Treatment of natto, a fermented soybean preparation, to prevent excessive plasma vitamin K concentrations in patients taking warfarin

Abstract

The purpose of this study is to find a method of cooking natto that prevents the appearance of high-plasma vitamin K concentrations after the consumption of natto, so that patients taking warfarin can benefit from eating natto. Five cooking methods were examined to determine which could most effectively decrease the count of the living Bacillus subtilis in natto. Volunteers ate natto or treated natto, and their plasma vitamin K level was measured at 5, 8, 24 and 48 h thereafter. One gram of natto contained 9.7±0.1 Log cfu/mL of Bacillus subtilis. Boiling significantly reduced the Bacillus subtilis count to 5.1±0.3 Log cfu/mL, and concomitantly reduced the content of menaquinone-7 (MK-7), which is a form of vitamin K synthesized by Bacillus subtilis, from 660.40±65.32 ng/mL to 78.50±11.12 ng/mL. Untreated natto increased the MK-7 concentration in blood from 1.86±1.51 ng/mL to 14.54±4.12 ng/mL at 5 h after intake, and the MK-7 concentration remained elevated at 8, 24 and 48 h (7.29±2.20, 6.97±2.60, and 5.37±1.94 ng/mL, respectively). In contrast, boiled natto increased plasma MK-7 only mildly (from 1.61±1.11 to 4.02±0.82 ng/mL at 5 h) and the concentration remained relatively stable up to 48 h (3.46±0.83, 4.22±1.51 and 2.77±0.75 ng/mL at 8, 24 and 48 h, respectively). In conclusion, boiled natto did not cause a marked increase in the plasma concentration of vitamin K in subjects who consumed it. Thus, patients on warfarin may be able to eat boiled natto without ill effects.