Natto benefits: the bone trials split between three years and one

Key benefits

1. 01Early research

   MK-7 over three years: spine and femoral neck improved, total hip did not. Over one year: nothing did

   These two have to be read together.

   The positive one — 2013, Osteoporosis International

   244 healthy postmenopausal women received placebo or 180 µg of MK-7 daily for three years. Bone mineral density of the lumbar spine, total hip and femoral neck was measured by DXA, bone strength indices of the femoral neck were calculated, and vertebral fracture was assessed by DXA.

   • Vitamin K status improved significantly.
   • The age-related decline in bone mineral content and density slowed significantly at the lumbar spine and femoral neck.
   • It did not at the total hip.
   • Bone strength moved favourably, and the loss of vertebral height at the mid-site of the lower thoracic vertebrae fell significantly.

   The authors' own conclusion is careful: MK-7 supplements may help postmenopausal women prevent bone loss — and whether this extrapolates to other populations is a separate question.

   The null one — 2010, Osteoporosis International

   334 healthy Norwegian women aged 50–60, one to five years past menopause, were randomised double-blind to 360 µg of MK-7 as natto capsules or identical-looking placebo capsules containing olive oil, for one year.

   • There were no statistical differences in bone loss rates at the total hip or at any other measurement site.
   • Yet carboxylated osteocalcin rose and undercarboxylated osteocalcin fell (p < 0.001).

   The authors' conclusion: MK-7 taken as natto over one year reduced undercarboxylated osteocalcin but did not influence bone loss rates in early menopausal women.

   The honest tally:

   	2013	2010
   People	244	334
   Dose	MK-7 180 µg	MK-7 360 µg (natto capsules)
   Duration	3 years	1 year
   Population	Postmenopausal	1–5 years past menopause
   Vitamin K marker	Improved	Improved
   Bone density	Spine and femoral neck improved; total hip did not	No change anywhere

   The higher dose is the one that failed — that contrast is the point. What is left to explain it is duration and population. Neither trial settles the question, and that is why this site grades it early.

   [1][2]
2. 02Early research

   Nattokinase capsules for eight weeks: systolic −5.55 mmHg, with an interval that nearly touches zero

   2008, Hypertension Research. A randomised, double-blind, placebo-controlled trial from Yonsei University and Mizmedi Hospital in Seoul.

   The design:

   • 86 adults aged 20–80 with untreated systolic blood pressure of 130–159 mmHg — prehypertension or stage 1 hypertension
   • Nattokinase 2,000 FU capsules or placebo for eight weeks
   • 73 completed the protocol — thirteen did not

   The results:

   • Net change in systolic pressure −5.55 mmHg (95% CI −10.5 to −0.57), p < 0.05
   • Net change in diastolic pressure −2.84 mmHg (95% CI −5.33 to −0.33), p < 0.05
   • Renin activity −1.17 ng/mL/h, p < 0.05

   Look at the confidence interval. The systolic upper bound is −0.57. Significant, yes, but pressed right up against zero — meaning this data is compatible with a true effect near −0.57 mmHg, which would be clinically meaningless.

   Four things to read alongside it.

   First, what was given was not natto. It was a 2,000 FU nattokinase capsule. Nothing here shows that eating natto delivers a comparable amount of active nattokinase.

   Second, 13 of 86 dropped out — 15%.

   Third, eight weeks. Shorter than the window used to judge blood pressure drugs, and no hard outcomes such as cardiovascular events were measured.

   Fourth, it is a single trial. No large meta-analysis has yet reproduced it.

   Nattokinase is an enzyme with reported fibrinolytic activity. That is why it becomes a caution for anyone on anticoagulants, noted below.

   [3]
3. 03Nutrition data

   The composition: 8.6 mg of iron, and how to read the vitamin K figure

   Per 100 g of natto (about two shop packs):

   • 211 kcal, protein 19.4 g, fat 11.0 g, carbohydrate 12.7 g
   • Fibre 5.4 g
   • Iron 8.6 mg — above the 8 mg requirement for adult men
   • Calcium 217 mg, magnesium 115 mg, potassium 729 mg
   • Manganese 1.53 mg, zinc 3.03 mg, copper 0.667 mg
   • Sodium 7 mg — very low. The sachet of soy sauce supplies nearly all the sodium
   • Vitamin K (phylloquinone) 23.1 µg

   The 8.6 mg of iron stands out. But plant iron is non-haem and poorly absorbed; eating it alongside something with vitamin C raises the uptake.

   The vitamin K figure has to be read precisely. The 23.1 µg above is phylloquinone, K1. The menaquinone-7 that natto is famous for does not appear in the USDA record. So this site does not print a number for natto's MK-7 content — there is no source to print. And the trial doses above (180 µg, 360 µg) are, again, supplement doses.

   That natto is a fermented food does not show up well in a composition table. Bacillus subtilis fermenting soybeans produces the sticky threads (poly-gamma-glutamic acid) and nattokinase. The process is said to break down some of the soy's phytate and so favour mineral absorption, but that has not been confirmed in randomised human trials.

   Against plain soybeans, the protein is similar and the fibre is lower. The real difference between boiled soybeans and natto is not in the composition table but in what the fermentation makes.

How to eat it well

What to watch out for