The Economist has an interesting article on the science behind cognitive nutrition.
Some foods […] are like pharmaceutical compounds; their effects are so profound that the mental health of entire countries may be linked to them.
Last year, for example, the Lancet published research showing that folic-acid supplements—sometimes taken by pregnant women—can help those between 50 and 70 years old ward off the cognitive decline that accompanies ageing. In a study lasting three years, Jane Durga, of Wageningen University in the Netherlands, and her colleagues found that people taking such supplements did better on measures of memory, information-processing speed and verbal fluency. That, plus evidence that folate deficiency is associated with clinical depression, suggests eating spinach, orange juice and Marmite, which are all rich in folic acid.
via Mind Hacks