Interactions with Blood-Thinning Medications
One of the most significant disadvantages of natto concerns its high vitamin K2 content. A single 100-gram serving can contain over 600 micrograms of vitamin K2, far exceeding the daily recommended intake. Vitamin K plays a crucial role in blood clotting, and while beneficial for most, it can have serious consequences for individuals on anticoagulant or blood-thinning medications, such as warfarin.
The Warfarin and Vitamin K Connection
Warfarin works by inhibiting vitamin K-dependent clotting factors. A sudden, high intake of vitamin K from natto can counteract the medication's effect, making it less potent and increasing the risk of dangerous blood clots. For this reason, anyone taking blood thinners should consult with a doctor before adding natto to their diet to prevent potential complications. In fact, cases have been reported where patients who replaced warfarin with nattokinase supplements experienced serious adverse events.
Potential for Thyroid Interference
Natto is made from soybeans, which contain compounds called goitrogens. These substances can interfere with the thyroid gland's normal function, particularly in individuals with pre-existing thyroid conditions, such as hypothyroidism. While the impact is generally minimal for healthy individuals, those with impaired thyroid function should limit their intake or seek medical advice.
How Goitrogens Affect the Thyroid
Goitrogens can inhibit the thyroid gland's ability to produce hormones by interfering with iodine uptake. For a person with an underactive thyroid, this could worsen their condition or require an adjustment to their medication dosage. The fermentation process in natto does not completely eliminate goitrogens, so this risk remains, especially with high consumption.
Allergy Risks
As a soy-based product, natto poses a risk for individuals with a soy allergy or intolerance. While most fermented soy products are generally well-tolerated, rare but serious cases of late-onset anaphylaxis specifically linked to natto have been reported. The allergen, poly-gamma-glutamic acid (PGA), is found in the sticky, viscous mucilage of natto and has been shown to cause delayed allergic reactions, sometimes occurring hours after ingestion.
Natto Allergy vs. Soy Allergy
It's important to differentiate between a standard soy allergy and a natto-specific allergy. Studies have found a connection between natto allergy and exposure to jellyfish stings, which also contain PGA. This can cause sensitization to the protein, leading to an allergic reaction when natto is consumed later. Therefore, a history of exposure to jellyfish can be a risk factor for developing this specific, delayed-onset allergy.
Digestive Upset and Side Effects
Despite being a fermented food containing probiotics, natto can cause digestive discomfort in some individuals. This can be due to its high fiber content or a simple adjustment period as the body gets used to the new probiotics introduced into the gut microbiome.
Common Digestive Side Effects
- Bloating and gas: High-fiber foods can cause gas and bloating, especially when first introduced to a diet.
- Diarrhea: Overconsumption can lead to digestive issues, including diarrhea, as noted in some sources.
- Indigestion: While fermentation makes natto easier to digest for most, some may still experience a reaction, potentially related to an underlying sensitivity.
Acquired Taste and Texture
For many people outside of Japan, the most immediate disadvantage of natto is its taste, smell, and texture. The flavor is often described as pungent, earthy, and nutty, while its signature sliminess is a significant hurdle for Western palates. This can make it difficult for new consumers to incorporate it into their diet, regardless of the potential health benefits.
Comparison of Natto's Characteristics
| Feature | Description | Common Perception |
|---|---|---|
| Flavor | Strong, pungent, nutty, umami. | Considered an acquired taste; off-putting for many beginners. |
| Smell | Very strong, distinct, and savory. | Often described as unpleasant or cheesy by those unfamiliar with it. |
| Texture | Viscous, sticky, and stringy. | Can be challenging and unappealing for newcomers. |
Practical Disadvantages
Beyond the physiological and sensory aspects, there are a few practical downsides to consider.
- Availability: Outside of Asian grocery stores or specialized online retailers, natto can be difficult to find in many parts of the world.
- Preparation and Storage: While it can be bought pre-packaged, it must be thawed and handled correctly to maintain its properties. It also requires specific fermentation conditions if one chooses to make it at home, which can be a multi-day process.
- Calorie Density: While nutritious, natto is relatively high in calories and should be consumed in moderation to avoid unintentional weight gain.
Conclusion: Weighing the Risks and Rewards
While natto is a superfood packed with protein, fiber, and potent nutrients like vitamin K2 and nattokinase, it is not without its drawbacks. Potential issues with blood-thinning medication, possible thyroid function interference, rare but serious allergic reactions, and its divisive taste and texture mean it is not suitable for everyone. Individuals on medication or with specific health conditions must consult a doctor before consumption. However, for most healthy people, enjoying natto in moderation as part of a balanced diet offers more benefits than risks.
