Miso

Health Benefits
(Source: World's Healthiest Foods, George Mateljan)

In comparison to research on soybeans and their overall health benefits, research specific to miso is much less common. In addition, research on miso is complicated by the fact that human intake of miso can be difficult to measure in isolation since miso is usually consumed as part of a soup, stir-fry, or other dish. (For example, when participants in a study provide information about their intake of miso soup and scientists analyze the nutritional and health benefits provided by miso, the outcome can be confusing since other foods contained in the miso soup like tofu or sea vegetables might also have been responsible for the nutritional and health benefits.) Still, as an overall observation, we believe that intake of soy miso shows a preponderance of health benefits and perhaps even stronger health benefits than soyfoods in general. We also suspect that these potentially stronger health benefits may be related to fermentation of soy miso by micro-organisms like the fungus Aspergillus.

Overall Nutrient Benefits

Soy miso is a very good source of manganese and copper as well as a good source of zinc (all three are important mineral antioxidants). It is also a very good source of the mineral phosphorus as well as a good source of protein and dietary fiber. In addition to these conventional nutrients, soy miso is also an important source of phytonutrient antioxidants including phenolic acids like ferulic, coumaric, syringic, vanillic, and kojic acid.

An especially interesting group of antioxidants in soy miso is the group of antioxidants related to miso fermentation. In several recent studies, the amount of some specific antioxidants in soy miso appears to increase when fermentation is carried out for a longer period of time. DDPH (2,2,-diphenyl-1-picrylhydrazyl) antioxidants appear to be one category of miso antioxidants that increase with prolonged fermentation. These particular free radical scavengers may be more plentiful in miso that has been fermented for many months—or even several years—in comparison to miso that has only been fermented for one to three months. food.

Like other soy-based foods, miso can provide us with a wide variety of phytonutrients. Many of these phytonutrients can function as antioxidants and anti-inflammatory substances. Due to differences in starting ingredients, micro-organisms used for fermentation, and total fermentation time, not all of the phytonutrients listed below are contained in all varieties of soy miso. But the list below provides a good overview of substances found in all forms of soy miso combined:

  • Flavonoids and Isoflavonoids

    • daidzein

    • genistein

    • malonylgenistin

    • malonyldaidzin

  • Phenolic Acids

    • Caffeic acid

    • Coumaric acid

    • Ferulic acid

    • Gallic acid

    • Sinapic acid

  • Phytoalexins

    • glyceollin I

    • glyceollin II

    • glyceollin III

  • Phytosterols

    • beta-sitosterol

    • beta-stigmasterol

    • campestrol

  • Proteins and Peptides

    • defensins

    • glycinin

    • conglycinin

    • lunacin

  • Saponins

    • soyasaponins (group A and group B)

    • soyasapogenols

Cardiovascular Benefits

Most of the heart-related research that we have seen on miso has involved population groups in Japan, and so it is difficult to say if these research findings also hold true for other population groups. In one large study involving 40,462 Japanese participants and lasting for more than 10 years, intake of miso was found to associated with decreased risk of one major type of stroke (cerebral infarct, or CI). This health benefit from miso held true despite miso also emerging as a major source of dietary salt in the study. These findings are fascinating, because ordinarily, a certain percentage of the Japanese population would be expected to be salt-sensitive and to have blood pressure undesirably increased by high intake of salt. Increase in blood pressure, in turn, would be expected to be a possible risk factor for CI. Yet miso was been found to lower risk of CI—a result that is unusual for a high-sodium food.

Animal studies have repeatedly shown this same result for miso versus table salt intake. Animals consuming a 2.3% table salt (sodium chloride) diet and animals consuming a 2.3% salt-from-miso diet have not experienced the same results. Salt-from-miso diets have not been found to raise blood pressure, even when they provide an equal amount of salt as high table salt diets. Researchers speculate that the difference may be related to a combination of factors, including soy proteins, peptides, isoflavones, and diverse antioxidants found in soy miso. In addition, researchers point to the potentially key role of fermentation in transforming soybean content and rendering it more capable of cardiovascular support. Further research is needed to clarify all of these issues.

Anti-Cancer Benefits

This area of soy miso research remains controversial. On the one hand, we know that soy miso contains the isoflavone genistein, and we know that dietary intake of genistein is associated with decreased risk of certain cancers, including prostate cancer. Yet at the same time, we also have studies on soy miso intake that show no association between prostate cancer risk and level of soy miso consumed.

