What are the side effects of eating natto?

November 14, 2025 •

3 min read

Natto, a traditional Japanese food, offers various health benefits but might cause side effects for certain individuals. Understanding what are the side effects of eating natto, is important for those with health conditions or taking specific medications.

Quick Summary

This article explores the health risks and adverse reactions associated with consuming natto, focusing on its vitamin K2 content, potential allergies, and digestive issues. Learn about the factors to consider before adding natto to your diet.

Key Points

Blood Thinner Interaction: The high vitamin K2 content in natto can counteract the effects of blood-thinning medications like warfarin, increasing blood clot risk.
Nattokinase Bleeding Risk: The enzyme nattokinase may increase bleeding risk in people with bleeding disorders or those on anticoagulant drugs.
Soy Allergy Risk: Individuals with a soy allergy should not consume natto, and even those without can experience a rare, delayed allergic reaction called late-onset anaphylaxis.
Digestive Issues: New consumers may experience gas, bloating, or stomach upset due to the high fiber and probiotic content as their system adjusts.
Gout Precaution: Natto contains purines, which can elevate uric acid levels, potentially posing a risk for individuals with gout or high blood uric acid.
Pre-Surgery Warning: Natto should be avoided at least two weeks before any scheduled surgery to prevent potential bleeding complications.
Hypothyroidism Concern: Soy is a goitrogen and may interfere with thyroid function in individuals with hypothyroidism.

Exploring the Potential Side Effects of Natto

Natto, a fermented soybean dish, is known for its nutritional benefits, including probiotics and the enzyme nattokinase. However, it can also have negative health consequences. Adverse effects can range from mild discomfort to serious health complications.

Interactions with Medications: Blood Thinners

A key concern with natto consumption is its high vitamin K2 content. Vitamin K helps blood clot, and high levels can interfere with blood-thinning medications.

Impact on Warfarin: Warfarin doses are carefully calculated. A sudden or high intake of natto can introduce excessive vitamin K2, which can counteract the medication. This increases the risk of blood clots.
Nattokinase's Effect: Nattokinase in natto has fibrinolytic properties, which help break down blood clots. This effect requires caution for those with bleeding disorders or those taking blood thinners, as it could increase the risk of bleeding. Medical supervision is essential for anyone taking these medications.

Allergies: Soy and Beyond

Since natto is made from soybeans, it can be an allergen. A rare reaction called late-onset anaphylaxis has been reported.

Late-Onset Anaphylaxis: This reaction can occur hours after eating natto. The allergic trigger is often poly-gamma-glutamic acid (PGA), a component of natto's sticky mucilage, rather than the soybean itself. Symptoms can include hives, swelling, wheezing, and in severe cases, a drop in blood pressure.
Unique Risk Factors: Research suggests a link between natto allergy and epicutaneous sensitization to PGA from jellyfish stings, particularly among those who engage in marine sports. This demonstrates that allergic reactions can be complex and are not limited to just dietary exposure.

Digestive Issues: Common Discomfort

Those new to natto, or with a sensitive digestive system, may experience gastrointestinal issues.

Gas and Bloating: As with high-fiber foods, introducing natto quickly can increase gas and bloating. This is usually temporary and manageable by starting with small portions.
Other Gastrointestinal Symptoms: Some may experience stomach cramps or diarrhea, especially when consuming larger amounts or if their gut is not accustomed to the probiotics in natto.

Gout Concerns

Natto contains purines, which can raise uric acid levels.

Increased Uric Acid: In those with high blood uric acid or a predisposition to gout, excessive consumption of high-purine foods like natto could trigger an attack.

Summary of Natto Risks vs. Benefits


Aspect	Potential Risk	Potential Benefit
Blood Clotting	High vitamin K2 and nattokinase can interfere with blood-thinning medication.	Nattokinase and K2 support healthy blood flow and cardiovascular function in healthy individuals.
Allergies	Severe, potentially delayed allergic reactions (anaphylaxis) can occur, triggered by either soy or PGA.	No specific benefit, but most people do not experience allergic reactions.
Digestion	New consumers may experience gas, bloating, and discomfort.	Provides probiotics for improved gut health and nutrient absorption.
Gout	High purine content may be a concern for individuals with gout or high uric acid levels.	Soy products are generally not associated with an increased risk of gout.
Dietary Intake	Excessive intake can lead to high calorie consumption and potential weight gain if not managed.	High in protein and fiber, which promotes satiety and can aid in weight management.

Conclusion

Natto offers health benefits but is not universally recommended. Considerations include high vitamin K2 and nattokinase content, which pose a direct risk for individuals on blood-thinning medication. Allergic reactions, particularly late-onset anaphylaxis, should also be considered. For healthy people, digestive side effects are the most likely and can be managed by starting with small amounts. Individuals with health conditions should consult a healthcare provider before making significant dietary changes. The risks for certain populations underscore the importance of informed eating, ensuring that natto's health benefits are pursued without compromising personal health and safety.

Here is a useful resource for understanding the specifics of nattokinase and medication interactions

Frequently Asked Questions

Individuals on blood-thinning medication like warfarin, people with a soy allergy or bleeding disorders, those with hypothyroidism, and those with a history of gout should exercise caution or avoid natto.

Yes, natto's high vitamin K2 content can interfere with blood-thinning medications like warfarin, potentially making them less effective. Nattokinase also has blood-thinning properties, which can be dangerous when combined with medication.

No, late-onset anaphylaxis caused by natto is a very rare but potentially serious allergic reaction. It is often triggered by poly-gamma-glutamic acid (PGA) in the natto's sticky component and can occur several hours after consumption.

Yes, for individuals not accustomed to fermented or high-fiber foods, eating natto can cause temporary digestive issues like gas and bloating as the gut flora adjusts.

Due to its purine content, natto could theoretically raise uric acid levels and potentially trigger a gout attack. While the evidence is mixed, individuals with gout are advised to be cautious with their intake and consult a doctor.

For late-onset reactions, the allergen is often poly-gamma-glutamic acid (PGA), a substance found in natto's characteristic viscous texture, rather than the soybean protein itself.

No, it is recommended to stop eating natto at least one to two weeks before any scheduled surgery due to the blood-thinning effects of nattokinase, which could increase the risk of excessive bleeding.

To minimize digestive discomfort, start by consuming a very small amount of natto and gradually increase your intake over time as your body adjusts to the probiotics and fiber.