Natto is a traditional Japanese fermented soya bean food renowned for its exceptionally high vitamin K content, particularly vitamin K2 in the form of menaquinone-7 (MK-7). Whilst natto has been a dietary staple in Japan for centuries, it remains relatively uncommon in the UK, though increasingly available in specialist shops. A single serving of natto can provide several times the recommended daily intake of vitamin K, raising important considerations for patients on anticoagulant therapy and those monitoring their vitamin K consumption. This article examines the vitamin K content of natto, its potential health implications, and essential safety guidance for UK patients.

What Is Natto and Why Is It High in Vitamin K?

Natto is a traditional Japanese fermented food made from soybeans that have been inoculated with the bacterium Bacillus subtilis var. natto. The fermentation process typically takes 12–24 hours at controlled temperatures, resulting in a distinctive sticky, stringy texture and a strong, pungent flavour that can be challenging for those unfamiliar with it. Whilst natto has been a dietary staple in Japan for over a thousand years, it remains relatively uncommon in the UK, though it is increasingly available in specialist Asian supermarkets and health food shops.

The exceptionally high vitamin K content in natto arises directly from the bacterial fermentation process. During fermentation, Bacillus subtilis produces substantial quantities of menaquinone-7 (MK-7), a specific form of vitamin K2. This distinguishes natto from most other dietary sources of vitamin K, which predominantly contain vitamin K1 (phylloquinone) found in green leafy vegetables. The bacterial synthesis of MK-7 occurs as a secondary metabolite during the organism's growth cycle, with concentrations varying depending on fermentation conditions, soybean variety, and bacterial strain used.

Natto contains approximately 900–1,000 micrograms of vitamin K2 per 100 grams, making it one of the richest natural dietary sources of this nutrient globally. For context, this represents roughly 10–15 times the safe intake recommendation for vitamin K in adults, which is about 1 microgram per kilogram of body weight per day. The MK-7 form found in natto has distinct pharmacokinetic properties compared to vitamin K1, including a longer half-life in circulation (approximately 72 hours versus 1-2 hours for K1), resulting in more sustained blood levels, which has generated considerable research interest in recent years regarding its potential health applications beyond basic nutritional requirements.

Vitamin K2 in Natto: Health Benefits and Evidence

Vitamin K2, particularly the MK-7 form abundant in natto, has been investigated for several potential health benefits beyond its established role in blood coagulation. The primary physiological function of vitamin K2 involves the carboxylation of specific proteins that regulate calcium metabolism in the body. These include osteocalcin (involved in bone mineralisation) and matrix Gla-protein (which inhibits vascular calcification). This mechanism has led researchers to explore whether dietary vitamin K2 might influence bone and cardiovascular health.

Bone health represents one area where evidence is emerging, though remains somewhat inconsistent. Several observational studies, predominantly from Japan, have suggested associations between natto consumption and improved bone mineral density, particularly in postmenopausal women. A systematic review examining vitamin K2 supplementation found some evidence for reduced fracture risk, though the quality of evidence was rated as low to moderate. It's worth noting that many positive bone studies used pharmacological doses of MK-4 (another form of K2) rather than the MK-7 found in natto. NICE guidance does not currently recommend vitamin K supplementation specifically for osteoporosis prevention, emphasising instead calcium, vitamin D, and lifestyle modifications as first-line approaches.

Regarding cardiovascular health, the proposed mechanism involves prevention of arterial calcification through activation of matrix Gla-protein. The Rotterdam Study, a large prospective cohort investigation, found that higher dietary intake of vitamin K2 (but not K1) was associated with reduced coronary calcification and cardiovascular mortality. However, subsequent randomised controlled trials have produced mixed results, and there is currently insufficient evidence to recommend vitamin K2 supplementation for cardiovascular disease prevention in UK clinical practice.

It is important to emphasise that whilst these associations are scientifically interesting, there is no official link established between natto consumption and guaranteed protection against osteoporosis or heart disease. Patients should not view natto as a therapeutic intervention but rather as part of a balanced, varied diet. Further high-quality randomised trials are needed before definitive clinical recommendations can be made.

