Nattokinase

Nattokinase is a serine protease enzyme produced by Bacillus subtilis natto during the fermentation of soybeans to make natto, a traditional Japanese food eaten primarily in the Kanto region. It was identified in 1987 by Sumi et al., who described it as having potent fibrinolytic activity — the capacity to dissolve fibrin, the structural protein of blood clots. This discovery generated substantial research interest in nattokinase as a potential cardiovascular intervention, and it is now widely available as a supplement marketed for thrombosis prevention, blood pressure management, and cardiovascular health. The clinical evidence base, while growing, does not yet support the broader claims attached to this marketing, and the safety profile requires more careful attention than most supplements of comparable popularity receive.

What it is

Nattokinase is standardised in Fibrinolytic Units (FU), a measure of enzyme activity rather than mass. This matters for comparing products and interpreting evidence — milligram quantities are not interchangeable with FU values, and products claiming identical milligram content can have vastly different enzyme activities. Most commercial products contain 2,000 FU per capsule; the evidence suggests this dose is insufficient for lipid-lowering outcomes and that blood pressure effects are more reliably demonstrated at higher doses.

The enzyme's primary fibrinolytic mechanism involves cleavage and inactivation of plasminogen activator inhibitor type 1 (PAI-1), which normally suppresses tissue plasminogen activator (tPA) — the body's primary clot-dissolving enzyme. By inhibiting PAI-1, nattokinase enhances the activity of endogenous tPA and thereby increases fibrinolysis. It also has direct proteolytic activity on fibrin, though this is estimated to be less potent than the PAI-mediated pathway. Additionally, nattokinase reduces clotting factor concentrations including fibrinogen, factor VII, and factor VIII, and may inhibit platelet aggregation through thromboxane pathway effects.

These mechanisms are well-characterised and plausible. The important interpretive distinction is between demonstrating these mechanisms in humans — which has been done — and demonstrating that they produce meaningful clinical outcomes such as reduced thrombotic events, which has not been done in adequately powered trials.

What the evidence shows

Blood pressure. The least inconsistent biomarker signal across the nattokinase RCT literature is blood pressure reduction in hypertensive and pre-hypertensive populations. Li et al. (2023, Reviews in Cardiovascular Medicine) conducted a systematic review and meta-analysis of six RCTs (n=546) and found significant reductions in both SBP (MD −3.45 mmHg, 95% CI −4.37 to −2.18) and DBP (MD −2.32 mmHg, 95% CI −2.72 to −1.92). The effect size is modest, the trial base is small, and the population is restricted — this should not be read as a well-established effect. Nattokinase is unusual among supplement ingredients in having a well-demonstrated biological effect in humans — measurable fibrinolytic and anticoagulant activity — without corresponding clinical outcome evidence to show this translates into meaningful cardiovascular benefit. A well-cited individual RCT by Kim et al. (2008, Hypertension Research, n=86) in pre-hypertensive and stage 1 hypertensive adults found net SBP reduction of −5.55 mmHg over eight weeks, with a mechanism partly mediated by reductions in renin activity.

These blood pressure effects are biomarker-level findings. The trials were not designed or powered to demonstrate reductions in cardiovascular events such as stroke or myocardial infarction, and they should not be extrapolated to clinical cardiovascular prevention.

Fibrinolytic biomarkers. Multiple small human trials have demonstrated that nattokinase supplementation reduces circulating fibrinogen, factor VII, factor VIII, and PAI-1 levels, and that single oral doses enhance fibrinolytic activity measurable in blood. Hsia et al. (2009, Nutrition Research) showed nattokinase decreased fibrinogen, factor VII, and factor VIII over eight weeks in 45 participants. Kurosawa et al. (2015, Scientific Reports) demonstrated enhanced fibrinolysis and anticoagulation profiles after a single oral dose in healthy volunteers. These are biomarker-level findings consistent with the proposed mechanism. They do not constitute clinical evidence that nattokinase prevents thrombotic events or can substitute for anticoagulant therapy. Critically, altering coagulation biomarkers without demonstrated clinical benefit introduces potential risk, not just uncertainty — an active fibrinolytic effect on the coagulation system can cause harm in some contexts even while appearing beneficial in others.

Lipid effects and atherosclerosis. Low-dose nattokinase at typical supplement doses (2,000–4,000 FU/day) does not reliably lower LDL cholesterol or total cholesterol in RCTs. The Li et al. 2023 meta-analysis found that low-dose supplementation had no significant lipid-lowering effect, and at some doses was associated with a slight increase in total cholesterol. The largest study reporting positive lipid and atherosclerosis outcomes — Chen et al. (2022, Frontiers in Cardiovascular Medicine, n=1,062) — used 10,800 FU/day over 12 months and reported reductions in LDL, atherosclerosis progression, and carotid intima-media thickness. However, this was a non-randomised clinical study, not an RCT, and the design limits causal inference. The lipid and atherosclerosis evidence should not be cited in support of typical-dose commercial products.

Five questions

Does low status cause harm? Nattokinase is not an essential nutrient and there is no deficiency state. Dietary nattokinase intake from natto consumption in Japan is not associated with a specific deficiency phenotype in populations that do not eat natto. There is no evidence that low nattokinase intake causes harm.

Does supplementation prevent disease? No controlled human evidence establishes that nattokinase supplementation prevents thrombotic events, stroke, myocardial infarction, or cardiovascular death. The fibrinolytic mechanism is plausible, but mechanism is not outcome. The trials conducted have not tested clinical disease endpoints and are not adequately powered to do so.

