Soybeans, which are the principal ingredient in natto, are known to be high in nutritional value.

An oft-cited example is the saying that soybeans are "the meat of the fields." The meaning is that soybeans are rich in proteins and lipids that are the most noteworthy nutrients contained in beef.

Above all, the quantity of protein contained in soybeans is considerably greater than that in other beans, and actually this is even cited as a reason that soybeans became the principal ingredient in natto... but that topic will be explored on a subsequent page.

Soybeans, the main ingredient in natto, are classified in the Fabaceae family in the order Fabales (scientific name: Glycine max). The soybean is thought to be native to areas of east Asia such as Japan, China and the Korean peninsula.

Archeological evidence has revealed that soybeans were already being cultivated in the middle of the Jomon period (around 4,000 years B.C.).

Since that time, soybeans have been one of the five grains eaten in Japan, along with rice, barley, millet and foxtail millet. To the Japanese, the five grains were not mere foodstuffs but were considered to have sacred power, and they have been prized since ancient times. For example, the reason that soybeans are thrown in the annual festival of setsubun which is held in February is that they were thought to have the power to ward off evil spirits. In addition to setsubun, other rites held in various locations in Japan rely on the spiritual power of soybeans.

To return to the topic of the soybean as a food, soybeans are a principal ingredients in various other traditional Japanese dishes in addition to natto, such as miso, shoyu and tofu. Traditional Japanese cuisine would not be what it is without soybeans.

In China, soybeans have also been known to be a natural medicine. In the long history of traditional Chinese medical science, doctors in ancient times focused on the medicinal properties of soybeans and used them for this purpose. Chinese herbal medicine, which blends together many different natural medicines, includes formulations that use soybeans (black soybeans) that are fermented and then dried.

In Japan as well, people soon recognized that soybeans contained ingredients that were good for the body. "The Heart of Medicine," a medical text written in the Heian period, contains a detailed description of the functions of soybeans.

• When steamed or boiled and eaten, its functions exceed those of rice.
• When eating boiled beans for the first time, the movements of the body feel sluggish, but after a year these become light and sexual intercourse is improved.
• When roasted and turned into powder (toasted soybean flour), the flavor is sweet, and it settles the stomach, reduces swelling and prevents paralysis.
• Placing roasted hot soybeans in alcohol and then drinking the alcohol is a treatment for palsy and other ailments,
• Eating boiled soybeans together with the liquid in which they were boiled prevents poisons from afflicting all parts of the body.
• Improves the passage of bowel movements and helps to prevent lumps in the belly.
• (freely translated from "The Heart of Medicine")

Moreover, in a surviving document from the Nara period, a certain monk writes the following request to a government office: "As I am suffering from beriberi, I have become unable to move freely even after getting up in the morning. So please provide one "sho" (1.8 liters) of soybeans so I can use them as medicine."

As a food rich in nutritional value...
...as an ingredient in traditional cuisine...
...as a sacred grain with spiritual powers...
...and as an all-purpose cure...
...soybeans are important to the Japanese people for many reasons.

Protein: (33.8%)
Carbohydrates: (29.5%)
Lipids: (19.7%)
Water: (12.4%)
Ash: (4.7%)

Soybeans (whole grain, yellow soybeans, domestically produced, dried) From "2020 Standard Tables of Food Composition in Japan (8th Revision)"

The first outstanding attribute of soybeans that should be mentioned is protein content. We said earlier that soybeans have been nicknamed "the meat of the fields." According to the 2020 Standard Tables of Food Composition in Japan (8th Revision), the quantity of protein contained in 100g of the edible portion of soybeans is much greater than that contained in meat.

Examples of protein content per 100g of edible portion
Soybeans (whole grain, yellow soybeans, domestically produced, dried) …33.8g
Beef (Japanese beef / shoulder / lean and fat / raw) …17.7g
Chicken (breast / with skin / raw) …19.5g
Pork (large type breed / shoulder / lean and fat / raw) …18.5g

In terms of nutrient value, beans can be generally divided into two groups:
one with abundant lipid content and another with abundant carbohydrate content. Soybeans belong to the former group. As shown in the figure above, lipids account for nearly 20% of a soybean. The carbohydrate content of soybeans is roughly 30%, roughly half that of the beans in the latter group.

