I once feared that if prolonged exposure to zero-g was toxic to the human metabolism, then manned space travel would never happen. In fact, early space explorers and planners feared this too. It was one of the "hazards" of space travel (like meteor strikes)that never fully materialized, but were anticipated with dread.

I guess the answer lies in that living organisms are essentially bags of colloidal solutions, and chemical reactions occur just as easily whether or not in the presence of a gravitational acceleration. Still, we do know there are long term negative metabolic effects of prolonged exposure to zero-g, even if they are not immediately fatal.