Nutritional epidemiology is a stickier than molasses because of the innumerable confounding variables present in studies on food consumption. This article is excellent You Can’t Trust What You Read About Nutrition , and here are my favorite excerpts from it:
“A few years back, Jorge Chavarro, a nutritional epidemiologist at the Harvard School of Public Health, advised that women trying to conceiveconsider swapping low-fat dairy foods for high-fat dairy products such as ice cream, based on FFQ data from an ongoing study of nurses. He and his colleague Walter Willett also wrote a book promoting a “fertility diet” based on the results. When I reached Chavarro this week to ask how confident he was in the link between dairy intake and fertility, he said that “of all the associations we found, this is the one we had the least confidence in.” It’s also, of course, the one that made headlines.
Nearly every nutrient you can think of has been linked to some health outcome in the peer-reviewed scientific literature using tools like the FFQ, said John Ioannidis, an expert on the reliability of research findings at the Meta-Research Innovation Center at Stanford. In a 2013 analysispublished in the American Journal of Clinical Nutrition, Ioannidis and a colleague selected 50 common ingredients at random from a cookbook and looked for studies evaluating each food’s association to cancer risk. It turned out that studies had found a link between 80 percent of the ingredients — including salt, eggs, butter, lemon, bread and carrots — and cancer.”