Protein, which forms the building blocks of bones, muscles, and hormones, is an essential nutrient for sustaining life. Even if you consume enough calories, a lack of protein can lead to low levels of albumin in the blood, causing symptoms such as swelling, abdominal bloating, and abnormalities of the skin and hair. For this reason, we often hear the advice, “Make sure to consume high-quality protein.” But what exactly does “high-quality protein” mean?
Proteins are made up of many amino acids, which are divided into essential and non-essential amino acids. Essential amino acids are those that cannot be synthesized in the body. Humans require nine essential amino acids, all of which must be obtained from the diet. The protein we eat is not absorbed as intact protein, but is broken down into amino acids, absorbed, transported throughout the body, and then reassembled into the proteins the body needs. In other words, the nutritional value of protein lies in its ability to supply essential amino acids that the body cannot produce on its own.
Therefore, the key measure of “high-quality protein” is how well it contains essential amino acids and how efficiently they can be consumed. There are several methods of evaluation, but one commonly used indicator is the amino acid score. If a protein meets the required amounts of all essential amino acids, it receives a score of 100.
Soybeans, often called “meat of the fields,” have an amino acid score of 100, including processed products such as tofu and natto. Legumes generally have high amino acid scores, although some are low in certain essential amino acids. However, in Japanese cuisine, these deficiencies can easily be compensated for. Legumes tend to be low in methionine but rich in lysine, whereas grains are often low in lysine but rich in methionine. Therefore, eating rice together with miso soup or other soybean products allows these foods to complement each other, making up for the essential amino acids each lacks.