Dietary guidelines for prevention of cancer: are they justified?

Although dietary guidelines for reduction of cancer risk have been issued and well publicized, there is still controversy within the scientific community about whether such guidelines are appropriate. Given that the scientific evidence of a causal role for diet in carcinogenesis is suggestive but not conclusive, is it more responsible to promulgate guidelines that might be ineffective or to delay communicating findings to the public until all the evidence is in-a process that could take decades? Because the public is already familiar with guidelines intended to minimize the risk of cancer, how should they be promoted by health professionals given the strength of the evidence on which they are based?