Natto

Natto, the sticky Japanese fermented soybean, has the biggest single-food number in the fermented-food research: about 40% lower death rates in one cohort of older Japanese men. It is also by far the richest food source of vitamin K2. But the mortality evidence is observational and comes from Japanese populations only, and its famous clot-dissolving enzyme did nothing in the one long trial.

If you like natto, a pack several times a week is a reasonable habit. If you take warfarin or any other blood thinner, talk to your prescriber before making it a daily food.

What it is

Natto is whole soybeans fermented by a bacterium, Bacillus subtilis var. natto. A standard pack is about 40 g. The taste and smell — and the stringy texture — are an acquired preference.

The fermentation gives natto three things that plain soy lacks, and each has its own evidence: a large dose of vitamin K2, a clot-dissolving enzyme called nattokinase, and a set of Japanese cohort studies tying it to lower death rates. It is one of the foods discussed on Fermented foods.

Vitamin K2: the solid part

Natto carries an order of magnitude more of the vitamin K2 form called menaquinone-7 (MK-7) than any other commonly eaten food. Vitamin K2 is needed to activate osteocalcin, the protein that directs calcium into bone, and matrix Gla-protein, which keeps calcium out of artery walls.

Habitual natto eaters have more MK-7 in their blood, more fully activated osteocalcin, and modestly higher bone mineral density. Pooling four Japanese cohort studies, the bone-density difference was real but modest once one outlying result was left out.^[1] That analysis measured no artery or calcification outcome at all. The "keeps calcium out of your arteries" half of the story rests on K2 supplement trials, and those are split — see Vitamin K2.

Living longer: big numbers, narrow evidence

The mortality claim rests on Japanese cohorts, the three below. All are observational, so they show that natto eaters die less often, not that natto is the reason.

Older men, all causes. In 1,548 older Japanese men followed for about 12 years on average, eating natto several packs a week went with about 40% lower all-cause mortality. At the very highest intake, a pack a day or more, the association weakened and was no longer statistically significant, so the signal sits at moderate rather than maximal intake.^[2]
The largest: fermented versus plain soy. Across about 93,000 Japanese adults followed for nearly 15 years, fermented soy, but not unfermented soy, was tied to lower all-cause mortality, and natto specifically to lower cardiovascular mortality.^[3]
Adults, heart disease and stroke. In a study of 29,079 Japanese adults followed for 16 years, the highest natto eaters had about 25% lower cardiovascular mortality and roughly a third lower stroke mortality than the lowest. Total soy protein showed no link with cardiovascular death overall, but for stroke it too went with about 25% lower mortality — so natto looks like a stronger version of a soy effect rather than something entirely separate.^[4]

These are the largest single-food associations in the fermented-food literature, from a narrow base: a few cohorts, all in one country, a diet and lifestyle that differ in many ways from those of people who don't eat natto.

Nattokinase: interesting chemistry, no proven benefit

The natto bacterium makes nattokinase, an enzyme that breaks down clots (it is "fibrinolytic"). It is genuinely active in laboratory tests on blood plasma. But the one long trial gave 265 adults 2,000 fibrinolytic units of nattokinase daily for a median three years and found no effect on the progression of artery disease or on any laboratory measure.^[5] Treat it as interesting chemistry, not a demonstrated heart benefit.

The anti-ageing claim is thinner still

A natto extract extends adult lifespan in the roundworm Caenorhabditis elegans.^[6] The popular explanation — that natto compounds switch on the cell's energy-and-stress-sensing "longevity pathways", the sirtuins and AMP-activated protein kinase (AMPK) — is not something any natto paper has tested. There is no rodent lifespan data and no human trial.^[7]

Who should be careful

Anyone on a blood thinner — warfarin, a direct oral anticoagulant, aspirin or clopidogrel. Natto is a problem on two counts. Its vitamin K2 directly opposes warfarin and the other vitamin-K-antagonist drugs; the newer direct oral anticoagulants are not expected to be affected. And nattokinase dissolves clots and has never been tested alongside any anticoagulant or antiplatelet drug, so the combination is untested rather than shown safe — a question that applies to the direct oral anticoagulants too. Raise it with your prescriber before making natto a daily habit.
Anyone taking a classic monoamine-oxidase inhibitor (MAOI) antidepressant — phenelzine, tranylcypromine, isocarboxazid, or high-dose selegiline. On these drugs, tyramine in food can push blood pressure up sharply, and fermented soy is among the ferments richest in it. Modern production has cut tyramine in most foods, but clear this one with a prescriber.^[8]
Active small intestinal bacterial overgrowth (SIBO) or severe bloating with irritable bowel syndrome. Ferments can trigger flares; test one food at a time.
Histamine intolerance or mast cell activation syndrome. Fermented soy averages about 74 mg of histamine per kilogram, more than aged cheese or sauerkraut on average, though histamine varies more within a food than between foods.^[9]

What to do

If you enjoy it, eat a pack several times a week. That is where the mortality association sits; at a pack a day or more it weakened.
On warfarin or any other blood thinner, check with your prescriber first.
If you can't stand the taste, a daily MK-7 supplement of 90–180 µg covers the vitamin K2, but not the nattokinase or anything else in the food. On warfarin, the supplement needs the same prescriber check as natto. The hard-outcome evidence for MK-7 itself is split; see Vitamin K2.
Don't buy nattokinase capsules for your heart. The one long trial was null.