Why nutrition science often isn't.

I asked 8 researchers why the science of nutrition is so messy. Here’s what they said. - Vox: "Nutrition science... is filled with contradictory studies that are each rife with flaws and limitations. The messiness of this field is a big reason why nutrition advice can be confusing. It's also part of why researchers can't seem to agree on whether tomatoes cause or protect against cancer, or whether alcohol is good for you or not, and so on, and why journalists so badly muck up reporting on food and health. To get a sense for how difficult it is to study nutrition, I spoke to eight health researchers over the past several months. Here's what they told me. 

 1) It's not practical to run randomized trials for most big nutrition questions...  Take the Women's Health Initiative, which featured one of the biggest and most expensive nutrition studies ever done. As part of the study, women were randomly assigned to two groups: One was told to eat a regular diet and the other a low-fat diet. They were then supposed to follow the diet for years. The problem? When researchers collected their data, it was clear that no one did what they were told. The two groups basically had followed similar diets. "They spent billions of dollars," says Walter Willett, a Harvard physician and nutrition researcher, "and they never tested their hypothesis.""

2) Instead, nutrition researchers have to rely on observational studies — which are rife with uncertainty...

3) Another difficulty: Many nutrition studies rely on (wildly imprecise) food surveys...  people lie about what they eat, offering answers that are more socially acceptable. Or it may be a simple failure of memory. Whatever the cause, this leaves researchers in a tricky place...

4) More complications: People and food are diverse...

5) Conflict of interest is a huge problem in nutrition research...