Nattokinase comes from fermented soybean and is a common food in Japan.  If you’ve ever seen Natto, you’ll know that it doesn’t look particularly appealing (I tend to think of a bad cold every time I see it).  I have never tried it myself so cannot comment on the taste.  However, I found this entertaining video of a guy trying Natto for the first time.

After he finishes, I thought he was going to bring it up again

If you do eat Natto, and want some serving suggestions, this lady shows you different ways to eat Natto.