Glycogen is the storage form of glucose and it’s the primary source of muscle energy during intense exercise. Carbohydrate loading, or carbo-loading, is the practice of increasing your muscles’ glycogen stores before a race through changes in diet and training. The more glycogen you have in your muscles, the more endurance you’ll have on race day.

When I was a young runner, many endurance athletes still practiced the old-school method of carbo-loading—and it wasn’t pretty. This method was developed by a Swedish physiologist named Gunvar Ahlborg, who discovered that muscles are able to store above-normal amounts of glycogen when high levels of carbohydrate consumption are preceded by severe glycogen depletion. So Ahlborg came up with a seven-day carbo-loading plan in which an exhaustive bout of exercise was followed by three or four days of extremely low carbohydrate intake (10 percent of total calories) to starve the body of glycogen. This depletion phase was followed by three or four days of extremely high carbohydrate intake (90 percent of total calories) and very light training. The athletes who tried this method in an experiment nearly doubled their glycogen stores and had significantly greater endurance in a performance test.

After these results were published, endurance athletes across the globe began to use Ahlborg’s carbo-loading method before races lasting 90 minutes or longer. There were, however, some obvious drawbacks to this proven plan, the first of which being that many athletes weren’t wild about doing an exhaustive workout just a week before a big race. There was also the fact that maintaining a 10-percent carbohydrate diet for three or four days carried some nasty consequences including lethargy, cravings, irritability, lack of concentration, and increased susceptibility to illness. Although I’ve never tried Ahlborg’s plan myself, I’ve talked to many athletes suffering through the depletion phase. Trust me—they’re truly miserable.

Fortunately, later research showed that you can increase glycogen stores significantly without first depleting them. A newer carbo-loading strategy based on this research recommended athletes eat a normal diet of 55 to 60 percent carbohydrate until three days before racing, then switch to a 70 percent carbohydrate diet for the final three days, plus race morning. As for exercise, this friendlier carbo-loading method suggested one last longer—but not exhaustive—workout done a week from race day followed by increasingly shorter workouts throughout race week.

But the most recent research indicates that you can slash the time you devote to carbo-loading to a single day. A 2002 study by Australian researchers showed that just one day of high-carbohydrate eating combined with complete rest was sufficient to raise glycogen levels as high as other more extensive carbo-loading methods. Even better, glycogen levels remain elevated for five days after your day of carbo-loading as long as you continue to train lightly. That means you don’t have to carbo-load the day before a race. If it’s more convenient for you to carbo-load on the Tuesday before a Sunday race, go right ahead.

When you do the one-day carbo-load, here’s what you need to remember:

• Take the day off from training—completely.
• Try to consume roughly 4.5 grams of carbohydrate per pound of body weight (or 10 grams per kilogram). So if you weigh 150 pounds, scarf down 675 grams of carbohydrate between the time you get out of bed and the time you crawl back in. That’s a lot of carbs.
• Because you will find it difficult to take in this much carbohydrate in a day, you will need to be very picky about what you put on your plate. Virtually everything you eat and drink should be carb-packed. Fat and protein can wait for tomorrow. Good choices are oatmeal, bagels, bananas and fruit juice for breakfast, energy bars and fruit for snacks, tomato soup and peanut butter and jelly sandwiches for lunch and pasta or rice with vegetables for dinner. You can also get more carbs from high-carb meal replacement drinks.
• Be aware that for every gram of carbohydrate the body stores, it also stores 3 to 5 grams of water, which leads many athletes to feel bloated after carbo-loading. But there’s no need to worry as the water weight will be long gone by the time you finish your race.

Speaking of the race itself, be sure to consume carbs in the form of sports drink or energy gels throughout your race. Truth be told, this is an even more effective way to delay glycogen depletion than carbo-loading. Some research suggests that athletes can maximize endurance performance by consuming adequate carbohydrate during exercise even without prior loading. But despite the fact that the additive effect of carbo-loading and consuming carbs during races might be small, it’s still worth doing both. Being an endurance athlete is all about doing lots of small things that add up.