Thanks to Bacillus subtilis fermentation, natto may be THE most concentrated known food source of Vitamin K2 (1). Vitamin K2 is increasingly being recognized as a critical micronutrient which most people are not getting enough of through our modern eating habits. Vitamin K2 is vital to both bone and cardiovascular health.
Most of us appreciate already the importance of calcium in the diet; however, calcium can do as much harm than good if not appropriately transported from food (via the bloodstream) to bones. It is emerging that perhaps the most important function of Vitamin K2 is to help deliver and deposit calcium to right places in the body (skeleton). Lack of Vitamin K2 can lead to accumulation of calcium in blood vessels, (calcification) and therefore, development of heart disease and osteoporosis. A large body of credible in vitro, in vivo, and epidemiological studies support the notion that increasing dietary Vitamin K2 intake is highly beneficial in promoting both heart and bone health (2,3,4).
This video is a bit long but is an excellent, detailed discussion of Vitamin K2's health benefits...
References: (1) http://www.scientificamerican.com/article/vitamin-k-and-natto-what-s-the-connection/ (2) http://chriskresser.com/vitamin-k2-the-missing-nutrient/ (3) Tsukamoto, Y. et al.(2000) Intake of fermented soybean (natto) increases circulating vitamin K2 (menaquinone-7) and gamma-carboxylated osteocalcin concentration in normal individuals. J Bone Miner Metab. 2000;18(4):216-22. http://www.ncbi.nlm.nih.gov/pubmed/10874601. (4) Beulens, JW et al. (2010) The role of menaquinones (vitamin K₂) in human health. Br J Nutr. 2013 Oct;110(8):1357-68.