Question explored with the scientific record
what do you have about natto
Natto is a fermented soybean food rich in vitamin K2 (menaquinone-7, MK-7) and soyasaponins, with the strongest evidence pointing to bone health benefits in premenopausal women and anti-inflammatory effects in lab studies.
The evidence on natto comes from food chemistry and fermentation optimization studies, not from large human trials on disease outcomes. One human study in 73 premenopausal Japanese women found that eating natto three times per week for a year was linked to higher bone formation markers compared to eating none, but it did not change bone stiffness (a direct measure of bone strength) [4]. That study was small and observational, not a randomized controlled trial. A rat study showed natto diets prevented bone loss after ovary removal, but animal results do not automatically apply to humans [1].
The main active compound is MK-7, a form of vitamin K2 that stays in the blood much longer than vitamin K1 — about three days versus one to two hours [2]. This matters because vitamin K is needed to activate proteins that move calcium into bone and keep it out of arteries. Natto is by far the richest food source: one analysis found about 902 micrograms of MK-7 per 100 grams [3], and dried natto reached nearly 4,000 micrograms per 100 grams [5]. For comparison, most other foods contain negligible amounts.
| Food | MK-7 content (µg/100g) |
|---|---|
| Natto (fresh) | 902 [3] |
| Dried natto | 3,998 [5] |
| Fermented soybean paste | 12–86 [3] |
| Kefir | 0 (MK-9 only) [3] |
Lab studies on cells show that MK-7 and related vitamin K2 forms can reduce inflammatory signals. In human intestinal cells, MK-7 cut the inflammatory protein IL-8 by up to 80% [6]. In mouse immune cells, MK-9 reduced TNF-alpha by 95% [6]. These are promising mechanisms, but they come from petri dishes, not from people.
Natto also contains soyasaponins at about 1,005 nmol per gram, similar to tofu [8], and fermentation changes the isoflavones into forms the body may absorb differently [7]. The evidence does not cover long-term safety, interactions with blood thinners (vitamin K can interfere with warfarin), or effects in men, children, or people with existing health conditions.
My call: natto is a legitimate food source of vitamin K2 with plausible bone and anti-inflammatory benefits, but the human evidence is thin and limited to bone markers in one small study. Confidence: low.
Sources used 8
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Prolonged intake of fermented soybean ( natto ) diets containing vitamin K 2 (menaquinone-7) prevents bone loss in ovariectomized rats
This study investigates the preventive effects of prolonged intake of fermented soybean (natto) diets containing vitamin K2 (menaquinone-7) on bone loss in ovariectomized rats, demonstrating significant benefits in maintaining bone density and mineral content.
DOI: 10.1007/s007740050014 -
Vitamin K–containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7
This study compares the absorption and efficacy of synthetic vitamin K1 and natto-derived menaquinone-7 (MK-7) in healthy volunteers, revealing that MK-7 has a longer half-life and greater effectiveness in carboxylating osteocalcin, which may impact anticoagulant therapy.
DOI: 10.1182/blood-2006-08-040709 -
Determination of vitamin K composition of fermented food
This study developed a rapid UHPLC-APCI-MS/MS method for analyzing vitamin K compounds in fermented foods, revealing significant levels of phylloquinone and menaquinones in various products.
DOI: 10.1016/j.foodchem.2018.09.136 -
Promotion of Bone Formation by Fermented Soybean (Natto) Intake in Premenopausal Women
In healthy premenopausal Japanese women, graded intake of Natto (a vitamin K2–rich fermented soybean) over one year was associated with dose-dependent increases in bone formation markers and changes in bone turnover markers, without a significant change in calcaneal stiffness, s…
DOI: 10.3177/jnsv.50.114 -
Development of vitamin K analysis method using column switching high-performance liquid chromatography method and analysis results of various food items for vitamin K content
This study developed a column-switching high-performance liquid chromatography (HPLC) method for the sensitive analysis of vitamin K compounds in various food items, revealing significant differences in vitamin K content across different food sources.
DOI: 10.1007/s12161-024-02643-4 -
The impact of gut bacteria producing long chain homologs of vitamin K2 on colorectal carcinogenesis
Among 86 gut-derived bacterial strains, anaerobic and microaerophilic species produced the most vitamin K2 (Lactobacillus plantarum up to 4.6 mg/L), and in vitro VK2 homologs, especially long-chain MK7/MK9, inhibited pro-inflammatory cytokine secretion in Caco-2 and RAW 264.7 ce…
DOI: 10.1186/s12935-023-03114-2 -
Les isoflavones du soja dans la filière aliment santé
Soybean isoflavones are bioactive phytoestrogens with potential health benefits for cardiovascular disease, osteoporosis, and hormone-dependent cancers, but their bioavailability, metabolism, processing-related variability, and safety require cautious, long-term human evaluation.
DOI: 10.1051/ocl.2000.0287 -
The content of soyasaponin and soyasapogenol in soy foods and their estimated intake in the Japanese
This study developed a high-performance liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS) method to quantify the content of soyasaponins and soyasapogenols in various soy foods and estimated their daily intake among the Japanese population.
DOI: 10.1002/fsn3.107