You need energy to exercise. Here are some tips from the American Heart Association for active people:

• Eat a diet fairly high in carbohydrates. Complex types like whole-grain breads and cereals, starchy vegetables, and legumes are especially good.
• Drink fluids before, during and after a workout that lasts 30 minutes or more, especially in warm weather.
• If you exercise vigorously for 30-60 minutes or more, drink 6-8 oz of fluid every 10-15 minutes.
• Avoid eating a lot of sugar before you exercise. It may cause dehydration, stomach discomfort, or low blood sugar.
• Eating large portions of high-fat foods can delay digestion. So, before a workout, eat carbohydrates, which are high-energy foods.
• Make sure you get your vitamins and minerals - by eating a well-balanced diet that includes all kinds of foods.

Related information Lifestyle changes: Exercise