Who Cannot Take Natto: A Guide to Health Considerations

November 14, 2025 •

3 min read

Natto is a traditional Japanese food made from fermented soybeans with notable health benefits, yet it is not safe for everyone. Its high vitamin K2 content and fibrinolytic enzyme, nattokinase, pose risks for individuals on anticoagulant medications, those with soy allergies, or people with certain thyroid conditions.

Quick Summary

This guide outlines the specific health risks associated with natto consumption, focusing on individuals with soy allergies, thyroid issues, and those taking blood-thinning medications, detailing key considerations for dietary safety.

Key Points

Blood Thinner Interaction: Individuals taking anticoagulant drugs like warfarin must avoid natto due to its high vitamin K2 content, which can promote blood clotting and interfere with medication efficacy.
Soy Allergy Risk: Anyone with a soy allergy should not consume natto, as it is made from fermented soybeans and can trigger allergic reactions ranging from mild symptoms to severe anaphylaxis.
Thyroid Concerns: Natto contains goitrogens, which can affect thyroid function, making it unsuitable for those with hypothyroidism or impaired thyroid function.
Digestive Sensitivity: The high fiber and probiotic load in natto can cause gas, bloating, or discomfort for people with sensitive digestive systems, especially upon first consumption.
Pre-Surgery Precautions: Due to its blood-thinning properties from nattokinase, natto should be avoided at least two weeks before any surgical procedure to minimize the risk of excessive bleeding.
Uncommon Allergies: A rare, delayed-onset allergy to natto's poly-gamma-glutamic acid (PGA) exists, particularly in individuals with a history of jellyfish stings.

Natto and Anticoagulant Medication

For individuals on blood-thinning medication, such as warfarin, natto consumption is a significant concern. The primary issue revolves around natto's exceptionally high concentration of vitamin K2, specifically menaquinone-7 (MK-7), which promotes blood clotting. While beneficial for bone health, this clotting property directly interferes with the function of anticoagulant drugs designed to prevent blood clots.

How Natto Affects Blood Clotting

The enzyme nattokinase, also present in natto, has fibrinolytic properties, meaning it can break down fibrin, a key protein in blood clots. This seemingly contradictory effect means the overall impact on blood thinning is complex. The high dose of vitamin K2, however, is a more dominant factor in counteracting the medication. For those managing their International Normalized Ratio (INR), a measure of blood-clotting time, a sudden increase in vitamin K intake can destabilize their levels, leading to an increased risk of stroke or other complications. It is imperative for these individuals to consult their healthcare provider before adding natto or nattokinase supplements to their diet.

Soy Allergies and Intolerances

Since natto is made entirely from soybeans, anyone with a soy allergy or intolerance must avoid it. Soy allergies can cause symptoms ranging from mild digestive upset to severe, life-threatening anaphylaxis. In rare cases, some individuals with a history of jellyfish stings have developed a delayed-onset allergy to natto's poly-gamma-glutamic acid (PGA), an allergen also found in jellyfish tentacles. This unusual form of food allergy can result in severe symptoms several hours after consumption.

Symptoms of Soy Allergy

Mild: Itchy mouth, rash, hives, or swelling of the lips and tongue.
Moderate: Digestive issues like bloating, gas, nausea, and stomach cramps.
Severe (Anaphylaxis): Difficulty breathing, wheezing, tightness in the throat, and a drop in blood pressure.

Hypothyroidism and Goitrogens

Soy is considered a goitrogen, a substance that can interfere with the normal function of the thyroid gland. This is particularly relevant for individuals with hypothyroidism or an already impaired thyroid function. While the fermentation process may reduce some of the goitrogenic compounds in soybeans, they are not completely eliminated. Healthy individuals are unlikely to be affected by moderate intake, but those with pre-existing thyroid conditions should exercise caution and discuss natto consumption with their doctor.

Other Considerations and Side Effects

Though generally well-tolerated, natto can cause some digestive discomfort for certain individuals. Its high fiber and probiotic content can lead to gas and bloating, especially for those not accustomed to a diet rich in fermented foods. Moderate intake is advised when first trying natto to gauge your body's reaction.

Natto and Surgery

Due to its potential blood-thinning effects from nattokinase, doctors recommend that individuals undergoing surgery, including dental procedures, should discontinue natto consumption at least two weeks beforehand to reduce the risk of excessive bleeding.

Comparative Overview of Natto Risk Factors


Risk Factor	Why Natto is a Concern	Associated Health Conditions
Blood Thinners (e.g., Warfarin)	High vitamin K2 content promotes blood clotting, counteracting medication's effects.	Atrial fibrillation, blood clot history, heart conditions.
Soy Allergy	Natto is made from soybeans, a common allergen.	Standard soy allergy, or rare delayed-onset allergy due to jellyfish sting sensitization.
Hypothyroidism	Soy is a goitrogen, which can interfere with thyroid function.	Impaired thyroid function, managing thyroid medication.
Digestive Sensitivity	High fiber and probiotic content can cause gas and bloating.	Irritable bowel syndrome (IBS), sensitive stomach.
Pre-Surgery	Nattokinase has blood-thinning properties.	Upcoming surgical procedures, including dental work.

Conclusion: Consult a Professional

While natto offers significant nutritional benefits, including probiotics, fiber, and vitamin K2, it is not a suitable food for everyone. The most critical restrictions apply to individuals on blood-thinning medication due to the high vitamin K2 content and potential interactions with the nattokinase enzyme. Those with soy allergies, hypothyroidism, or digestive sensitivities should also proceed with caution. The risk of delayed anaphylaxis from PGA, particularly for individuals with prior jellyfish exposure, highlights the need for a thorough health history. As with any dietary change affecting medication or pre-existing conditions, consulting a healthcare provider is the safest course of action. This ensures you can enjoy the health benefits of fermented foods without compromising your safety.

For further reading, consult resources like the Memorial Sloan Kettering Cancer Center regarding nattokinase interactions with medications.

Frequently Asked Questions

No, you should not eat natto while taking blood-thinning medication like warfarin. Natto's high vitamin K2 content can interfere with these medications and promote blood clotting.

No, it is not safe to eat natto if you have a soy allergy, as natto is made from fermented soybeans and can trigger an allergic reaction.

People with hypothyroidism or impaired thyroid function should be cautious with natto. Soy, the main ingredient, contains goitrogens that can interfere with thyroid function.

Some people may experience digestive discomfort, such as gas and bloating, when first eating natto due to its high fiber and probiotic content.

Yes, though rare, a severe, delayed-onset anaphylaxis has been reported in individuals with a specific allergy to natto's poly-gamma-glutamic acid (PGA), which can be linked to prior jellyfish exposure.

Yes, it is recommended to stop eating natto at least two weeks before any scheduled surgery, including dental procedures, due to the blood-thinning effects of the nattokinase enzyme.

The key component is vitamin K2 (MK-7), which is produced during the fermentation process and can promote blood clotting, counteracting anticoagulant medication.