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Indispensable amino acids
Body protein - Daily requirement - Indispensable amino acids - Protein quality - PDCAAS - Optimum intakeBy definition, essential or indispensable amino acids are those that your body cannot synthesize, thus it has to obtain them preformed from food. Out of 22 amino acids commonly occurring in foods, 8 are essential for adults - isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine - and two more - histidine and arginine - are also essential for children and infants.
The FAO/WHO recommended safe intake of indispensable, or essential amino acids (EAA) is given in the table below. Histidine is essential for infants and small children (decreasingly with age), while arginine is made by the body at all ages, only at a slower rate in the early years. Numbers in brackets are the relative fraction of the specific EAA in the total.
The bottom row gives corresponding ideal fraction of the essential amino acids (EAA) in the total of amino acids, or per gram of protein (gP). It is obtained as a ratio of the EAA total vs. recommended daily protein intake per kg of body weight, which is 0.75g, 1g, 1.2g and 2g for adults, ~10 year old, ~2 year old and infant, respectively.
The recommended adult EAA intake level is determined experimentally, by feeding study subjects with mixtures of EAA in various proportions, until achieving nitrogen balance without the signs of deficiency.
As a side note, the table illustrates why are so called branched chain amino acids (BCAA), isoleucine, leucine and valine, important for muscle-building; they make only about 1/30 of the total protein intake, but up to 1/3 of the muscle protein amino acids.
For children, EAA requirement is based on the positive nitrogen balance estimated, or observed to be adequately supporting average growth rate (there are indications that the actual needs are lower, but the old recommendation still holds officially), and for infants also on fairly dated observational studies, where the adequate level of EAA intake was determined based of its level found to be seemingly supporting normal growth.
No other nutrient attracts as much attention as proteins and their building blocks, amino acids. Also, no other nutrient seems to be the subject of so many speculations and confusion. Part of it originates from the early standards for human protein needs in general, and needs for indispensable (essential) amino acids in particular, being derived from the experiments on rats. Since plant proteins couldn't support optimum growth in rats, they were dimmed "inferior", and even suspected of not containing all essential amino acids.
The main reason why experimental results on rats are not applicable to humans is that protein needs of growing rats are much higher than those of growing humans - let alone adults. While needed time to double birth weight is 180 days in humans, it is only 4.5 days in rats. This is reflected in the respective milk protein content for the two species, which is ten times higher in rats (12% vs. 1.2%). Plants, on average, simply supply insufficient quantity of total protein to optimally support growing needs of rats.
Human experiments have shown that both, human needs for proteins and patterns of amino acid synthesis are quite different than those in rats. Amino acid contents of many food proteins have been established, showing that all plant proteins
contain all amino acids essential for humans,
and, with the exception of some fruits, in quantities and proportion needed to support optimum health. The latter is also confirmed by numerous human experiments. The fact is that
there is no difference in the chemical structure of an amino acid,
essential or non-essential, whether it comes from an animal, or plant.
Another protein myth is that we need to have all the essential amino acids in nearly optimal proportion at the same meal. The facts don't support this assumption. Your body constantly brakes down and synthesizes proteins,
effectively forming "amino acid pool",
which mostly consists of body's own proteins. Less than 1/5 of the total daily protein use, on average, comes from ingested food. All that matters to the body is that it gets all the amino acids it needs; variations in the proportion of essential amino acids at which they come with different foods are of no practical importance.
Use of indispensable amino acids by the body
is effectively limited by the lowest in proportion amino acid relative to
the ideal proportions. No single food protein, plant or animal in origin,
has what is considered perfect essential amino acid profile.
Protein and essential amino acid assimilation
Another constantly perpetuated misconception relates to protein assimilation (digestibility). The notion is, again, that animal protein is generally better absorbed, and rather significantly so, with plant proteins being, therefore, inferior. In fact, the difference in the rate of absorption is small, and doesn't necessarily favor animal protein sources.
For instance, protein absorption rate for meats, poultry and fish varies from 85% to 99%. And, guess what - other major foods, plant or animal in origin, are pretty much within that same range. Protein absorption rate is 97% for eggs, 81% for cow's milk, 91%-95% for wheat, 90% for corn and soybeans and 85%-93% for legumes.
Considering that the average nominal protein intake in the U.S. is leaning toward over-consumption, you are probably better off with less proteins absorbed into the bloodstream anyway. A big unknown is how much of your nominal protein intake is actually assimilated at the cellular level. This depends not only on your individual genetics, digestion, intestinal absorption and cellular metabolism, but also on your choice of foods and diet.
For instance, predominantly raw food diet will likely give you better assimilation rate than predominantly cooked/processed food diets.
In other words, if your diet is predominantly based on cooked and processed foods, what seems to be relatively large protein consumption may be effectively considerably smaller.
table shows essential amino acid content of some common foods.
