The beneficial—and sometimes paradoxical—effects of miso

Miso is a traditional Japanese condiment and one of the hallmarks of umami,¹ the fifth basic taste alongside sweet, salty, sour, and bitter. In the Land of the Rising Sun, it is best known as the star ingredient of the famous soup that bears its name.¹ Since the 1970s, it has steadily made its way into Western kitchens, where it is used as a versatile condiment.² 

What is miso?

Miso is a fermented soybean paste produced in two stages:^1,2 

• koji preparation: an initial substrate (usually rice, barley, or soybeans) is inoculated with a microscopic filamentous fungus—most commonly Aspergillus oryzae. This produces what is known as ‘koji’.
• final fermentation: the koji is then mixed with a puree of cooked soybeans and salt. This mixture undergoes a second fermentation, this time involving yeasts and bacteria (including lactic acid bacteria). This stage can last anywhere from a few weeks to three years.

There are four main categories of miso based on its ingredients: rice miso (which accounts for around 80% of production in Japan), barley miso, soybean miso, and blended miso.^1,2 Its flavor (ranging from sweet to salty) and color (white, red, or black) depend on the ingredients used, the proportion of koji, and how long it is left to ferment.^1,2 

Health benefits of soy

Miso, whose main ingredient is soy, retains many of soy’s nutritional benefits. Soy is rich in high-quality protein (twice as much as most other legumes) because it contains all nine essential amino acids.^3,4 Soy is also rich in fiber, which helps feed our gut bacteria, and in minerals. Miso also contains isoflavones, plant compounds that can bind to human estrogen receptors. These are sometimes referred to as phytoestrogens. 

This has raised concerns:⁵ could these compounds affect hormone-dependent cancers, fertility, or hormonal development? While these concerns were once legitimate, current evidence is entirely reassuring: research has found no increased risk and even suggests that soy consumption may have a potential protective effect against cancer,⁶particularly in women.^7,8

One final point to keep in mind: miso, like any soy-based food, contains several allergenic proteins.⁹ Although fermentation can significantly reduce their allergenic potential by breaking down complex proteins, the risk remains for people with a high sensitivity to soy.⁹ 

Fermentation that boosts health benefits

Miso’s double-fermentation process transforms soybeans into a food that is not only easier to digest but also richer in bioactive compounds.^3,9,10 For example, fermentation improves the digestibility of soy proteins by breaking them down into smaller molecules that are easier to absorb.³ It also increases the bioactivity of the well-known isoflavones, transforming them into a form of phytoestrogens that is more bioavailable and active in the body, with potential health benefits, including :

• protection against certain cancers,
• improved lipid profile,
• and beneficial effects on cardiovascular health—although the evidence remains inconclusive.^3,10–12 

It should be noted, however, that because these compounds have estrogen-like activity, consuming them in excessively high amounts could potentially pose toxic risks to the reproductive system, particularly in pregnant women, women of childbearing age, and prepubescent children (ANSES, French National Agency for Food, Environmental and Occupational Health & Safety).¹³
Miso is naturally rich in diverse and varied microorganisms, including the trio Aspergillus oryzae derived from koji), the yeast Zygosaccharomyces rouxii and the bacterium Tetragenococcus halophilus.^1,2,14 This microbial community enriches the gut microbiota,¹⁵ promoting its diversity and stability, boosting certain beneficial bacteria while reducing less favorable ones. This rebalancing of the microbiota is intriguing enough that some researchers have suggested a potential protective effect against diabetes mellitus—^10,15 but for now, that hypothesis still needs to be substantiated. 

Finally, an improved gut flora means an increased production of key compounds, particularly short-chain fatty acids (sidenote: Short chain fatty acids (SCFA) Short chain fatty acids (SCFA) are a source of energy (fuel) for an individual’s cells. They interact with the immune system and are involved in communication between the intestine and the brain. Silva YP, Bernardi A, Frozza RL. The Role of Short-Chain Fatty Acids From Gut Microbiota in Gut-Brain Communication. Front Endocrinol (Lausanne). 2020;11:25. )  (SCFAs).¹⁵
These small molecules work in our favor: they nourish the cells of our colon, help strengthen our intestinal barrier, support immune function, and may help reduce inflammation.^4,9,15  

The salt paradox

The WHO and public health campaigns regularly remind us: we should cut back on salt. Yet miso is one of the main sources of salt in the Japanese diet. And here comes the paradox: despite its high salt content, this Japanese specialty does not seem to have the same harmful effects as plain old salt: consuming it may have no effect on blood pressure.^2,16,17 It may even lower heart rate¹⁶and increase salt excretion through urine!¹⁸ 
How can this surprising result be explained? One possibility is potassium. Soybeans and the other ingredients commonly found in miso soup (vegetables, fish...) are rich in potassium, which may help counteract some of salt’s blood-pressure-raising effects.¹ According to some experts, the key may therefore be not just how much sodium we consume, but the balance between sodium and potassium.^1,2,19

Thus, thanks to its double fermentation process, miso becomes easier to digest and richer in potentially beneficial compounds. When consumed in moderation—high daily intake of 1 to 5 bowls per day appears to be associated with an increased risk of stomach cancer in men–⁷ it may offer a range of health benefits, despite its high salt content.