Not taking one side or the other, it's important to note that when it comes to nutrition, we know very little. Or rather, we know many disparate facts but very little about how they interact. The reason is that there are very few good "unit tests" for human physiology, and we're not even sure about what they really mean (for example, current research suggests that what we thought we knew about the significance of blood cholesterol levels might be wrong). And if all we have are "integration tests" (like mortality rates), the number of variables affecting them becomes so huge, that in order to get truly statistically significant results we would need to conduct randomized trials on very big populations. Add the difficulty of conducting a double-blind randomized nutrition trial even on a small population, and you get results that have little actual "results" (although a randomized double-blind trial with vitamin pills is possible).

When John Ioannidis proved that most published medical research is false[1], he singled out genetics and nutrition[2] as the two most problematic fields, where very, very few findings are true. The fact is that we know alarmingly little about what constitutes a healthy diet. Let me clarify: we know hardly anything about significant difference in total health one relatively "normal" diet has over another. Ioannidis's advice: completely ignore any nutritional "scientific" claims because they are anything but. Any supposed claims for "healthy" eating are no more than wishful thinking, pseudoscience and good-old snake-oil salesmanship.

What is interesting from a scientific point of view, and this is one subject we know a lot more about than actual nutrition, is why people who are otherwise scientific minded skeptics are superstitious when it comes to nutrition (if you believe that eating more green vegetable or less, consuming more vitamin C or less, more saturated fat or less is actually better for you, then you are superstitious, unless you are knowingly placing a scientifically unfounded bet on one diet over another).

Obviously, the well-known psychology of magical thinking applies to everyone, and even skeptics are not immune to it (Charles Darwin, if one anecdote about him is correct, quite amazingly identified the effect magical thinking had on his own research, and took measures to counteract it). But why is food such a focus of superstition among skeptics? It probably has to do with their political leaning.

Jonathan Haidt hypothesizes that the value of "purity" is ingrained in human existence, and every society seeks to fill it with particular meaning[3]. Because liberals wholly reject giving "purity" sexual or racial content, they choose to purify themselves through health, and in particular, food. It seems that people have such a strong desire to feel pure, that those that cannot achieve it through religious means, try to achieve it with food. And because food is, after all, subject to scientific study (though mostly bogus), it gives that little something to grab onto and construct magical thinking around.

Consider this if you don't believe me: pick one scientific nutritional finding that you particularly like, and have chosen to adhere to. Let's suppose that the finding is true as published. Now, look at what statistical effect-size that particular research claims your chosen diet has. For example, by how much does it prolong life? 10%? 5%? 3%? Or by how much does it reduce the risk of getting a particular form of cancer? Does it take it from 1/100 to 1/10000? Or, perhaps, from 1/1000 to 1/1200?

Now think if in other areas of life, such effect-sizes would change your behavior as much.

Theres definitely a lot of psychology when it comes to people's diets. Science? Not so much.

[1] http://www.plosmedicine.org/article/info:doi/10.1371/journal...

[2] http://www.theatlantic.com/magazine/archive/2010/11/lies-dam...

[3] http://www.ted.com/talks/jonathan_haidt_on_the_moral_mind.ht...