Does Carb-loading Really Work? Yes and No.

By Erica Andersen and Dara Farhadi
BU News Service

If you’re trying to lose weight, managing a chronic health condition or simply looking to live a healthier lifestyle, you might try to limit your carbohydrate intake. But the lure of carbs can be hard to ignore. Some runners are attracted to the practice of “carbo-loading” before a big race. But does it work?

Current sports nutrition guidelines state that carbo-loading, or consuming a high-carbohydrate diet prior to a race, can provide a performance benefit for athletes competing in endurance events lasting at least an hour and a half.

The Boston Marathon certainly falls into that category, and the Boston Athletic Association helps competitors with their carbo-loading by organizing a pasta dinner the night before the race.

But one carb-heavy meal 12 hours before the big event is not a sports nutritionist’s definition of carbo-loading. The Mayo Clinic recommends that an athlete increase his or her carbohydrate intake for three to four days prior to a competition, while also tapering workouts to a full rest day before the event. In studies that have shown improved performance as a result of carbo-loading, the benefits came at the end of the race, around the time when runners encounter “the wall” — the fatigue and negative thoughts associated with running out of glycogen.

Glycogen is a molecule that our bodies use to store energy in the muscles in the form of glucose, a simple carbohydrate that’s found in foods like pasta, bread, and rice. If a runner can fully stock his or her glycogen stores before a race, that can lead to a slight improvement in performance.

“The body can only store around 2,000 calories of glycogen at a time, which fuels both the working muscles and brain,” wrote Andrea Cullers, a registered dietician, in an article on the American College of Sports Medicine’s website. When glycogen runs low, both the muscles and the brain tell the body to slow down.

Those glycogen stores typically last for about 90 minutes, which is why runners carbo-load before a long-distance event like the marathon. So it’s not that eating carbs is a bad idea, but it is important to do it right.

For example, it is a bad idea to eat differently than usual right before a big race. That can lead to gastrointestinal woes, which certainly don’t improve performance on the big day. Studies on the practice have also mostly been carried out with male participants, which means that dietary requirements and performance benefits for female athletes might be different.

Additionally, according to Cullers, the benefits of carbo-loading are measured in seconds, which mostly benefit competitive rather than recreational athletes. One study found a performance improvement of two to three percent.

“It could be the difference between first, second, third place,” Cullers said, but “those few seconds aren’t really going to make that much of a difference for most of us.”

Rather, she stresses that proper nutrition throughout training and competition has much more of a positive impact on a runner’s performance than a meal or two right before the race.