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In the chaos of the real world, scientific findings that seem rock-solid in the lab often fall apart – and few places in the real world are more chaotic than the final kilometres of a marathon race.

The solution, increasingly, is for exercise physiologists to get out of the lab and study their subjects "in the wild." That's what a group of researchers in Britain did leading up to the London Marathon in 2009; the resulting data, published late last year, suggest that mid-pack marathoners need to pay more attention to what they eat the day before the race – and just slurping up a big bowl of spaghetti won't cut it.

During prolonged exercise, carbohydrates stored in your muscles (and, to a lesser extent, in your liver) provide the quickest and most easily accessible source of metabolic fuel. Once you run out of carbohydrate, you still have an essentially inexhaustible supply of fat to burn – but since fat can't be processed as rapidly, you're forced to slow down dramatically. That's what "hitting the wall" is all about.

The question is: What does it take to make sure your muscles are filled to the brim with carbohydrates on race day? The original carbo-loading lab studies, performed in Scandinavia in the 1960s, called for a depletion phase of several days, during which athletes would abstain from all carbohydrates; then an exhausting workout to empty out the muscles completely; and finally a three- to four-day carbohydrate binge.

This sort of weeklong ordeal, based on studies of sedentary subjects, has been largely discredited in the years since, says Trent Stellingwerff, a physiologist and nutrition researcher with the Canadian Sports Centre in Victoria, who works with aspiring Olympic marathoners. More recent studies suggest that you can ignore the depletion phase entirely and simply increase your carb intake for one or two days before the race.

To find out whether this revised advice works in practice, researchers in Britain followed 257 London Marathon participants for five weeks prior to the race, collecting data about their training and eating patterns. The runners had an average age of 39 and an average finishing time of 4 1/2 hours. The results were published in the International Journal of Sports Medicine.

Sure enough, day-before carbohydrate consumption mattered. Runners who ate more than seven grams of carbohydrate for every kilogram of body weight (g/kg) ran 13.4 per cent faster than a comparable group of runners who ate fewer carbohydrates but were otherwise identical in terms of age, body mass index, training and marathon experience. Surprisingly, the amount of carbohydrate consumed during the marathon didn't matter as much.

"What's cool is that this reconfirms what we see in the lab," Dr. Stellingwerff says.

It's not just that the low-carb group ran slower; they also slowed down more between the start and finish. The greatest difference in speed between the two groups occurred from the 35-kilometre mark to the finish – exactly when you'd expect a carbohydrate shortage to wreak havoc in a 42.2 km race.

If anything, the study underestimates the potential effect of carbo-loading. Based on the latest lab research, Dr. Stellingwerff tells the endurance athletes he works with to aim for 10 g/kg of carbohydrate the day before the race – an amount that virtually none of the amateur runners in the London study reached. Only 31 of the 257 runners even managed to reach seven g/kg – a clear indication, Dr. Stellingwerff says, that most of the runners didn't start the race with full fuel stores.

That doesn't mean that carbo-loading is an all-purpose magical performance booster. For most people, carbohydrate stores aren't depleted until after 90 to 120 minutes of vigorous exercise. That means you don't need to binge on pasta before a soccer game or 10-km race.

There are also legitimate concerns about how your stomach will respond to a superdose of carbs. The numbers are daunting: A 70 kg runner would have to down 16 cups of cooked pasta over the course of a day to get 10 g/kg of carbohydrate. So it's easy to understand why so many runners can't do it.

The simplest solution, Dr. Stellingwerff says, is to add up to two litres of sports drink (which typically contain about 55 to 60 grams of carbohydrate a litre) to your diet on the day before the race.

"For most people, I normally say that you should get your calories from food and your fluids from water," he says. But the day before a marathon is an exception: You should be relaxing, not trying to win a pasta-eating contest.

Tips for a full tank

Muscles: One day before the race, take in an additional 100 to 120 grams of carbohydrate by consuming a sports drink or similar products such as gels. This will fill up your muscles with carbohydrate, and they will remain full overnight.

Liver: The carbohydrate stores in your liver will be partly depleted overnight from performing essential functions such as keeping your brain going. Wake up a few hours before the race and take in some easily digestible carbohydrates such as oatmeal, a bagel or a sports drink.

Practise: Whatever nutrition plan you follow, make sure to try it out in the weeks before the competition. Don't do anything new right before the race.

Alex Hutchinson blogs about research on exercise at . His new book, Which Comes First, Cardio or Weights? , is now available.

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