A correlation study conducted with small group of 7 and 9 year olds demonstrated a positive relationship between diet quality, in particular dietary fibre content, and performance on a task designed to assess attention and the ability to disregard distracting stimuli.

65 children undertook a modified version of the Erikson Flanker Test in which they were asked to pay attention to the direction of a fish in the centre of a computer screen and to try to ignore other fish that appeared on the screen at the same time that were facing either the same direction or the opposite direction of the target fish. Their results were correlated against food diaries that tracked a range of nutrients and subsets of nutrients. For example fats were broken down in to saturated fatty acids, cholesterol, omega-3s and DHA .

They found that children who had higher fibre diets had significantly improved accuracy scores on the Flanker test. The results showed that soluble fibre was associated with overall accuracy and pectin was particularly linked to accuracy in the version of the test that required greater effort. The researchers postulate that the effects might be linked to better control of blood sugar and/or the interaction of the gut microbiota. Fibre cannot be assimilated into the body but is highly fermentable by the bacteria in the gut. Some of the important end products of this fermentation are short-chain fatty acids, which have been linked to reduced inflammation in the brain and increased BDNF, compound associated with the creation of new brain cells.

Whilst this study was conducted on children it is highly likely that the outcomes will be similar in adults.

High soluble fibre foods: Oats and oat bran, beans, lentils, apples, pears.
High insoluble fibre foods: Wheat bran and wholegrains, brown rice, seeds, fruit and vegetable skins.
High pectin foods: Apples, pears, quince, peaches, plums, oranges, lemons, grapefruit, gooseberries, apricots, guava, carrots, tomatoes.

Reference

Khan, N. A., Raine, L. B., Drollette, E. S., Scudder, M. R., Kramer, A. F., & Hillman, C. H. (2015). Dietary Fiber Is Positively Associated with Cognitive Control among Prepubertal Children. The Journal of Nutrition, 145(1), 143–149. http://doi.org/10.3945/jn.114.198457