How Much Natto a Day Is Linked to Fewer Fractures?

About one pack a day, roughly 40 grams. In a 15-year Japanese study, women who ate 7 or more packs a week had about 44% fewer osteoporotic fractures than women who ate less than one pack a week. That study observed people’s habits rather than testing natto in a trial, so it shows a link, not proof.

Natto is a traditional Japanese food made by fermenting soybeans with a bacterium called Bacillus subtilis. It is sticky, strong smelling and an acquired taste. It is also the richest natural food source of menaquinone-7 (MK-7), a form of vitamin K2. Your body uses vitamin K2 to switch on osteocalcin, a protein that helps bind calcium into bone.

What the Data Show

The newest summary of the research is a meta-analysis of six Japanese studies with 2,327 people. A meta-analysis pools the results of several studies. It compared people who ate natto habitually with people who ate little or none.

Regular natto eaters had much higher blood levels of MK-7. They had more activated osteocalcin and less of the inactive form, called undercarboxylated osteocalcin, which is a sign of low vitamin K. Their bone mineral density, the measure of bone strength on a DEXA scan, was modestly higher across skeletal sites. The authors rated the certainty of the evidence as moderate for blood MK-7 and low or very low for the bone outcomes.

The strongest single study followed 1,417 postmenopausal Japanese women for a median of 15.2 years. During that time, 172 of them had an osteoporotic fracture, meaning a break from a minor fall or strain. After the researchers accounted for age, bone density, weight, smoking, alcohol, calcium intake and other soy foods, women eating 7 or more packs a week had about 44% lower fracture risk than women eating less than 1 pack a week. Women eating 1 to 6 packs a week had a smaller drop that was not statistically clear. Tofu and other soy foods showed no link with fractures.

That last detail matters. Tofu and natto are both soy, but only natto is loaded with vitamin K2. If soy itself were protecting bone, tofu should have shown a benefit too.

Does Natto Help Prevent Osteoporosis in Women?

The evidence in women is the most complete. In the JPOS study of 944 Japanese women, postmenopausal women who ate more natto had higher hip bone density at the start. Over the next 3 years, they also lost less bone at the top of the thigh bone (the femoral neck) and at the forearm. The femoral neck link held up after adjusting for other factors. Tofu and other soy products showed no link with bone loss.

Does Natto Help Men’s Bones Too?

Yes, the same pattern shows up in men. In a study of 1,662 Japanese men aged 65 and older, greater natto intake was linked with higher bone density and a lower chance of low bone density at the hip. When the researchers adjusted for blood levels of inactive osteocalcin, the link disappeared. That suggests natto’s effect on bone runs through its vitamin K.

A study comparing regions points the same way. Women in Tokyo, where natto is eaten often, had average blood MK-7 levels of 5.26 ng/mL. Women in Hiroshima, where it is seldom eaten, averaged 1.22 ng/mL, and British women averaged 0.37 ng/mL. Across Japan, prefectures that ate more natto had fewer hip fractures in women. Comparisons between regions can be explained by many things besides diet, so treat this as a clue rather than evidence on its own.

How Much Vitamin K2 Is in Natto?

A lot. A study of natto and men’s bones listed 380 micrograms (μg) of vitamin K2 and 20 μg of vitamin K1 per pack. A study by researchers at a natto company measured regular natto at 775 μg of MK-7 per 100 grams. Pack sizes and brands vary, so treat these as rough numbers.

For comparison, the MK-7 supplement that slowed bone loss in a 3-year trial (below) gave 180 μg a day. A single pack of natto supplies about twice that.

Do You Need to Eat Natto Every Day?

No trial has tested a specific daily amount of natto. The best guide is the fracture study. Its lowest-risk group ate about one pack a day, roughly 40 grams, or 7 or more packs a week.

Because MK-7 lasts a long time in the body, you may not need it every single day. A study comparing vitamin K supplements found MK-7 has a very long half-life, which gives steadier blood levels, and during prolonged intake it built up to levels 7 to 8 times higher than vitamin K1. In practice, a few packs a week will raise your vitamin K2 levels, and a pack most days matches what the lowest-risk group in the fracture study ate.

One pack is a normal serving. There is no reason to eat several a day.

Do Vitamin K2 (MK-7) Supplements Work as Well as Natto?

The trial results are mixed.

In a 3-year randomized trial of 244 healthy postmenopausal women, 180 μg of MK-7 a day slowed the age-related loss of bone density at the spine and the femoral neck, but not at the total hip. Bone strength measures also improved.

In a 1-year randomized trial of 334 Norwegian women who were 1 to 5 years past menopause, 360 μg of MK-7 a day, given as natto capsules, improved osteocalcin activation but made no difference in bone loss at the hip or any other site.

