The human body can absorb some of the that is synthesized by intestinal bacteria.

The answer to this question would be: vitamin

Some bacteria can produce vitamin K2 in the intestine. Other bacteria also able to change vitamin K1 into vitamin K2. But the amount produced is not much, so human still need to fulfill it from the diet. Vitamin K has a role in calcium regulation and in blood clotting pathway.

Dthrough a slow process