Examples of the nutrient value of beans
Chickpeas
Lipids: 5.2%
Protein: 20％
Carbohydrates: 61.5%

Green peas
Lipids: 2.3%
Protein: 21.7%
Carbohydrates: 60.4%

Peanuts
Lipids: 47.5%
Protein: 25.4%
Carbohydrates: 18.8%

Soybeans
Lipids: 19.7%
Protein: 33.8%
Carbohydrates: 29.5%

Carbohydrate is a general term for organic compounds made up of monosaccharides.
While controlling the intake of sugars is currently recommended to maintain health, the components of carbohydrates that cannot be digested by the digestive enzymes in the human stomach are not included in these sugars.
Soybeans contain a larger quantity of dietary fibers than other beans, so they can be said to be low in sugar.

There are many opportunities to tout the abundance of nutrients contained in soybeans, but the things that soybeans do not contain are also worth considering.

Protein is one of the types of compounds that are called nutrients. Proteins are made up of 20 types of amino acids, comprising between 50 and 1000 organisms, which are linked in various sequences.

There are said to be 100,000 types of proteins, but in simple terms, each of these is an aggregation of 20 types of amino acids that differ in terms of number and sequence.

The protein contained in beans, wheat, rice and other vegetables is called vegetable protein. The protein contained in meat, fish and shellfish, eggs, and dairy products is called animal protein. The insects that have attracted attention recently as a sustainable food are also rich in animal protein.

Whether protein comes from animal sources or vegetable sources is not a question of good or bad, but there are differences in terms of the types and balance of essential amino acids contained in these types of protein.
Although the scientific basis has not yet been adequately established, the results of multiple cohort studies suggest that "excessive intake of total protein and animal protein may pose the risk of Type 2 diabetes and other lifestyle-related diseases, while vegetable protein showed either no correlation or a possible preventive effect."
(from Japan Vegetable Protein Food Association

Vegetable protein and health See Japan Vegetable Protein Food Association (protein.or.jp)

In addition to the essential nutrients such as protein, lipids, vitamins and minerals, foods contain various other types of compounds. Many of these have functions that are said to promote health, and these are generally referred to as "functional components." Isoflavone is a functional component that is already well-known. It is an organic compound that is found only in vegetables. Large quantities of this compound are found in the embryo buds of many plants in the Fabaceae family, of which soybeans is a prominent member.

Polyphenols
↓
Typical polyphenols
Flavonoid / oleuropein / lignin (sesamin) / curcumin
↓
Typical flavonoids
Isoflavone
Catechin
Anthocyanin
Rutin
Tannin

The molecular structure of isoflavone resembles that of the female hormone estrogen, and accordingly it is also known as "phytoestrogen." When taken into the human body, it is known to have an effect that is similar to estrogen.

Estrogen has numerous important effects on the human body, such as the development of secondary sexual characteristics, regulation of the menstrual cycle, and stabilization of the autonomic nerves. It is also important in preventing osteoporosis. In recent years, it has also been in the spotlight for its expected ability to prevent breast cancer in women and prostate cancer in men.

In this way, estrogen has various important effects on the human body. However, the quantity of estrogen in the body peaks at the age of around 20. By the time a person is 45 to 50 years old, the quantity of estrogen that is secreted has decreased dramatically. Accordingly, isoflavonen— which has a similar effect on the body — has been recognized as a component that can supplement the functions of estrogen.

In the past, the excessive intake of isoflavone was pointed out as being problematic. This is because excessive intake is known to have some negative effects.
With regard to the assessment of the safety of foods for specified health use including soybean isoflavone, the Food Safety Commission established by the Ministry of Health, Labor and Welfare has issued a basic policy and indicated the daily recommended allowances for adults.
However, these guidelines are ultimately for foods and supplements with increased isoflavone content. With regard to traditional soybean processed food products, the Commission says the following.

"Japanese people have long experience when it comes to eating traditional soybean foods such as tofu, natto, boiled soybeans and miso in their daily dietary lifestyles. No adverse health impacts due to soybean isoflavone as a result of eating these soybean foods have been reported, and there is no cause for concern."

In other words: There is no cause for concern as long as soybean isoflavone is consumed in in moderation.

