Why Nutrition Is So Confusing - NYTimes.com

"And they do what are called observational studies, observing populations for decades, documenting what people eat and what illnesses beset them, and then assume that the associations they observe between diet and disease are indeed causal — that if people who eat copious vegetables, for instance, live longer than those who don’t, it’s the vegetables that cause the effect of a longer life. And maybe they do, but there’s no way to know without experimental trials to test that hypothesis.

"The associations that emerge from these studies used to be known as “hypothesis-generating data,” based on the fact that an association tells us only that two things changed together in time, not that one caused the other. So associations generate hypotheses of causality that then have to be tested. But this hypothesis-generating caveat has been dropped over the years as researchers studying nutrition have decided that this is the best they can do."