There are three studies which have appeared in the Neurology journals linking cognition with diets. One such study links increased body fat with declining mental function. If this is true, they are practically saying that overweight people are less intelligent then skinny ones. Um, I think this can lead to controversy.
Another study found that the Mediterranean style diets can actually help in preventing the Alzheimer’s disease. You can see the full story here.

The last study found that increased body weight can lead to poor cognitive function. This study was done with about 2,000 healthy men who were in the ages of 30 and 60. In general, the researchers found that people with a high BMI had lower test scores than those with a lower Body Mass Index.

Overall, they found that your diet can have a direct impact on your brain.