There's evidence that eating more fruits and vegetables is good for you (see, e.g., [1]), but buried in [2] under point 8 of "Needs for Future Research" is the admission that
The mechanism(s) of action for the effects of vegetables and fruits have not been determined and, therefore, may vary for different health outcomes. The observed effects could be a simple displacement of these foods with other foods that cause poorer outcomes…
In other words, the command to eat more plants, constantly reverberating through the echo chamber of conventional wisdom on nutrition as it's parroted by everyone from the USDA itself all the way down to Michael Pollan, could with equal justification be replaced with the command to just eat less.
Or we could try to give laymen an accurate impression of the state of nutritional science, to admit what we don't know as readily as we boast of what we do know, but persuading people to do what we think is in their best interests is clearly more important than helping them understand the world.
[1] http://www.nutritionevidencelibrary.com/topic.cfm?cat=3238
[2] http://www.nutritionevidencelibrary.com/topic.cfm?cat=2854
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What atrocities have been committed in the name of backwards compatibility!