First case in point: the same scientists who documented the reality of non-celiac gluten sensitivity in a study a few years ago have gone back and done a more rigorous study. The result:
In contrast to our first study… we could find absolutely no specific response to gluten.And for our second case, let's consider the ongoing bruhaha over whether resveratrol (the stuff in red wine) is good for you. After so many positive indications, a new study in JAMA Internal Medicine found no effect at all, adding to the confusing literature on this embattled molecule.
Here's one piece of simple wisdom you can use: when the studies are all over the place, you can conclude that the effect being studied is not very large. When the effect of something on health is big -- like the effect of smoking, moderate regular exercise, or very high salt intake -- there is no disagreement among scientists about the effect. It is simple to measure from an actuarial table.
The health effects of things like chocolate, red wine, the Atkins diet, yoga, and so on may or may not exist, and they may be positive or negative, but we know for certain that for most people they are not very big.