Essential amino acid contents for selected foods in mg/100g8. Numbers in brackets are % of the specific essential amino acid in their total for the food protein (limiting amino acid for each food, based on the official recommended intake, is in red for adults, in blue for preschool children, and in green when identical for both); gP stands for "gram of protein", EAA/P for EAA-to-total-amino-acids ratio, unadjusted for limiting EAA. AAS and AAS is amino acid score based on the official pre-school child and adult requirements, respectively. Protein % is by the weight, and calories % is for the protein calories in the total of calories.
For really no justifiable reason, the official reference amino acid pattern for determining protein quality is that for preschool children (2-5 years, highlighted in blue). Since it differs significantly from the official reference amino acid pattern for adults, especially in the methionine/cysteine requirement, protein quality ratings for adults are often in disaccord with the official recommended intake for individual amino acids.
Possible origin of this contradicting, illogical official stance is the discrepancy between adult recommended values based on nitrogen balance method, and those more recent, based on carbon tracing oxidized amino acids. Faced with the strong evidence of significantly higher essential amino acid requirements for adults than what the current official recommendation is, with the needed "perfect" essential amino acid (EAA) pattern being likely closer to that established for pre-school children, FAO/WHO officials appear to be taking the middle road. They wouldn't go so far as to change the official adult requirement, but when it comes to determining protein "quality", they act as if, in fact, it isn't valid anymore.
Just how hard it is to satisfy your protein/EAA needs? Not as hard as you may think. Even taking a food with the lowest protein content in the above table, baked potato, as the only food consumed, you need not to worry. If you are an average 75kg male, consuming all your daily 2800 calories from baked potato (about 3kg of it) would provide you with a total of 23g of essential amino acids, and 62g of total protein.
Your total protein intake from potato alone, at 0.83g/kg, is 10% greater than the safe recommended level of 0.75g/kg. It is even better with indispensable amino acids supplied: corrected for the limiting amino acid ratio of 7/16 (methionine + cysteine), it comes to 10g of the effective EAA supply, or 60% over the recommended safe minimum of 6.3g a day.
However, potato-only-diet would fall short in supplying non-essential amino acids, providing 39g vs. officially recommended 50g. While 39g would still be within satisfactory for most individuals, it would probably be sub-optimal for some.
Still, not bad for a food at the bottom of the official "protein
quality" list. Obviously, if your
caloric intake is significantly lower than average, you do need foods with
higher protein and EAA content to satisfy your body's needs.
The limiting amino acid concept
The effective essential amino acid content of any particular food is determined based on its lowest in proportion individual essential amino acid. In other words, amino acid with relative proportion most below that in the ideal amino acid pattern profile, will reduce the use of all the others essential amino acids from that food protein by that ratio. Nominal content of other essential amino acids exceeding the level determined by the lowest one, are effectively wasted.
The amino acid that is lowest with respect to perfect (reference) EAA proportions is called the
limiting amino acid,
and the ratio of its relative proportion vs. relative proportion of that same EAA in the ideal (reference) protein is so called amino acid score (AAS).
In the above table, the limiting amino acid (the numbers highlighted in red, green and blue) for adults is methionine/cysteine in all foods but wheat, for which the limiting amino acid is lysine.
Note that these EAA proportions are based on the official recommendations for each separate group. If the proportions for 2-year old are applied to all - as the FAO/WHO, contradicting itself, does when it comes to rating protein quality - the relative need for methionine/cysteine diminishes, resulting in better overall utilization of most food proteins by the body.
There is an obvious discrepancy between the nitrogen-balance-optimized essential amino acid profile (first column), and that of the egg, regularly referred to as a "model protein". The latter is significantly lower in both, methionine/cysteine and tryptophan (although all other foods here, except wheat, are even lower in methionine/cysteine, which seems to be the limiting essential amino acid for most foods, for adults).
However, if more recent scientific estimates of the ideal EAA profile are more accurate - which is more likely than not - foods that are presently on disadvantage due to the "official" lack of methionine/cysteine are actually nearly as good EAA-wise as egg protein.
It not only illustrates how questionable are the current standards, it also shows that
there is no perfect food, not even for protein intake alone.
The closest to it - among the foods listed - is a wheat/egg combination. However, their respective PDCAAS values don't indicate this.
It is another drawback of the PDCAAS concept which is an erroneous, misleading concept from the standpoint of a healthy diet. It made many people believe that the best protein source for them are highly-concentrated protein foods, with high proportion of essential to non-essential amino acids. The truth is that making such foods the primary protein source inevitably leads to
over-consumption of proteins and unbalanced, unhealthy diets.
While the debate on how much of what amino acid is needed by the body continue, you don't need to be overly concerned about it. Unless you are on a severely calorie-restricted diet, you are likely getting all the protein you need, and then some. As mentioned before, nominal protein intake in the U.S. significantly exceeds the recommended safe intake levels.
Keep in mind, however, that despite an adequate nominal intake you still may be protein-deficient.
CONTINUES: Protein quality
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