Protein Quality
The concept of protein quality, how it is calculated, and misconceptions about it.
Overview
One often hears the claim that plant protein is a low-quality incomplete protein, and that if one only follows a vegetarian diet, one’s health may worsen due to protein deficiency. But such claims either lack a nutritional basis or are grounded in outdated research.
Most people who eat a Western diet are already consuming excessive amounts of protein. Because of the protein myth spread by the meat industry, dairy industry, and so on, many people end up eating excessive amounts of meat, but according to numerous studies, animal-based diets (cows, pigs, chickens, eggs, cow’s milk, etc.) cause or worsen heart disease, obesity, diabetes, breast cancer, prostate cancer, colon cancer, autoimmune disease, kidney disease, and eye disease, whereas vegetarianism, especially a whole-food plant-based diet, is effective in preventing or improving these diseases, so if you care about your health, it is better to reduce meat consumption as much as possible and, if possible, eat a vegetarian diet, especially a whole-food plant-based one.
Protein, essential amino acids, and amino acid composition
Human beings need various kinds of amino acids to sustain life. Some of these amino acids are not produced sufficiently within the body, or are not produced at all, and therefore must be obtained through food. These are called essential amino acids.
Amino acids are consumed through protein. Protein is a molecule that looks like a long bundle of threads made by linking together many different kinds of amino acids. When protein is consumed, it is broken down into amino acids during the process of digestion. Depending on the type of protein, there are differences in which amino acids it contains and in what amounts. This is called the protein’s amino acid composition.
Amino acid composition and limiting amino acids
Because the minimum amount humans need differs for each kind of essential amino acid, there is a need to quantify this. For example, by World Health Organization standards, isoleucine must be consumed in twice the amount of histidine.
It would be best if the ratio of essential amino acids contained in a given protein exactly matched the ratio humans need, and the only such ideal protein is human flesh. But since we do not eat humans (in most circumstances), we cannot match this ratio exactly.
Instead, if we know which amino acid in a protein source is present in the lowest relative amount compared to what humans need, we can know how much of that protein must be consumed in order to obtain essential amino acids without deficiency. This amino acid with the lowest relative content is called that protein’s limiting amino acid.
Limiting amino acids and protein quality
If protein is the signboard and amino acids are the alphabet letters contained in the signboard, then one can think of the following analogy.
The human body is a signboard recycling factory. Every day, this factory has to extract the letters from signboards collected from outside (consumed protein) (break them down into amino acids) and make ten new signboards containing the word “HELLO” (the various proteins needed in the body). To do that, it needs 10 each of the letters H, E, and O, and 20 of the letter L every day.
If this factory collected 10 signboards saying “HOLE,” then it could only make 5 signboards saying “HELLO.” That is because it lacks Ls. Therefore, in order to produce its daily quota of 10 “HELLO” signboards, it would have to collect 20 signboards saying “HOLE.”
In the analogy above, if “HOLE” is the protein we consume, then the limiting amino acid of this protein is “L,” and the protein quality of this protein is 0.5. That is because the limiting amino acid is only half of the required amount (1/2 = 0.5).
Protein quality is an index that reveals the composition of the amino acids obtained by digesting a protein in the human body, especially its limiting amino acid. Therefore, one should not think in terms of the amino acids absorbed through so-called “low-quality” protein somehow being low in quality. There is no such concept as the quality of an amino acid. A particular amino acid is exactly the same molecule whether it is consumed through animal protein or through plant protein.
Other indicators for calculating the nutritional value of protein
There are various indices for evaluating the value of protein depending on how one calculates the ratio of essential amino acids needed by humans, how one measures the degree to which protein is digested and amino acids are absorbed into the human body, and so on.
At present, the standard used by the Food and Agriculture Organization under the UN and the World Health Organization (FAO/WHO), as well as the U.S. Food and Drug Administration, is PDCAAS (Protein Digestibility Corrected Amino Acid Score). PDCAAS assumes that all amino acids consumed, except those excreted, have been absorbed. However, it has been criticized as lacking accuracy because it does not take into account cases where they are absorbed by gut microbes, among other things.
In 2013, a new index called DIAAS (Digestible Indispensable Amino Acid Score) was proposed to address the shortcomings of the existing PDCAAS. DIAAS is more accurate because it measures the amino acids actually absorbed in the small intestine.
By PDCAAS standards, soy protein is relatively high at between 0.91 and 1, and pea protein is 0.89. By DIAAS standards, soy protein is 0.9, and pea protein is 0.82. (For reference, beef is 0.92 by PDCAAS standards.)
Complete protein and incomplete protein
The belief that plant protein is incomplete protein and animal protein is complete protein is a myth. In reality, almost all protein we eat in everyday life is complete protein, and the only incomplete protein is gelatin. Gelatin is an animal protein.
For details, see complete protein.
Undigested protein and protein excess
Not all consumed protein is always well broken down and absorbed. Most undigested protein is excreted through the large intestine, but in diets centered on meat-eating, there is a risk that, because of a lack of fiber, it stays in the large intestine for too long and causes cancer. In infants, undigested protein can enter the bloodstream, and this can also be a cause of allergic reactions.
If protein is consumed in amounts exceeding what is needed, too much ammonia can be produced in the body, which can put strain on the liver and kidneys and become the cause of various diseases.
In South Korea and most other developed/developing countries, people are already consuming more protein than necessary, yet many people worry only about protein deficiency. What is commonly called a “low-protein diet” is in fact an “adequate-protein diet.”
Plant protein and animal protein
As examined above, the distinction between complete protein and incomplete protein is not very meaningful. From a nutritional perspective, a far more important distinction than that is plant protein-animal protein.
Animal protein worsens kidney disease, whereas plant protein has been shown through a large-scale study (11,952 people) to be effective in preventing kidney disease. This is because of the inflammatory response that occurs when animal protein is consumed. In fact, there is also a study result showing that when animal protein is consumed together with anti-inflammatory drugs, it does not put strain on the kidneys.
In addition, animal protein tends to contain more sulfur-containing amino acids than plant protein, and in the metabolic process this generates sulfuric acid. This too is one of the factors that burdens the kidneys.
Since most people living in affluent countries, including South Korea, are already consuming enough protein, rather than calculating protein quality, one can obtain sufficient essential amino acids simply by not being excessively picky in one’s diet. If you care about your health, stopping meat consumption and eating a whole-food plant-based diet is far more important than calculating protein quality.
Conclusion
To summarize, it is as follows.
- Complete protein and incomplete protein are a false dichotomy.
- The “quality” of protein merely refers to differences in the amino acid composition obtained by digesting protein; there is no such concept as a “high-quality amino acid” or a “low-quality amino acid.”
- In South Korea and most other developed/developing countries, the problem is not protein deficiency but excessive intake of animal protein due to meat-centered diets.