Miso is one of the saltiest foods in the Japanese kitchen. A single serving of miso soup carries about 1 to 2 grams of salt, and paste-based dishes push the number higher.
Yet in a Japanese clinical trial published in Hypertension Research, eight weeks of daily miso did not raise blood pressure in adults with high-normal readings. It quietly lowered it at night.
The trial randomized 40 adults aged 40 to 69 with high-normal blood pressure or stage I hypertension into a miso group or a control soy-food group. Both groups were tracked for eight weeks.
The miso group consumed about 3.8 grams of salt from miso alone each day. The control group consumed 0.2 grams. Daytime clinical blood pressure did not change in either group.
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Nighttime blood pressure, measured at 2 and 4 a.m., fell significantly in the miso group. Pulse rate did not shift. The paper concluded that daily miso did not raise blood pressure in the participants.
Why salt in miso behaves differently
A 2020 review in Environmental Health and Preventive Medicine, written by Koji Ito, covered the same paradox.
Ito cited evidence that the intake of fermented soy products, such as miso and natto, is inversely associated with developing high blood pressure in Japanese adults with normotension, while unfermented soy products are not.
The proposed reason lies in fermentation itself, which appears to generate bioactive peptides that inhibit angiotensin-converting enzyme, or ACE. That is the same target as several common blood pressure medications.
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The review also linked habitual miso intake to lower heart rate and reduced activity in the sympathetic nervous system, which drives fight-or-flight responses.
What fermentation adds
Fermentation reshapes the soybean itself. A 2023 review in the Journal of Diabetes Investigation noted that the process "increases the nutritional value of soybeans by increasing the content of vitamins, essential amino acids, and fatty acids".
The same review described how fermentation converts bound isoflavones into their more absorbable aglycone forms, genistein and daidzein.
Those isoflavones are the plant compounds tied to modest cardiovascular and bone-density benefits in soy research.
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Miso is made using the koji mold Aspergillus oryzae. Animal studies suggest habitual miso intake shifts the gut microbiome in directions linked to lower inflammation, though most of that evidence is still preclinical.
The sodium caveat
None of this rewrites the standard sodium advice. The 2020-2025 Dietary Guidelines for Americans recommend "less than 2,300 mg per day" of sodium, roughly one teaspoon of table salt. Average U.S. intake sits well above that.
The trial dose was about one bowl of miso soup a day for eight weeks. People with kidney disease, heart failure or salt-sensitive hypertension should talk to a doctor before making it a daily habit.
Packaged miso paste lists sodium per tablespoon on the label, so a quick check keeps a bowl within a daily sodium budget.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.