How Much Vitamin K Is in Natto?

The vitamin K content of natto is remarkably high compared to other dietary sources, though exact concentrations vary depending on production methods and fermentation conditions. Standard natto contains approximately 900–1,100 micrograms of vitamin K2 (MK-7) per 100-gram serving, with some analyses reporting values exceeding 1,000 micrograms. A typical Japanese serving portion of natto is around 40–50 grams (often sold in small polystyrene containers), which would provide approximately 360–550 micrograms of vitamin K2.

To contextualise these figures, the UK safe intake for vitamin K (combining K1 and K2) is approximately 1 microgram per kilogram of body weight daily. For a 70-kilogram adult, this equates to roughly 70 micrograms per day. A single standard serving of natto therefore provides 5–8 times this reference value. By comparison, other vitamin K-rich foods contain substantially lower amounts: 100 grams of kale provides approximately 390 micrograms of vitamin K1, whilst hard cheeses (another source of K2) typically contain about 20–80 micrograms per 100 grams, varying significantly by cheese type.

The bioavailability of MK-7 from natto is notably high, with studies demonstrating a longer circulation time compared to vitamin K1 from vegetables. MK-7 has a plasma half-life of approximately 72 hours, compared to 1–2 hours for vitamin K1, resulting in more sustained elevation of vitamin K status. This extended circulation time means that even infrequent consumption of natto can significantly influence overall vitamin K status.

For patients monitoring their vitamin K intake—particularly those on warfarin therapy—it is crucial to recognise that natto represents an extremely concentrated source that can substantially affect international normalised ratio (INR) values. Patients on warfarin should avoid starting natto consumption or varying their intake without consulting their anticoagulation clinic, as this could significantly reduce INR values and potentially increase thrombotic risk. Healthcare professionals should specifically enquire about natto consumption when assessing dietary vitamin K intake in relevant patient populations.

Safe Consumption of Natto: UK Guidance for Patients

For the general population without contraindications, natto can be consumed safely as part of a varied diet, though its unfamiliar taste and texture may require gradual introduction. There is no established upper tolerable limit for vitamin K in the UK or EU, as toxicity from dietary sources has not been documented. However, several important patient safety considerations warrant emphasis, particularly regarding drug interactions and specific clinical scenarios.

Patients taking anticoagulant medications require particular caution. Those prescribed warfarin must maintain consistent vitamin K intake, as fluctuations can significantly affect INR and anticoagulation control. Given natto's exceptionally high vitamin K2 content, even small servings can substantially reduce warfarin efficacy, potentially increasing thrombotic risk. The MHRA and British National Formulary advise patients on warfarin to avoid significant changes in vitamin K consumption. Whilst complete dietary restriction is unnecessary, introducing natto or consuming it irregularly is inadvisable. Patients should discuss any dietary changes with their GP or anticoagulation clinic before consuming natto.

Regarding direct oral anticoagulants (DOACs) such as apixaban, rivaroxaban, edoxaban, and dabigatran, the interaction profile differs. These medications do not depend on vitamin K antagonism for their mechanism of action, and therefore dietary vitamin K intake does not directly affect their anticoagulant effect. However, patients should still inform their healthcare provider about significant dietary changes.

Additional considerations include:

• Pregnancy and breastfeeding: Natto is generally considered safe at normal dietary amounts, though pregnant women should ensure proper food hygiene given its fermented nature. There is no evidence suggesting harm from dietary vitamin K2 during pregnancy, but high-dose supplements should be avoided unless advised by a healthcare professional.

• Individuals with soy allergy: Natto should be avoided entirely, as it is a soy-based product.

• Patients with thyroid conditions: Fermented soy products contain compounds that may interfere with thyroid hormone absorption; consumption should be separated from levothyroxine administration by at least 4 hours.

Patients experiencing unusual symptoms after consuming natto, or those uncertain about its suitability for their individual circumstances, should contact their GP for personalised advice. Healthcare professionals can assess potential interactions with existing medications and medical conditions to provide appropriate guidance tailored to individual patient needs. Patients should report any suspected side effects from medicines, including interactions with foods like natto, via the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk or the Yellow Card app).

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