Researchers in Japan investigated the absorption of Nattokinase across the rat intestinal tract.

They introduced nattokinase into the duodenum of the rats at a dose of 80mg/Kg and monitored fibrinogen in the plasma.  There was a breakdown of the fibrinogen suggesting that nattokinase had been absorbed across the intestinal tract and itself remained intact to carry out it’s protease activity.

Transport of nattokinase across the rat intestinal tract.