It’s been suggested that eating healthy can have a positive effect on exercise performance. How quickly you might notice a difference is investigated in a study from Saint Louis University that was published in the Journal of the American College of Nutrition.
Researchers had 7 women and 5 men run 5 kilometers on a treadmill after 4 days of adhering to a Mediterranean style diet and 4 days after consuming a typical Western style diet. There was more than a week between tests. On average, subjects ran 6% faster after consuming a Mediterranean diet of whole fruits and vegetables, nuts, olive oil and whole grains.