The problem is that people have been saying that fruits and vegetables are good for you for a long time, so conscientious who exercise and go to the doctors and

The problem is that people have been saying that fruits and vegetables are good for you for a long time, so conscientious who exercise and go to the doctors and take the medicines they're prescribed will also disproportionately eat more fruits and vegetables. With health, the conventional wisdom is the hardest thing to test and is where you really need controlled trials, or at least natural experiments where people eat more or less vegetables for extrinsic reasons.