“You don’t have to necessarily eat more calories,” said Trent Stellingwerff, chief performance officer for the Canadian Sport Institute Pacific in Victoria, British Columbia.

“You’re just going to fill more of your plate up with carbohydrates.”

Carb-loading doesn’t improve performance for athletes running distances such as a 5km or 15km, because they are unlikely to burn out of glycogen in the same way a marathoner running 42.1km over several hours might.

How to avoid hitting the wall

Humans can only store so much glycogen in their muscles and liver – and it’s not enough to run a marathon. Trained endurance athletes can maintain their pace for about 30 to 35km, depending on the runner, said Louise Burke, the chair of sports nutrition at Australian Catholic University.

By then, the body’s glycogen runs low and the marathon “starts getting really tough”.

This is what runners refer to as hitting the dreaded wall.

“We just can’t store enough in our muscles and liver to make it through all the way,” said David Nieman, a professor of biology at Appalachian State University who has run 58 marathons. “If you can get more carbohydrate – glycogen – into the muscles and liver, you can maintain your racing pace longer.”

There are three ways runners try to avoid hitting the wall, Nieman said.

First, by training. The more a runner trains, the more glycogen their muscles can store. Second, by tapering. Runners cut back their mileage before a marathon to give their muscles a chance to stockpile the fuel. And third, by eating more carbohydrates before and during a race.

To replenish muscles’ glycogen stores ahead of a marathon, researchers say athletes should ingest around 10g of carbohydrates per kg of body weight for up to two days beforehand.

For someone who weighs 70kg, that’s about 700g of carbohydrates a day, or about the same as consuming two bagels, two bananas, a 375ml glass of orange juice, four cups of white rice, around three dozen gummy bears and two 625ml sports drinks.

Instead of sipping water the day before a marathon, opt for a sports drink with carbohydrates, Stellingwerff said. Or, instead of filling half your plate with carbohydrates, aim for closer to three-quarters of the plate – and less protein or fat.

The amount of carbohydrates a runner should eat to fuel a marathon depends on body type, training and how fast a person is planning to run, experts say.

“Athletes who are pushing the boundaries of what they can do,” such as by running for a personal record of some kind, are going to burn more carbohydrates during the marathon, said Patrick Wilson, an associate professor of exercise science at Old Dominion University.

What carbs to eat before (and during) a race

Carb-loading can leave runners feeling bloated and heavy before a race. That’s partly because, with every gram of glycogen, the body stores three grams of water – and more weight.

“Off the start line, you might feel a bit sluggish,” Stellingwerff said. “But as you proceed through the race, obviously, you’re going to use that glycogen, and you’re also going to use that water for sweating.”

As you carb-load for a marathon, try to reduce the fibre in your diet; foods high in fibre can add to bloating or, potentially, unexpected bathroom breaks during a race. Burke, who’s running the New York City Marathon in November, said she eats breakfast cereals, such as Rice Krispies, rather than fruits, granola, whole wheat or vegetables.

“This is the time when you try to strip all that away,” Burke said.

High amounts of lactose from milk and fructose from certain types of fruits or juice can also cause bloating or gas, Wilson said. He recommends runners stick to carbohydrates they’re used to, and if they feel bloated from carb-loading, they can instead focus on fuelling during the race.

Marathon runners have to learn how to eat during a marathon, Clark said. During training, start with a small breakfast – say, half an English muffin – before a long run. Then eat animal crackers, gummy bears or sports gels during the run.

“From the day you start training for the marathon, you start training your gut,” Clark said.

The Auckland Marathon is on Sunday, November 3