DC hits the back of the Internet in 'We know little about the effect of diet on health. That’s why so much is written about it':
More than in any other field it is hard to do the RCTs that could, in principle, sort out the problem. It’s hard to allocate people at random to different diets, and even harder to make people stick to those diets for the many years that are needed.
We can probably say by now that no individual food carries a large risk, or affords very much protection. The fact that we are looking for quite small effects means that even when RCTs are possible huge samples will be needed to get clear answers. Most RCTs are too short, and too small (under-powered) and that leads to overestimation of the size of effects. That’s a problem that plagues experimental pyschology too, and has led to a much-discussed crisis in reproducibility.