we don't need it anymore, but our bodies - and our minds - haven't gotten enough time to factor it in.

Turning back to protein "quality" indicators, let's look at them for what they are: quantitative protein indicators. None of the three is reliable even as a quantitative indicator, for the following reasons.

∙ All three, PER, BV and NPU are determined mainly on rats, which have different (higher) protein requirements and metabolism than humans

∙ Protein metabolism turned out to be more complex than a simple "linear" processing. Measurements of both, BV and NPU yielded unexpected results: a zero AAS proteins (with one of the essential amino acids entirely missing), would still result in BV as high as 0.4 (40%) and 0.25 NPU. The reason is body's metabolic ability

to adjust to the reduced intake of one or more
essential amino acids

and manage to retain more of it than in the normal circumstance - a pattern seemingly common to several species, including humans19.

∙ Another factor that BV/NPU do not account for is that the ratio of protein absorption/utilization varies with the protein intake level. In general, it is at the highest with intakes well below the protein maintenance level, dropping as the intake increases. For instance, milk protein BV is near 1 (100%) at the intake level of 0.2mg/kg, but only about 0.7 at the near-maintenance level of 0.5mg/kg.

Protein Amino Acid Score (AAS), while not perfect, is considered to be more reliable indicator of protein quality than either PER, BV or NPU. However, that itself doesn't make it reliable. A frequent problem encountered here is the

indiscriminate use of the "reference protein",

which is often times assumed to be egg protein (sometimes cow milk), instead of the FAO/WHO essential amino acid reference pattern. This hides inefficiencies of the "reference protein" (and to a certain extent animal proteins in general), while creating superficial deficiencies in plant proteins for those essential amino acids that are proportionately excessive in the "reference protein".

Probably the most graphic illustration of how grossly approximate are the official protein-intake-related figures is FAO/WHO decision to base their general protein "quality" indicator, PDCAAS, on the official essential amino acid needs of pre-school children, despite it being both significantly higher overall, and structurally different than officially still standing essential amino acid intake recommendation for adults (bottom two rows of the table).
 

There is also the problem of the

neglect of the indispensable amino acids
to total protein (E/T) ratio.

By definition, the AAS shows the ratio of the lowest in proportion (or limiting) indispensable, or essential amino acid (EAA) in a protein versus the lowest EAA in the reference pattern. With the FAO/WHO's 84mg/kg/day of total daily EAA requirement for adults, the adequate EAA daily intake for a 75kg person is 6.3g. At the same time, the corresponding total protein intake (including both, essential and non-essential amino acids) required for maintaining body's protein levels is 750mg/kg/day. This puts near optimum proportion of the essential amino acids in the total of amino acids at about 11%.

Egg protein - which is often quoted as "reference protein" - has the essential-to-total amino acids proportion of 0.46, or 46% - nearly fourfold higher. With the limiting amino acid (Tryptophan) reducing it effectively by a factor of 0.63 for adults, it effectively becomes 29% and, after recalculating with the remnant of 54% of the non-essential amino acids, 35% in the total. This is still more than threefold higher essential amino acid proportion than officially desired. As a result, at the intake level where the need for essential amino acids is satisfied, the need for non-essential amino acids is not, and requires additional ~37% intake of the protein. This makes egg protein

27% less efficient protein source

than when based on satisfying the essential amino acid requirement alone. Also, it can never supply the amino acids in near-optimal proportion.

Relative abundance of essential amino acids at the expense of non-essential ones lowers the efficiency of animal proteins, with higher essential amino acid proportion, more than it, on the average, does with plant proteins. However, the difference is not dramatic. Beef comes at 26% effective essential amino acid content, cheddar cheese also at 26% and tuna (in oil) at 32%. On the other hand, plant sources are closer to the ideal proportion: brown rice is at 27%, kidney beans at 24%, whole wheat 22%, tofu 23%, potato 20%, and so on.

In all, Amino Acid Score as a protein quality indicator has three major drawbacks. It

∙ neglects the non-essential amino acid intake requirement
∙ neglects non-linear body response to the limiting amino acid level
    ∙ neglects variations in protein absorption with the protein intake level

Nevertheless, the present FAO/WHO food protein quality categorization uses Amino Acid Score, corrected for digestibility factor alone (Protein Digestibility Corrected Amino Acid Score, or PDCAAS). Evidently, it is not very reliable even as a quantitative indicator, and can be seriously misleading when presented as a protein "quality indicator". 

CONTINUES:  Protein Digestibility Corrected Amino Acid Score