It seems like every other week the news is telling me that I need "X" or "Y" nutrient to live a longer and healthier life. Then 3 weeks later I read eating "X" and "Y" nutrients will make me die a long painful death. Over the last 10 years eggs, wine, caffeine, low-fat foods, high fat foods and more have all been considered healthy and unhealthy by our conflicting nutritional data. The goal of nutritional research should be the search for truth using the best available methods. Unfortunately, that does not always happen because academics need to publish, journals need revenue, news outlets need new or conflicting stories, and commercial interests need to sell pharmaceuticals, vitamins etc.
Perhaps the worst nutritional research is an "observational study". Even though many top medical journals include observational epidemiology (the study of causes of disease in populations) studies on a regular basis, many scientists believe this research is simply not good science! No matter how well designed and how many subject they may include, observational studies cannot determine causal effects. NYC in August has a spike in ice cream sales and also violent crime therefore they may be related, or not. Observational studies can give researchers the ammunition for their hypothesis but fall way short of beyond a reasonable doubt. These studies never even get to the "experiment" stage in the scientific method! Not good science.
"Every time that these Harvard researchers had claimed that an association observed in their observational trials was a causal relationship...the folks from Harvard got it wrong. Not most times, but every time." - Gary Taube
Some of the other major problems with nutritional research:
We can not isolate a nutrient's effect on the body.
Data collection (usually food diaries or questionnaires) in nutritional research tends to be flawed because people underestimate and don't remember what they ate.
Nutrient content in food varies due to freshness, the way it is prepared, how it was grown etc.
Genetic differences in the subjects involved in the diet will skew results.
Most research can't control for all lifestyle factors like exercise, meditation, drug interactions etc.
We all have a unique bacterial makeup which metabolizes nutrients in differing ways.
Just because a poor mouse contacted type 2 diabetes from eating tons of "Ring-Dings" doesn't necessarily mean humans will react the same way.
Researchers can't completely control of what the subjects eat, breath and drink (unless the subjects are prisoners or voluntarily housed in a ward) .
Most nutritional studies lack a randomized control group. It is hard to compare apples and oranges.
View nutritional advice with with a hypercritical eye. My advise from my grandparents is to eat real foods in moderation, drink lots of water and exercise on a regular basis. If a nutritional theory is compelling, but unconvincing run your own personal experiment and see if it works for you.