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Whether you're a recreational athlete or a marathon runner, what you eat and when you eat it can affect your performance and the way you feel while you're exercising.


Timing is everything

You've probably noticed that if you eat a large meal before you exercise, you may feel sluggish or have an upset stomach, cramping and diarrhea. That's because your muscles and your digestive system are competing with each other for your body's resources.

Your body can digest food while you're active, but not as well as it can when you're not exercising - partly because your blood is trying to do two jobs at once, according to Stephen DeBoer, a registered dietitian at Mayo Clinic, Rochester, Minn.

"Your blood goes to whatever part of your body needs it," DeBoer says. When you start exercising, some blood gets taken away from your stomach. Both jobs get done, but not as effectively or as side-effect-free as you'd like.

On the flip side, not eating before you exercise can be just as bad as eating too much. Low blood sugar levels can make you feel weak, faint or tired, and your mental abilities may be affected as well, making you slower to react.

So how do you strike a balance? Everybody's different, but here are some general rules for timing your eating and exercise:
  • Eat a full breakfast. Wake up early enough to eat a full breakfast. "Most of the energy you got from dinner last night is used up by morning," says DeBoer. "Your blood sugar is low. If you don't eat, you may feel sluggish or lightheaded while exercising." If you plan to exercise within an hour after breakfast, eat a smaller breakfast or drink something to raise your blood sugar.
  • Time your meals. Eat large meals at least 3 to 4 hours before exercising. If you're having a small meal, you should eat at least 1 to 2 hours before exercising. Most people can eat snacks right before and during exercise. The key is how you feel. Some people feel lightheaded if they eat an hour or less before exercise. Do what works best for you.
  • Don't skip meals. Skipping meals may cause low blood sugar, which can make you feel weak and lightheaded. The longer your exercise session, the more important that last meal is for keeping your energy level up.
  • Eat after your workout. To help your muscles recover and to replace their fuel stores, eat a meal that contains both protein and carbohydrates within 2 hours of your exercise session if possible.

Carbohydrates count

You'll feel better when you exercise if you eat foods high in carbohydrates and low in fat. That's true whether you're a marathon runner or a walk-the-office-stairwell person. Carbohydrates keep your blood sugar normal and fuel your muscles during exercise. "Your body gets used to using whatever you feed it," says DeBoer. "The more carbohydrates you eat, the more efficiently your body uses carbohydrates."

Cereals, breads, vegetables, pasta, rice and fruit are good carbohydrate sources. But right before an intense workout, avoid carbohydrates high in fiber, such as beans and lentils, bran cereals, and fruit. The fructose in fruit can increase the tendency to have diarrhea with hard exercise, and high-fiber foods may give you gas or cause cramping.

If you don't like to eat solid foods before exercising, drink your carbohydrates in sports beverages or fruit juices. "Research shows it makes no difference in performance whether you drink your carbohydrates or eat them," says DeBoer. Do what feels comfortable to you.

When you exercise intensely for more than an hour, you'll need to consume additional carbohydrates or you'll start to lose energy. Sip a sports drink or chew on a sports bar high in carbohydrates. If you're exercising at any pace for 1 hour or less, these probably won't make a difference.

Low-carbohydrate diets - those in which fewer than 25 percent of calories come from carbohydrates - impair your body's ability to replace glycogen, your muscles' fuel. A diet containing at least 40 percent to 50 percent carbohydrates allows your body to store glycogen, but if you're a long-distance runner or you exercise for long periods of time, you might want to consume more carbohydrates regularly and consider carbohydrate loading before a big athletic event.


Protein and fat are important, too

Protein isn't your body's food of choice for fueling exercise, but it does play a role in muscle repair and growth. If you exercise three times a week, a diet containing 0.5 to 0.75 grams of protein per pound of body weight is sufficient. If you're a competitive athlete, up your intake to 0.6 to 0.9 grams per pound of body weight. Most people can easily get the protein they need from such foods as poultry, meat, dairy foods and nuts and don't need additional protein supplements.

Fat is an important, although smaller, part of your diet. Fats, along with carbohydrates, provide fuel for your muscles during exercise. A good rule of thumb is to multiply your desired weight by 0.45, and the result is the number of fat grams to consume in a day. Try to get most of your fat from sources such as nuts, fatty fish or olive oil. Avoid fatty foods just before exercising, though. Fats sit in your stomach longer, causing you to feel less comfortable.


Don't forget to drink

To stay well hydrated, drink six to eight glasses of fluids daily, plus consume at least 4 to 10 additional ounces of water for every 15 minutes of exercise. Electrolytes - elements such as potassium, sodium and chlorine - are depleted when you sweat. If you don't replace the fluid you lose during exercise, your heart rate increases and your temperature rises, putting you at risk of heat illness as well as compromising your workout.

If you're a competitive athlete, weigh yourself before and after you exercise. For every pound you've lost, DeBoer recommends drinking up to 24 ounces of fluid.

Water is generally the best way to replace lost fluid, unless you're exercising for more than 60 minutes. In that case, sip a sports drink to help maintain your electrolyte balance and give you a bit more energy from the carbohydrates in it. The sodium in sports drinks helps you rehydrate more quickly, notes DeBoer.


Tips for preventing cramps

No one knows for sure what causes muscle cramps, but the following suggestions might help prevent them:
  • Drink enough fluids so that you have to urinate every 2 to 4 hours.
  • Consume at least two servings of calcium-rich foods a day, such as low-fat milk or yogurt.
  • Don't restrict sodium intake too much, especially if you're a competitive athlete.
  • Eat potassium-rich fruits and vegetables, such as bananas and potatoes.

Eating on the run

If you're short on time before your workout and aren't sugar sensitive, and your choice is candy or nothing, eat the candy because it can improve your performance compared with eating nothing. But keep in mind, all candy is high in sugar and low on nutrients, so a snack of yogurt and a banana would be a better choice.

Energy bars are another popular alternative because they're easy to carry, high in carbohydrates, and some brands also contain protein and fat. But they're often high in calories and expensive, and they really don't provide an advantage over regular snacks, such as fresh fruit or a bagel. Check the nutrition label before you buy any nutrition bar. Anything over 250 calories per serving is more than you need. Try a different brand.

You can avoid the candy-bar-at-the-convenience-store trap by having your own nutritious snacks on hand. Here are some ideas:
  • Low-fat or fat-free yogurt
  • Low-fat granola bars
  • Fruit or fruit juices
  • Sports drinks
  • Low-fat cereals
  • Bagels, rice cakes, breadsticks, graham crackers
  • Fig bars, vanilla wafers, animal crackers

Listen to your body

When it comes to eating and exercise, everyone is different. Pay attention to your performance, listen to your stomach, and learn from experience what pre- and post-exercise eating habits work for you.

Adapted from Mayo Foundation for Medical Education and Research (MFMER).