Or, to take another controversial example: if individuals who consume daidzein (an isoflavone found in soyfoods and other types of food) have the right type of intestinal bacteria inside of their digestive tract, we know that they will end up having some of this dietary daidzein transformed by their intestinal bacteria into a related compound called equol. We also know that micro-organisms used to ferment soy miso (including many different strains of the fungus Aspergillus oryzae) are capable of turning daidzein into equol during miso fermentation. In this way, soy miso can end up containing equol even before we eat it!

This conversion of daidzein into equol by Aspergillus during miso fermentation was originally thought to have some reliable cancer-preventive benefits, but studies in this area have not reflected such a result. For example, in a study measuring risk of breast cancer among Japanese women, intake of miso soup was not found to reduce risk. Other studies have show a similar lack of risk reduction for colorectal cancer and prostate cancer based on intake of miso soup.

We have not seen any research studies specific to miso showing increased risk of cancer in any form. However, we are also unable to describe any clear anti-cancer benefits for soy miso based on the studies that we have seen. It's difficult for us to imagine negative consequences when taking a whole food like whole soybeans and allowing them undergo natural fermentation under safe conditions. We would expect a naturally fermented whole food to provide outstanding health benefits, including anti-cancer benefits.

However, soy miso is definitely a challenging food to research. As mentioned earlier in this section, consumption of miso can be difficult to measure in isolation since miso is usually consumed as part of a soup, stir-fry, or other dish. In addition, miso is often consumed in relatively small amounts in comparison with other foods. We hope that researchers will continue to investigate possible anti-cancer benefits from miso intake, and devote more attention to this uniquely fermented form of soy.

Digestive Benefits

In some ways, it's accurate to think about soy miso as a food that has been "pre-digested." That's because Aspergillus and other micro-organisms used in fermentation of soy miso can help metabolize proteins, carbohydrates, and fats found in soybeans and transform them into smaller molecules that may be more easily digested. In addition, depending on processing and fermentation methods, some soy miso can contain "friendly" bacteria like lactic acid bacteria (including various species of Lactobacillus) that might be helpful in supporting intestinal microflora. We have not seen large-scale, human research studies showing benefits of soy miso consumption for the digestive tract, but we would expect such studies to show digestive benefits based on the above factors. Hopefully, more attention will be given in future research studies to soy miso and its potential digestive benefits.

Fermented Soy Foods and Vitamin K

As described throughout this Health Benefits section, much of the nutrient support we get from fermented soy miso depends on the micro-organisms used in fermentation. While Aspergillus oryzae is often a premier micro-organism used in miso fermentation, other micro-organisms—including bacteria—can be highly important. Bacillus subtilis is one of these bacteria, and it is more commonly used in fermentation of Chinese miso than in Japanese, Korean, or Indonesian versions of this fermented soy food.

From a health standpoint, one of the reasons that Bacillus bacteria are so interesting is their ability to create a form of vitamin K called menaquinone-7 (MK-7). Vitamin K (in all forms) is an important nutrient for bone health. Sufficient intake of vitamin K is associated with decreased risk of osteoporosis, since this vitamin is involved with maintenance of bone mineral density and also with shaping of bone structure (through gamma-carboxylation). In the case of MK-7 (the form of vitamin K produced by Bacillus bacteria, and a member of the vitamin K2 menaquinone family), we know that higher levels of MK-7 in the blood correspond to lower risk of hip fracture in older Japanese women, and that higher MK-7 levels correspond to increased intake of soy foods that have been fermented with Bacillus bacteria. One fascinating aspect of Bacillus-fermented soy foods is the potential ability of these bacteria to stay alive in our lower intestine after these foods are consumed. We've seen one study in which 1.6-20 million Bacillus bacteria (per gram of feces) were found to remain alive up to 6 days following consumption of natto. If Bacillus bacteria from fermented soy foods can remain alive in our digestive tract, they may keep providing us with vitamin K benefits many days after their consumption.

It's important to remember that Bacillus bacteria are not used in production of all miso, and it can be difficult to determine which miso products have been fermented with the help of these bacteria. (Sometimes a call to the manufacturer may still not result in a clear answer here.) Our recommendation here is that you consider selection of Chinese miso products if you want to enjoy miso in your meal plan and you are paying special attention to your vitamin K intake, since Chinese miso is more likely to have been fermented with Bacillus bacteria.

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