Body proteins Amino acids in mother's milk History of amino acids Source of life

	Water accounts for 60-70% of our body weight, followed by amino acids (proteins) with 20%. Thus, a person weighing 50 kg consists of about 10 kg of proteins, which make up important tissues of the body such as muscles, gastrointestinal tract, internal organs, hemoglobin in the blood, and the collagen of our hair and skin. The constituents of these proteins are amino acids.

About 500 kinds of amino acids have been discovered in nature. However, only 20 kinds (Table) serve as the constituents of our body proteins. Complicated combinations of these 20 kinds produce as many as 100 thousand kinds of various proteins. When we eat food such as meat, fish, and cereals, the proteins contained in it are first degraded to the 20 kinds of amino acids, and then reassembled into proteins in the body.

Valine, Leucine, Isoleucine, Alanine, Arginine, Glutamine, Lysine, Aspartic acid, Glutamate, Proline, Cysteine, Threonine, Methionine, Histidine, Phenylalanine, Tyrosine, Tryptophan, Asparagine, Glycine, Serine

Amino acids are always synthesized and decomposed time and again in the body. Of the 20 kinds of amino acids that serve as the building blocks, 11 kinds can be synthesized in the human body when needed, whereas the remaining 9 kinds can not be synthesized in the body, so we must take them from our food. These are called essential amino acids. "Essential" means that we must necessarily take them from food. Although the other amino acids are called nonessential amino acids, they are also vital to make up the body. New born babies can thrive on mother's milk only. Mother's milk contains abundant amounts of amino acids, including essential amino acids (Figure).

In 1806 an amino acid was first discovered from asparagus shoots in France, and was named asparagine. After this, cysteine, glycine, and leucine were found from urinary calculus, gelatin, and muscles/wool, respectively. All the protein-constituting amino acids were discovered by 1935. In 1866 glutamate, which is familiar to us, was isolated by Ritthausen (Germany) from gluten, a wheat protein. In 1908 Dr. Kikunae Ikeda (Japan) discovered that glutamate is the Umami component of sea tangle. After amino acids were found to be responsible for the secret of deliciousness, the various properties of amino acids were studied in Japan in great depth.

Many theories have been presented to explain the origin of life: Some claim that life is of extraterrestrial origin, some believe that life began in the atmosphere, and some hold that the sea is the cradle of life. In all cases, however, amino acids are said to be the source of life. Some meteorites which collided with the earth after a long journey from the remotest corner of the universe contain amino acids. Trace amounts of glycine, alanine, glutamate, and -alanine were detected in a meteorite that struck Murchison (Australia) in 1969. The amino acids in meteorites are considered a trace of life elsewhere in the universe. A trilobite fossil dating back to 500 million years ago was found to contain amino acids such as alanine. Science continues its search for an answer to the intriguing mystery of the origin of life by studying the amino acids detected in fossils and meteorites. Amino acids are important substances which create life itself.

A Good Combination of dishes, for example, rice and miso soup that are the staples of the Japanese diet represents an ideal balance of amino acids. Rice and wheat contain 7% and about 11% of proteins, respectively. However, their proteins are different in amino acid profile. Since wheat contains 3 essential amino acids, lysine, methionine, and threonine in small amounts, it is necessary to compensate the deficient amino acids by eating meat and dairy products. Beans contain a large amount of lysine which tends to be insufficient in rice. Conversely, rice contains a large amount of methionine which is lacking in beans. Therefore, it can be said that steamed rice and bean products such as miso and tofu traditionally eaten in Japan constitute an ideal combination to make sure that all the essential amino acids are ingested. Traditional dietary habits in various parts of the world seem to reflect the secrets and wisdom of ancient people in matters of health.