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The workout: A vigorous morning run or swim.
If you're going to get your heart pumping for at least 30 minutes, have a snack first, says Christine Rosenbloom, PhD, the sports dietician for Georgia State University athletics. You've been fasting all night, so you need some carbohydrates to top off your depleted glycogen stores. The ideal snack will be low in fat and protein, which take longer to digest, and have about 30 grams of carbohydrates.
The snack: A tube of energy gel, a packet of instant oatmeal (Rosenbloom says southerners like her may prefer grits) made with water, one cup of dry cereal or a plain mini-bagel. Wash this down with a 4-ounce glass of water.
When: Your body will be able to convert these carbs into energy if you eat 15 to 30 minutes before your workout. Don't worry if you're headed to the pool: The belief that you'll cramp up is an old wives' tale, says Rosenbloom.