Well, there's the problem that most of the effects are pretty small - most of the really dramatic effects have already been discovered. So it's not a matter of

Well, there's the problem that most of the effects are pretty small - most of the really dramatic effects have already been discovered. So it's not a matter of "People who eat X every day develop intestinal lesions and die within 18 months", it's more like "People who eat X every day have a lifetime increase in the incidence of toe cancer from .016 to .02". Combine that with the fact that these effects take years or decades to show themselves, it makes for an area of science with very few hard answers.