Does it affect biomarkers? Yes — modestly and selectively. Blood pressure is reduced by approximately 3–5 mmHg systolic in hypertensive populations in the most consistent evidence. Fibrinolytic biomarkers including PAI-1, fibrinogen, and clotting factors are reduced in multiple small trials. LDL and cholesterol are not reliably reduced at typical supplement doses. These are surrogate measures and should not be read as clinical cardiovascular benefit.

Does it help clinical populations? Hypertensive adults show the most consistent evidence for blood pressure reduction. No clinical population has been shown to have improved outcomes — reduced stroke, reduced thrombotic events, or improved cardiovascular prognosis — from nattokinase supplementation. The NAPS trial (Hodis et al., 2021, n=265, 3 years) examined nattokinase in older adults at risk for cardiovascular disease and found no significant differences in adverse events, but this was primarily a safety and feasibility study rather than a powered efficacy trial.

Does it benefit healthy individuals? Effects on blood pressure in normotensive individuals are not established. Fibrinolytic biomarker changes in healthy volunteers have been demonstrated at single doses, but whether these translate into any meaningful benefit in people without thrombotic risk is not known.

Individual variation

The blood pressure evidence is most consistent in pre-hypertensive and hypertensive adults; effects in normotensive individuals are not reliably demonstrated. Individuals who consume natto regularly as a traditional food will have some dietary nattokinase exposure, though supplemental doses deliver higher enzyme activity.

Soy allergy is a contraindication. Nattokinase is derived from fermented soybean and anaphylactic reactions have been reported in individuals with fermented soybean allergies.

The dose-activity relationship is more important for nattokinase than for most supplements because efficacy appears dose-dependent in a way that is clinically meaningful — the typical 2,000 FU commercial dose sits below the threshold at which most outcomes beyond modest blood pressure effects have been demonstrated. This should inform purchasing decisions for anyone choosing to supplement.

Testing and status assessment

There are no clinically available tests for nattokinase status or fibrinolytic activity relevant to supplementation decisions. Blood pressure monitoring is the most practically useful measure for tracking any expected effect. Coagulation parameters including INR should not be self-monitored as a guide to nattokinase dosing — clinical supervision is required for anyone with coagulation-related health concerns.

Safety

Nattokinase has one of the more substantive safety profiles among supplements in this library, and the relevant concerns deserve explicit treatment.

The fibrinolytic and anticoagulant properties that give nattokinase its proposed mechanism also create meaningful clinical risk when combined with anticoagulant or antiplatelet medications, or in populations with intrinsic bleeding risk. Most published clinical trials systematically excluded participants on anticoagulant therapy, which means the safety profile in those populations is based on mechanistic reasoning and case reports rather than trial data.

The most clinically important case is that nattokinase must not be used as a substitute for prescribed anticoagulation. A documented case (Elahi et al., 2015) describes a patient who independently discontinued warfarin after mechanical aortic valve replacement and substituted nattokinase. After nearly a year of this substitution, the patient developed prosthetic valve thrombosis requiring repeat valve replacement — a life-threatening complication. No adequate anticoagulation for mechanical heart valves has been found among natural supplements.

A case of fatal internal bleeding has been reported in an elderly woman who took over-the-counter nattokinase for atrial fibrillation without other anticoagulants. This case is notable because it occurred in a patient not on concurrent blood thinners — the nattokinase itself appears to have contributed to the fatal outcome through its intrinsic fibrinolytic activity.

Additive bleeding risk with concurrent anticoagulants (warfarin, direct oral anticoagulants including apixaban and rivaroxaban) and antiplatelet agents (aspirin, clopidogrel) is mechanistically expected and supported by case reports. The frequency and magnitude of these risks are not quantified in controlled trials — most of which excluded anticoagulated participants entirely. Nattokinase should not be combined with these agents without explicit clinical supervision.

Pre-surgical discontinuation is recommended. The fibrinolytic activity of nattokinase creates bleeding risk in surgical contexts; discontinuation at least two weeks prior to elective procedures is appropriate.

Vitamin K note: whole natto food contains high levels of vitamin K and Bacillus subtilis continues to synthesise vitamin K in the intestine following natto consumption, which can interfere with warfarin. Nattokinase supplement preparations are typically processed to remove or reduce vitamin K content, but this should be verified on product labels, particularly for warfarin users.

Pregnancy safety data are absent. Avoidance in pregnancy is appropriate.

What can reasonably be concluded

Nattokinase has a well-characterised fibrinolytic mechanism and a modest but consistent blood pressure signal in pooled RCT data. It is one of the few supplement ingredients where the proposed mechanism — direct enzymatic activity on the coagulation system — is demonstrably active in humans at supplemental doses.

The gap between mechanism and clinical outcome evidence is the defining limitation. No controlled trial has demonstrated that nattokinase supplementation prevents stroke, myocardial infarction, or other thrombotic events. The lipid-lowering evidence is unreliable at typical commercial doses. Blood pressure effects are real but modest and biomarker-level only.

The safety concerns are more concrete than the efficacy evidence and should receive proportional attention. Nattokinase is not a benign supplement in high-risk populations — the documented fatal cases and valve thrombosis case establish it as an agent with real potential for harm when used inappropriately or in the wrong population. For otherwise healthy adults without anticoagulant therapy or high bleeding risk, the short-term safety profile from trials is reassuring. For anyone on anticoagulants, with a mechanical heart valve, with significant bleeding risk, or planning surgery, nattokinase requires explicit clinical discussion before use.

Where evidence is limited or outcomes are uncertain, conclusions should be treated as provisional and subject to revision as the evidence base develops.