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Should one eat before or after a workout and does it change if you are lifting weights or running?

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Twenty years ago, when I was misspending my youth training for 10K races and the occasional marathon, runners and other endurance athletes were strongly advised to avoid eating in the hour or so before exercise.

We were told that pre-exercise calories would lead to a quick increase in blood sugar — a sugar high — followed by an equally speedy blood-sugar trough, known as “rebound hypoglycemia,” which would arrive in the middle of our race or workout and wreck performance. This idea grew out of decades-old studies showing that blood-sugar levels and performance tended to decline if athletes ate or drank sugary foods or drinks just before exercise.

But newer experiments have found that, while rebound hypoglycemia can occur, it is rare and doesn’t usually affect performance. When, for instance, a group of British cyclists gulped sugary drinks before a workout, a few of them experienced low blood sugar in the first few minutes of a subsequent, exhausting 20-minute ride, but their blood sugar levels then stabilized and they completed the ride without problems. Other studies have found that eating easily digestible carbohydrates in the hour before exercise generally enables athletes to work out longer.

As for after a workout, by all means, indulge — provided your session has lasted for at least 45 minutes or longer. (If it’s shorter than that, you will likely ingest more calories than you have burned.)

Both runners and those lifting weights vigorously should ingest carbohydrate-rich foods or drinks within an hour after a workout, said John L. Ivy, a professor of kinesiology at the University of Texas at Austin who has long studied sports nutrition. During that time, muscles are “primed” to slurp blood sugar out of the bloodstream, he said, replenishing lost fuel stores. If the food or drink also includes protein, the muscle priming is prolonged, Dr. Ivy has found, meaning you can store more fuel and be better prepared for your next workout. Protein also aids in rebuilding muscle fibers frayed during the workout, he said.

There is little evidence, however, that weight trainers need more protein after exercise than runners or other endurance athletes. “Protein supplements are often used” by weight trainers after exercise, according to the latest edition of Sport Nutrition, the definitive textbook on the subject, “but they are not necessary.”

Chocolate milk, on the other hand, is, at least at my training table. Inmultiple recent studiesvolunteers who drank chocolate milk within an hour after working out had higher muscle fuel stores, less body fat and a greater, overall physiological response to exercise than those who recovered with water or a sports drink.

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