Carb loading for a half marathon: do you need it?

Your muscles store carbohydrate as glycogen. On a long, steady effort you burn through it, and when it runs low the pace gets hard to hold. Carb loading fills those stores to the top before the start. Trained runners can do this by eating more carbohydrate and easing off training over about two days.

Most recreational runners finish a half marathon in more than 90 minutes, so most of them are in the group that can benefit. A full load is a lot of food, though. If it upsets your stomach, a smaller increase on the day before is still useful.

That is far more carbohydrate than most people eat on a normal day. You get there by making carbohydrate most of each plate, adding carb-rich snacks and drinks, and cutting back on fat and fiber so you are not overfull.

For a 150-pound runner aiming at about 10 g/kg. Grams are rough; check labels for your own foods.

• Breakfast: a large bagel with jam, a banana, a 12-ounce glass of orange juice (about 140 g).
• Snack: a cup of low-fat fruit yogurt and a granola bar (about 70 g).
• Lunch: a large turkey sandwich on white bread, pretzels, a sports drink (about 150 g).
• Snack: rice cakes with honey, a handful of raisins (about 80 g).
• Dinner: two cups of pasta with tomato sauce and chicken, bread, a glass of juice (about 170 g).
• Evening: a bowl of low-fiber cereal with milk (about 60 g).

That adds up to roughly 670 grams. Repeat a similar day the day before, then eat a familiar breakfast on race morning: the position paper suggests 1 to 4 g/kg in the 1 to 4 hours before a long event.

• White rice, pasta, bagels, white bread, pancakes and low-fiber cereal.
• Potatoes without the skin, and pretzels for salt.
• Juice, sports drinks and chocolate milk, which add carbs without bulk.
• Honey, jam, maple syrup and dried fruit in small amounts.
• Less of: big salads, beans, bran, whole nuts, fried and creamy foods.

THOR is a half marathon you can spread over 20 days, and most people do. Short daily runs need no carb loading at all. If you choose to run the full 13 miles (21 km) in one go, treat it like a race: load over two days, eat a familiar breakfast, and fuel during the run. See energy gels for running and the virtual half marathon guide.

Carb loading fills your muscles. Breakfast tops up your liver, which empties overnight. The position paper suggests that carbohydrate eaten 1 to 4 hours before exercise can continue to raise these stores. For a 150-pound runner, a bagel with honey, a banana and a glass of juice three hours before is a reasonable choice. Avoid high-fat, high-protein or high-fiber breakfasts, and eat the same thing you tested before your long runs.

A full load does not suit everyone. Some runners feel heavy and bloated on 10 g/kg. If that is you, aim for the lower end, around 7 g/kg, from more frequent, smaller meals, and rely more on fuel during the race. Liquid carbohydrate, like juice and sports drinks, adds a lot without filling you up. Whatever you choose, try it before a long training run first, never for the first time before the race.

Smaller runners need fewer total grams, because the targets are per kilogram. A 120-pound runner aiming at 10 g/kg needs about 545 grams a day, far less than the 815 grams a 180-pound runner would.

Walkers still benefit from a tested breakfast and steady snacks. Pick foods that are easy to carry, like a sandwich, a banana and a bag of pretzels.

A half marathon walk takes many people four hours or more. Walking uses fuel more slowly than running, so a full load is less important. Eat a good carb-rich dinner and breakfast, and plan regular snacks during the walk. See walk a virtual half marathon.

• How do I know if the load worked? You feel strong in the first half, and you may weigh a pound or two more on race morning. That is water stored with glycogen.
• Do I need to deplete first? Older plans had a low-carb phase first. The position paper notes trained athletes can load without it.
• Should I carb load for a 10K? No. Normal meals are enough for runs under 90 minutes.
• Can I carb load on a low-carb diet? Sudden big changes can upset your stomach. Practice well ahead.
• Should I stop training while loading? Ease off. Short, easy runs are fine.

This is general information for healthy adults. If you have diabetes, heart or kidney disease, are pregnant, or follow a diet set by a doctor, ask your doctor or a registered dietitian first.

• Nutrition and Athletic Performance, joint position of Dietitians of Canada, the Academy of Nutrition and Dietetics and the American College of Sports Medicine (2016): carb loading for events over 90 minutes, daily needs for shorter events, loading over 48 hours, the pre-event meal.