“Physical fitness is not only one of the most important keys to a healthy body, it is the basis of dynamic and creative intellectual activity. Intelligence and skill can function at the peak of their capacity when the body is healthy and strong.”

What i think JFK is trying to tell us is that it is best for you to have a healthy body. Having a healthy body can help you do your best at all times. Not having a healthy body can prevent you from doing your best. It gives you more energy for things you thought you couldn't do. Not giving your full effort can show that you are not the best. Mental health is important as well. If you do not feed yourself properly you might feel lazy and tired, which will not help you get to the peak of your intelligence and skill.