A few weeks ago, a friend showed up for a run with a CamelBak — one of those humplike backpacks with a tube that allows you to sip liquid — and a belt containing food to eat along the way. Every 20 minutes or so as we ran, he stopped to eat and drink, sprinting afterward to catch up.

Now that is unusual, I thought. Does it really help to eat so often during a 16-mile run?

Certainly a lot of athletes believe they need constant nourishment. My friend and running partner Jen Davis, who has entered more races and run more than I ever have, once went on a 30-mile training run with a guy wearing a CamelBak and bearing snacks. He stopped every 20 minutes along the way and then, about halfway through the run, pulled out a turkey sandwich.

“I’m not sure if he ever actually ran an ultra race,” Jen said. “He may have gotten injured after carrying that heavy pack on those long runs.”

There is no end to the crazy foods people will eat at endurance events. At the J.F.K. 50-Mile in Maryland, boiled potatoes and chicken broth are provided at aid stations. At the Rocky Raccoon Endurance Trail Run in Texas, runners can choose rice and beans or pasta, along with snacks like pretzels, cookies and candy.

At a 100-mile bike ride my husband and I have done several times, pumpkin pie is offered about 25 miles from the finish line. (My husband tried it one year and felt ill the rest of the ride.)

For the athlete determined to munch on the go, there are shelves worth of prepackaged “energy gels” and bars, even jelly beans, promising to raise performance.

But most athletes are not running 30 or 50 or 100 miles, nor are they doing the equivalent amount of exercise in another sport, like cycling or swimming or skiing. So most of us really do not need to keep eating during a race to maintain energy and stamina, said Nancy Rodriguez, a sports nutritionist at the University of Connecticut.

Dr. Rodriguez reviewed published studies on nutrition and performance as part of a group of experts who wrote a position paper on the topic for the American College of Sports Medicine. Runners, for example, competing in a 5- or 10-kilometer race, she said, “don’t need the CamelBaks and don’t need to have that Hershey bar or Powerade or Clif shot.”

Even athletes who are fast and competitive may not always need to eat during a workout. There’s no set rule on what they should eat and drink before, during and after exercise, said Melinda M. Manore, a sports nutritionist at Oregon State University who was an author of the position paper.

“People have gotten the message that they have to eat something,” Dr. Manore said. They guzzle an energy drink or eat a sports bar, but that doesn’t help. And for the many who are trying to lose weight, the habit just adds extra calories.

What they need depends on what they ate before they started and how hard their workout is going to be, among other things, she explained. “If you can run six-minute miles or five-minute miles and you are going out for an hour, you do not need to be eating an energy bar during the workout,” Dr. Manore said.

Moderate athletes need to eat and drink after the workout, she said, but a healthy meal with plenty of fluids is sufficient. Indeed, for most of them, the most common error is to eat too much.

Dr. Manore follows her own advice. She hikes for an hour in the hills every morning, four to five miles. All she has before she goes out is a cup of tea with milk.

But anyone exercising for two hours or more does need to get carbohydrates, the muscles’ fuel, according to the position statement. That means eating before, and perhaps during, the workout.

Those who try to skimp can end up with a poorer performance, said Dan Benardot, a sports nutrition researcher at Georgia State University. A long workout, like a run that lasts more than two hours, is “an enormous drain on blood sugar,” he said.

If the body runs out of glucose for fuel, it will start breaking down muscle, which is counterproductive. Dr. Benardot’s research indicates that athletes do best when they never let themselves have more than a 400-calorie deficit during the day. That is, if you expend 1,500 calories on a two-hour run, you offset it with at least 1,100 calories in food that day.

That means it is a disadvantage to eat most of the day’s calories at one time — at night, for example. But athletes should make dietary changes gradually so their bodies can adapt to more frequent fueling, he said. Those who try sudden changes sometimes pay a price.

Dr. Benardot tells the story of a distance runner who was doing well and felt great the morning of a big marathon. Before the race began, she saw her chief competitor put packs of a sugary gel into her running bra to eat during the race.

The distance runner did the same, even though she had never before eaten during races or long runs. It was a disaster: She had diarrhea during the event.

The gels “were anything but a competitive advantage,” Dr. Benardot said.

“You have to let your body adapt,” he added. “And you have to find out what works for you.”

This post has been revised to reflect the following correction:

Correction: February 21, 2012

Earlier versions of this post misspelled the surname of a sports nutrition researcher at Georgia State University. He is Dan Benardot, not Bernadot or Benradot.

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Personal Best: Workouts May Not Be the Best Time for a Snack