Natto is a traditional Japanese food made from fermented soybeans, celebrated for its nutritional benefits, including a high concentration of vitamin K2 and the enzyme nattokinase. Despite its 'superfood' status, consumers must be aware of its potential side effects, which can range from minor digestive discomfort to serious drug interactions. The fermentation process, while beneficial for gut health, also creates unique compounds that can be problematic for certain individuals.
Blood Clotting and Medication Interactions
The most significant and well-documented side effect of natto is its interaction with medications that affect blood clotting. This is primarily due to its extremely high levels of vitamin K, particularly vitamin K2 (menaquinone-7).
- Vitamin K's Role: Vitamin K is essential for the production of several clotting factors in the liver. Its function is to promote blood coagulation, a natural process that stops bleeding.
- Warfarin and Anticoagulants: Individuals taking anticoagulant drugs, such as warfarin (Coumadin), are especially at risk. Warfarin works by interfering with vitamin K's ability to activate clotting factors. A sudden increase in vitamin K intake from natto can counteract the effects of warfarin, reducing its effectiveness and increasing the risk of dangerous blood clots. Healthcare providers must be consulted to manage medication and diet carefully.
- Nattokinase Risks: Nattokinase, an enzyme unique to natto, is known for its fibrinolytic (clot-dissolving) properties. This enzyme can also increase the risk of bleeding and bruising when combined with blood-thinning medications, including aspirin and ibuprofen. In rare cases, nattokinase supplements have been associated with bleeding issues, even without concurrent medication use.
Allergic Reactions and Sensitivities
Natto, being a soy-based product, can trigger allergic reactions in sensitive individuals. While most common allergies stem from the soybean itself, a unique form of natto allergy has been identified.
- Soybean Allergy: People with a known soy allergy should avoid natto entirely. Symptoms can range from hives and itching to severe anaphylaxis.
- Poly-γ-glutamic Acid (PGA) Allergy: A rare and unusual form of delayed anaphylaxis is linked to poly-γ-glutamic acid (PGA), a component of the natto mucilage. This type of allergy has a unique trigger, where sensitization to PGA can occur through environmental exposure, such as jellyfish stings, before causing a food-induced reaction hours later.
Digestive Issues
For some, the potent nature of fermented foods can lead to gastrointestinal discomfort, especially when first introduced to the diet.
- Gas and Bloating: The fermentation process creates beneficial probiotics, but it can also increase gas production, leading to bloating and discomfort. Those unaccustomed to fermented foods or high-fiber diets may experience this more acutely.
- Antinutrients: While fermentation reduces the levels of antinutrients found in soybeans, some residual compounds can affect digestion in susceptible individuals.
Thyroid Concerns
Soybeans, including natto, contain compounds known as goitrogens, which can potentially interfere with thyroid function.
- Goitrogenic Effects: Goitrogens can inhibit the body's ability to use iodine, which is crucial for thyroid hormone production. This is typically a concern for individuals with pre-existing thyroid conditions, such as hypothyroidism. A healthy thyroid can usually compensate, but those with impaired function may need to limit their intake.
Comparison: Natto Risks vs. Benefits
| Aspect | Potential Risk | Potential Benefit |
|---|---|---|
| Cardiovascular Health | Increased bleeding risk from nattokinase and high Vitamin K2 interaction with anticoagulant medications like warfarin. | Reduced risk of heart disease through the clot-dissolving action of nattokinase and the arterial health benefits of vitamin K2. |
| Digestive System | Gas and bloating, particularly for those new to fermented foods or sensitive to high fiber. | Improved gut health and balanced intestinal bacteria due to probiotic content. |
| Allergies | Allergic reactions to soy or the unique PGA compound, with a rare risk of delayed anaphylaxis. | None; natto provides no specific benefit for individuals with existing allergies. |
| Thyroid Function | Interference with thyroid function due to goitrogens, especially for people with hypothyroidism. | None; a healthy thyroid can mitigate risk, but it does not offer a specific benefit for this area. |
| Overall | Serious interactions for specific medical conditions necessitate caution and medical advice. | Broad nutritional support, including protein, fiber, and vitamins, for general wellness. |
Conclusion
While natto offers a rich source of beneficial nutrients and probiotics, it is crucial to recognize and respect its potential side effects. The most critical concerns are related to its high vitamin K2 content and nattokinase enzyme, which can interfere with blood-thinning medications. Additionally, those with soy allergies, pre-existing thyroid conditions, or sensitive digestive systems should approach natto with caution. For the majority of healthy individuals, consuming natto in moderation as part of a balanced diet poses minimal risk. However, anyone with a chronic health condition or taking regular medication should consult a healthcare provider before incorporating it into their diet to ensure safety and avoid adverse effects.
Revolution Fermentation's Article on Natto Benefits provides further reading on the positives, but always remember to weigh benefits against personal risks. Proper awareness allows for a safe and informed culinary experience.
How to safely try natto for the first time
For those who are not allergic and have no contraindications, introducing natto to your diet can be a great way to boost your nutrient intake. Start with a very small amount, perhaps a teaspoon, to see how your body reacts. As your digestive system adapts, you can gradually increase the portion size. Traditional Japanese preparation, served with rice and a simple topping like soy sauce and mustard, is often the most palatable way for beginners to enjoy it.