Proteins are made up of amino acids. A peptide is a chain of two to several dozen amino acids. As there are 20 types of amino acids, even a peptide that is made up of only two amino acids may comprise 20 x 20 = 400 types.

The unique properties of each peptide, such as neurotransmission action, antibacterial action and so on, will depend on the type and number of amino acids in the chain. For example, a certain peptide has a sedative effect, while a different peptide has a powerful sweetness and is commonly used as a sweetener.

Peptides consisting of chains of amino acids derived from soybeans are called soybean peptides. As in the case of peptides in other foods, these are combinations of various amino acids.

Based on recent research, there are high hopes for the action of soybean peptides in promoting the lipid metabolism as compared to animal proteins and their peptides. The use of soybean peptides is expected to expand as a source of protein for people who are dieting as a measure to prevent metabolic syndrome.

Soybean peptides are also known to maintain a good blood sugar balance by stimulating the hormone that governs the concentration of sugar in the blood.

In addition to these functions, it has been reported that soybean peptides accelerate fermentation by microorganisms. In one experiment in which bread dough using yeast (baker's yeast) grown in a culture that included soybean peptides was frozen, the yeast survival rate following defrosting was increased, improving fermentation capacity.

Research is still underway to determine what functions soybean peptides have when they are used to ferment various fermented foods. However, the words that are associated with soybean peptides — soybeans, fermentation, microorganisms — naturally make one think of "natto." Even if it was not added deliberately, soybean fermented foods containing soybean peptides are the logical result of the wisdom of our ancestors.

Soybeans are a healthy food that has abundant quantities of high-quality protein and is also low in fat.
However, on another page we also said that soybeans belong to the group "with abundant lipid content."
Fats and lipids are terms that resemble one another and are often used interchangeably, but here they will be used differently as follows.

Fatty acids, the principal component of lipids, can be generally divided into saturated fatty acids and unsaturated fatty acids. It is the latter type, unsaturated fatty acids, that are considered to be "essential fatty acids" that need to be ingested from food. Each type of unsaturated fatty acid has various positive effects on the body, such as preventing arteriosclerosis and blood clots, reducing blood pressure and LDL cholesterol and so on.

It is true that, compared to other beans, soybeans contain a greater quantity of lipids. However, the fatty acid composition is slightly more than 10% saturated fatty acids and around 80% unsaturated fatty acids. In other words, soybeans have a low proportion of the saturated fatty acids that need to be controlled, and therefore they are said to be a "low-fat" food.

In addition, of the fatty acids in soybeans, roughly half are linolic acid. Although linolic acid is an unsaturated fatty acid, it is said that excessive consumption should be avoided. However, this should not be a problem if normal quantities of soybean foods are consumed.

Of the 20 types of amino acid that make up proteins, 11 can also be synthesized in the human body. The remaining nine must be ingested from food. These nine amino acids are called essential amino acids. The key to maintaining health is to not target a specific amino acid and ingest large quantities of that type but rather to ingest a balance of all nine essential amino acids.

9 essential amino acids
Phenylalanine
Leucine
Valine
Isoleucine
Threonine
Histidine
Triptophan
Lysine
Methionine

The "amino acid score" is the score that indicates the quantity and balance of these nine amino acids that are needed by the human body and that must be ingested from food. The amino acid score will not be high if the quantity of one of the nine is significantly greater than the rest, or if the quantities of eight out of nine are high. The amino acid score is calculated based on the lowest value. The latest method that is used to calculate the amino acid score is the Digestible Indispensable Amino Acid Score (DIAAS), which was adopted in 2013 by the Food and Agriculture Organization of the United Nations (FAO). This method takes into account not merely the amino acid content but also the coefficient of digestibility in the human body, so it is held to be a more realistic indicator.

The amino acid score for the amino acids contained in soybean protein is the highest possible score of 100. The other sources of protein with perfect scores of 100 are animal proteins such as meat, fish and shellfish, milk, eggs and so on. Soybeans are the only staple crop that has a score of 100. This score is due to the outstanding amino acid balance in soybeans.

Top 5 foods from vegetable sources in terms of high amino acid score
1st place Soybeans (100)
2nd place Broccoli (80)
3rd place Potatoes (73)
4th place Carrots (55)
5th place Tomatoes (48)