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How long can I go before refuelling with carbs?

The question

How long can I go before I need to "refuel" with carbs, and how much do I really need?

The answer

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If you're spending nine hours doing an Ironman triathlon, you definitely need fuel. Samantha McGlone, the 2007 World Championship silver medalist, downs five sports drinks during the bike ride, and adds one or two energy gels an hour to her drinks during the run.

"But I have done Olympic-distance races [which take about two hours]on water," says the Montreal native, who represented Canada in triathlon at the 2004 Olympics. "Maybe not the best, but back in the day …"

Researchers have been arguing for years about whether carbohydrates make any difference during exercise lasting an hour or two and, if so, how much you need. Recent experiments suggest that during shorter workouts, carbs are for the brain rather than the muscles, and that during longer workouts, not all carbs are created equal.

The traditional advice has been you should ingest carbs - from a sports drink or energy bar to simple food such as a banana - during exercise lasting longer than about two hours, according to Asker Jeukendrup, a sports nutrition researcher at the University of Birmingham in Britain. For shorter exercise, it's commonly believed you should have enough carbs stored in your body. A typical 150-pound male, for example, can store about 2,000 calories of carbohydrate in his leg muscles alone; running a half-marathon requires only 1,500 calories.

But that's not what scientists have observed. In placebo-controlled studies, drinking a high-carbohydrate sports drink seemed to sometimes - but not always - offer a performance boost even in 60-minute exercise bouts.

Last year, researchers from Loughborough University in England published a study in the Journal of Sports Sciences that, combined with earlier results, explains this riddle. They found that runners could go farther in a 60-minute treadmill test if they drank a sports drink containing 6.4-per-cent carbohydrate - but only if they had fasted overnight. If they ate a meal three hours before the test, the sports drink didn't help.

It seems simple: Sports drinks only help during one-hour workouts if you're depleted at the start. But there's another wrinkle. Researchers get exactly the same results during a 60-minute treadmill test if the runners simply swish the sports drink in their mouth then spit it out, without swallowing: The subjects get a performance boost from the drink if they've been fasting, but none if they've had a meal. "This shows that it has nothing to do with energy absorption," Dr. Jeukendrup says. "It's in the brain."

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While this research is still new, brain-scanning studies by Ed Chambers, a colleague of Dr. Jeukendrup's in Birmingham, suggest that previously undiscovered carbohydrate sensors in the mouth send signals directly to the brain announcing the impending arrival of more fuel. (The sensors work even if the subjects can't taste the drink.) The brain then signals that you can go faster, even if the carbs never reach your muscles.

At the other end of the spectrum, your muscles really do need more carbohydrate during exercise lasting longer than two hours. Even the most quickly absorbed carbohydrates (glucose and maltodextrin) can only pass from your intestine into your blood stream at a rate of about 60 grams (a little more than 200 calories) an hour - not enough to keep up with demand during intense exercise.

To get around this limit, Dr. Jeukendrup realized he could mix different carbohydrates that are absorbed from the intestine through separate mechanisms. While fructose is only absorbed at 30 grams an hour, it doesn't interfere with glucose absorption. That means glucose and fructose mixed in a 2:1 ratio can be absorbed as quickly as 90 grams an hour.

This formula has been incorporated in drinks, bars and gels by PowerBar, which funds some of Dr. Jeukendrup's research (the funding only started after the initial results were published, he points out). Other companies are following suit by introducing products with rapidly absorbed carbohydrate blends.

For Ironman athletes such as Ms. McGlone, the ability to absorb up to 90 grams an hour of carbohydrate is crucial, though it takes practice to tolerate such a high intake without stomach upset. More generally, Dr. Jeukendrup says, the glucose-fructose mix is important only for sustained exercise lasting more than about three hours. But if you're trying to avoid bonking during a marathon or long bike ride, it could be the key.

Drinks, gels or bars?

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Taking in carbs only helps if you can keep them down - not always an easy task if you're exercising intensely. Marathon fans still remember Bob Kempainen at the 1996 U.S. Olympic trials, vomiting up a stream of fluorescent green Gatorade as he pulled away from his bewildered competitors to win. Typically, drinks containing less than about 8-per-cent carbohydrate (80 grams a litre) are easiest to absorb.

Depending on your sport and your preferences, you can also take in solid food. In November, Asker Jeukendrup and his colleagues published a pair of papers in Medicine & Science in Sports & Exercise comparing sports drinks, gels and bars. The result: Subjects were able to absorb and burn carbohydrates in all three forms at exactly the same rate. "Cyclists tend to eat more real food than runners, because of the amount of jostling in running," triathlete Samantha McGlone notes. "If I'm riding hard for more than five hours, I can stop at the store for Snickers and a Red Bull."

Alex Hutchinson blogs about research on exercise at

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About the Author
Jockology columnist

Alex Hutchinson writes about the science of fitness and exercise. A former national-team distance runner and postdoctoral physicist, he is the author of Which Comes First, Cardio or Weights? Fitness Myths, Training Truths, and Other Surprising Discoveries from the Science of Exercise. He is also a senior editor at Canadian Running magazine and a contributing editor at Popular Mechanics. More

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