In In Defense of Food, Michael Pollan explains what he sees as one of the problems with science-based dietary advice:
When Prout and Liebig nailed down the macronutrients, scientists figured that they now understood the nature of food and what the body needed from it. Then when the vitamins were isolated a few decades later, scientists thought, okay, now we really understand food and what the body needs for its health; and today it’s the polyphenols and carotenoids that seem to have completed the picture.
One has to wonder, if the scientists have long been so confident in the completeness of their understanding of human nutrition, why do they continue researching the subject?
It is true that scientists have sometimes expressed undue confidence in an idea. However, the suggestion that nutrition scientists see human nutrition as a solved problem is patently absurd. These scientists continue doing research precisely because they understand that there are many questions remaing about nutrition.
It is not now possible, and may never be possible, to specify a diet that would protect everyone against all forms of cancer. Nevertheless, the committee believes that it is possible on the basis of current evidence to formulate interim dietary guidelines that are both consistent with good nutritional practices and likely to reduce the risk of cancer.
The authors of the report thus readily acknowledged that they didn’t “really understand food.” They issued guidelines anyway on the belief that useful recommendations could be made based on what they did understand.