One year may be too short to see a change in bone. Still, these results show that raising vitamin K levels does not always translate into stronger bone.

For fractures, the most careful pooled analysis is a review of 36 randomized trials of vitamin K. In postmenopausal or osteoporotic patients, vitamin K lowered the odds of any clinical fracture by about 28%. When the analysis kept only the trials at low risk of bias, the drop shrank to about 24% and was no longer statistically clear. There was no difference in spine fractures. The authors concluded that vitamin K may reduce clinical fractures, but the evidence is insufficient to confirm it. That review covered several forms of vitamin K, not natto alone.

Can Natto Replace Osteoporosis Medication?

No. Natto has never been tested head to head against osteoporosis drugs, and the vitamin K supplement trials are too weak to stand in for them.

If a DEXA scan shows osteoporosis, or you have broken a bone from a minor fall, that is a medical diagnosis with proven drug treatments. Natto fits alongside treatment as a food, not in place of it.

Is Natto Safe If You Take Warfarin?

No. If you take warfarin (Coumadin), avoid natto unless your doctor says otherwise.

Warfarin works by blocking vitamin K, and natto delivers a large dose of it. The vitamin K supplement study warned that MK-7 at 50 μg a day or more may interfere with oral anticoagulant treatment in a clinically relevant way. One pack of natto has several times that amount.

A systematic review of vitamin K intake in warfarin users found that 25 to 325 μg a day was acceptable, with about 150 μg a day as a reasonable target. Among ordinary foods, natto and foods made with it were the only ones that had to be avoided. Leafy greens were acceptable in limited amounts.

The newer blood thinners such as apixaban (Eliquis) do not work through vitamin K, so the concern is different. I cover that in a separate article linked below.

What Are the Other Health Benefits of Natto?

The best data outside bone come from a Japanese study of 92,915 adults followed for 14.8 years. People in the highest fifth of fermented soy intake (natto and miso) had about 10% lower death rates from any cause than those in the lowest fifth, in both men and women. Natto in particular was linked with lower death from cardiovascular disease. The authors cautioned that other unmeasured differences between natto eaters and non-eaters might explain part of this.

Dr. Kumar’s Take

The fracture study is what makes this more than a vitamin story. Natto eaters did not just have better blood markers. They broke fewer bones over 15 years, even after the researchers accounted for bone density, and tofu eaters did not. That points to vitamin K2 rather than soy, and the men’s study points the same way. It is still observational evidence from Japan, and the supplement trials are mixed, so I would not call it proven.

For someone who can stomach natto, a pack most days is a cheap, food-based way to get vitamin K2 that lines up with the best data. It is not a substitute for weight-bearing exercise, enough protein and calcium, or osteoporosis treatment when you need it. The one real hazard is warfarin.

Practical Takeaways

Aim for about one pack (roughly 40 grams) most days if you want to match the lowest-risk group in the fracture study.
A few packs a week still raises vitamin K2, because MK-7 stays in the blood a long time.
If you take warfarin, avoid natto unless your doctor approves it and checks your blood clotting.
Do not count on natto to treat diagnosed osteoporosis. It is a food, not a replacement for treatment.
If you cannot eat natto, MK-7 supplements are an option, but the bone results from trials are mixed.
Watch the sauce packet if you limit salt.

FAQs

What is natto?

Natto is a traditional Japanese food made by fermenting soybeans with Bacillus subtilis. It has a sticky texture and a strong smell. It is the richest natural food source of vitamin K2 in the MK-7 form.

How much vitamin K2 is in one pack of natto?

A Japanese study listed 380 μg of vitamin K2 per pack. Another measured 775 μg of MK-7 per 100 grams of regular natto. Brands and pack sizes vary.

Is it OK to eat natto every day?

For most people, yes. A pack a day was the intake linked with the lowest fracture risk in a 15-year study of Japanese women. The main exception is anyone taking warfarin.

Is natto better than a vitamin K2 supplement?

Natto has the stronger long-term evidence for fewer fractures, but that evidence is observational. Supplement trials are randomized but mixed: one 3-year trial slowed bone loss, and a 1-year trial found no difference.

Does tofu or soy milk give the same benefit?

No. In the Japanese studies, tofu and other soy foods showed no link with fractures or bone loss. Natto’s fermentation is what produces the vitamin K2.

Bottom Line

About one pack of natto a day was linked with roughly 44% fewer fractures in Japanese women, and natto eaters in Japan have higher vitamin K2 and somewhat denser bones. The evidence is observational and comes from Japan, and vitamin K2 supplement trials are mixed. Natto is a sensible food for bone health, not a treatment for osteoporosis, and anyone on warfarin should avoid it.

How Much Natto a Day Is Linked to Fewer Fractures?