The Science Behind Natto and Bone Health
Natto's unique fermentation process, driven by the Bacillus subtilis natto bacteria, is responsible for its high concentration of beneficial compounds for bone health. Unlike other soy products, natto is a powerhouse of Vitamin K2, specifically the highly bioavailable menaquinone-7 (MK-7) form. This makes it a standout food for supporting bone mineral density.
Vitamin K2 and Osteocalcin Activation
The primary mechanism through which natto supports bone health is its role in calcium metabolism via Vitamin K2. The body requires Vitamin K2 to activate osteocalcin, a protein that is crucial for binding calcium to the bone matrix. Without sufficient Vitamin K2, osteocalcin remains inactive and unable to perform this vital function. This can lead to calcium being deposited in soft tissues, like arteries, rather than in the bones, where it is needed. By providing a rich source of bioavailable Vitamin K2, natto ensures that calcium is effectively directed to the bones, contributing to their strength and density.
Bioavailable Isoflavones
Beyond Vitamin K2, natto also contains isoflavones, plant-based compounds with a structure similar to estrogen. Isoflavones have been studied for their potential to help prevent bone loss, particularly in postmenopausal women. The fermentation process of natto enhances the bioavailability of these isoflavones, meaning the body can absorb and utilize them more efficiently than those found in non-fermented soy products like tofu. Research indicates that this combination of K2 and isoflavones may contribute to the maintenance of bone mass.
Geographic and Population Studies
Japanese population studies have consistently shown a strong correlation between habitual natto consumption and better bone health outcomes. The Japanese Population-Based Osteoporosis (JPOS) Study found that postmenopausal women with higher natto intake experienced less bone mineral density (BMD) loss at the femoral neck and wrist compared to those who consumed less natto. The Fujiwara-kyo Osteoporosis Risk in Men (FORMEN) study similarly demonstrated a positive association between natto intake and higher BMD in elderly Japanese men. Furthermore, an inverse association has been found between natto intake and the incidence of hip fractures in Japan.
Natto vs. Other Sources for Bone Health
| Feature | Natto | Aged Cheese (e.g., Gouda) | Leafy Greens (K1) | Supplements (K2) |
|---|---|---|---|---|
| Vitamin K2 (MK-7) Content | Exceptionally high; the richest food source | Moderate; content varies with aging | Negligible; contains K1, which is less effective for bone density | Highly variable based on dosage and form |
| Calcium Content | Good source, with ~217mg per 100g serving | Very high, but dependent on the type of cheese | Moderate to high, but often paired with high oxalate content inhibiting absorption | Only present if included in the formulation |
| Bioavailable Isoflavones | Yes, enhanced bioavailability due to fermentation | No, not a soy product | No, not a soy product | No, typically not included |
| Other Nutrients | Probiotics, nattokinase, iron | Protein, Vitamin A, healthy fats | Fiber, Vitamin A, Vitamin C | Varies widely by supplement blend |
| Overall Bone Health Impact | Strong evidence for increasing BMD and reducing fracture risk, especially in Japanese populations | Supports bone health but significantly lower K2 (MK-7) content than natto | Supports bone health generally but less direct impact on activating osteocalcin compared to K2 | Can be effective, but some studies suggest whole food natto offers greater benefits |
How to Incorporate Natto into Your Diet
For those new to natto's unique taste and sticky texture, here are a few simple ways to ease into it:
- Classic style: Serve over a bowl of hot rice with a dash of soy sauce and Japanese mustard.
- With eggs: Mix natto with scrambled eggs or serve it as a side dish for breakfast.
- In salads: Add a small portion of natto to your favorite salad for a nutritional boost.
- In sushi rolls: Use it as a filling for maki rolls, often paired with green onion.
- With toast: Spread natto on toast with avocado or other toppings.
- In soups: Stir natto into miso soup at the end of cooking to add a distinct umami flavor and texture.
- Condiments: Try mixing it with a little kimchi or fermented vegetables to complement the flavor.
Considerations and Potential Downsides
While natto offers significant bone health benefits, it is not for everyone. The most critical consideration is for individuals on blood-thinning medications, such as warfarin. Because Vitamin K is essential for blood clotting, consuming natto can interfere with the medication's effectiveness. It is imperative to consult a healthcare provider before adding natto to your diet if you take these medications.
Additionally, some people may be concerned about soy's status as a goitrogen, which can potentially interfere with thyroid function. For healthy individuals, the effect is likely minimal. However, those with pre-existing thyroid conditions should discuss it with their doctor.
Conclusion: Does Natto Increase Bone Density?
The scientific evidence, particularly from extensive Japanese population studies, strongly suggests that yes, natto can help increase bone density and, crucially, reduce the risk of osteoporotic fractures. The primary driver of this effect is natto's exceptionally high concentration of Vitamin K2 (MK-7), which effectively directs calcium to the bones to build and maintain a strong bone matrix. Unlike other soy products, the unique fermentation process makes its compounds highly bioavailable and potent. For those looking to proactively support their bone health through diet, habitual natto intake is a scientifically supported and powerful option.
For more in-depth research on the subject, see the study on natto intake and fracture risk in postmenopausal women here.
