If you already have pretty good stats (gpa and DAT) and demonstrate an ability in sciences, then an MPH will probably work in your favor. However, I know from experience that a friend of mine who was getting her MPH *at the school she applied to* did not really have a much better chance of getting in because they didn't see it as being overly relevant to a DDS program. Her stats were a bit lower than they would have liked and the MPH didn't really help her.
So, if you're looking to increase your chances by getting an MPH, my advice would be against it unless you're looking for a fallback. If you want an advanced degree to help you, I would recommend an MS in bio, chem or biochem.