Carb-Loading: What Science Says About Fueling for Exercise
By: Christie Aschwanden
Tags: Sports, Body, Test Gym, Food, Fitness

Carbo-Loading is Dead. Long Live Carbs!

What the science now says about housing pasta for peak athletic performance

Test Gym is a new Elemental column about the science of exercise.

When I was a high school cross-country runner a few decades ago, my team would gather for dinner the night before meets to load up on pasta and bread. We were taught that “carbo-loading,” as this pre-race feasting on carbohydrate-rich foods was called, would help us perform our best the following day.

The concept of carbo-loading arose from research done in the 1960s by Scandinavian scientists using needle biopsies to peek at what was happening in muscles. In this study, which made a splash after it was published in Nature in 1966, researchers Jonas Bergström and Eric Hultman used themselves as the sole test subjects to measure what happened to the stores of glycogen (sugar used to fuel exercise) in their leg muscles before and after an intense one-legged session on a cycling ergometer.

They found that immediately after the bout of hard exercise, muscle glycogen levels plummeted in the leg that had done the work, while glycogen levels in the other leg remained stable. Over the next three days, the men ate a high-carbohydrate diet and the glycogen levels in their exercised leg soared to about twice the amount in the unexercised leg. This led to the idea that depleting a muscle’s glycogen levels and then feeding them with a high-carb diet could lead to a “supercompensation” effect that packed your muscles with extra fuel. Carbo-loading was born.

At the 1969 European Marathon Championships, British runner Ron Hill posted a spectacular come-from-behind win and attributed it to the concept, which he claimed prevented him from slowing down like the runner he overtook.

Indeed, the benefit of carbo-loading, as understood at that point, wasn’t so much that it could make you run faster, but that it could help you run at your optimal pace for longer, says Louise Burke, chief of nutrition strategy at the Australian Institute of Sport.

And while it was believed that optimal carbo-loading required a depletion phase where you tried to drain the glycogen stores with exercise and carb restriction before loading up on carbs again, this kind of deplete/load/deplete cycle has fallen out of favor as researchers have realized that in a lot of cases, it’s neither necessary nor all that helpful. Particularly for elite athletes or those who are regularly doing hard training, there’s no need to undergo a deliberate depletion phase, Burke says, because the intense training that athletes already do naturally drains glycogen.

But even without prior glycogen depletion, carbo-loading is much less of a thing these days. Today, the goal is more often to match the fuel to the workout, rather than trying to overload before, and deliberately loading up on carbs should only be used for longer-distance events like a marathon or ultramarathon. Those first studies weren’t exactly wrong, she says, but their results didn’t generalize well to other situations.

Carbs are still great, though

There’s little doubt that carbohydrates are the body’s preferred fuel when engaging in high-intensity exercise. For instance, in a study published in 2015, researchers fed a carb-rich solution to competitive runners along with nicotinic acid to blunt their bodies’ use of fat. Then they had these volunteers run on a treadmill at a pace that was 95% of their personal best for a half-marathon. The results showed that blocking their ability to run on fat did not reduce the distance that they were able to go at that pace. They were running just fine on carbohydrates.

“There is still a time and a place for carbo-loading, but most contemporary sports nutritionists would recommend it a lot less than they would have 20 years ago.”

The idea behind carbo-loading is that it ensures that your muscle glycogen stores are at maximum capacity before an event so you’re less likely to run low on fuel. And it’s true that under the right circumstances, carbo-loading can pack something like an extra 10% to 15% of glycogen in the muscle stores, says Trent Stellingwerff, director of performance solutions at the Canadian Sport Institute Pacific.

In most cases, you have to exercise at race- or competition-level effort for 75 to 90 minutes before glycogen only becomes limiting, he says. So if you’re prepping for an event that’s shorter than that, you’re unlikely to enhance your performance by stocking up on carbs. Those big plates of pasta I ate prior to my 5K high school cross-country races probably didn’t help me much.

“There is still a time and a place for carbo-loading, but most contemporary sports nutritionists would recommend it a lot less than they would have 20 years ago,” Stellingwerff says. Today, the prevailing idea is that athletes should tailor their carb consumption to their training and competition plan so that it fuels the work required for a particular effort. Instead of trying to deplete or overload your glycogen resources, the aim is to keep them well-matched to your fuel needs.

Whereas early nutrition guidelines tended to take a one-size-fits-all approach, the newest ones attempt to provide more situation-specific ones, Burke says. She and Stellingwerff were the first and last authors, respectively, on a consensus statement on nutrition for athletes put out last spring by the IAAF (track and field’s international governing body). Having high carbohydrate availability is generally best during competitions, but what exactly that means will differ by event, Burke says.

Burke and her colleagues provide some general guidelines for carbohydrate consumption based on exercise type:

Low-intensity exercise (such as walking): three to five grams of carbohydrates per kilogram of body weight per day. (That’s equivalent to 202 to 338 grams of carbs per day for a 150-pound athlete. The Dietary Guidelines for Americans recommend that a diet of 2,000 calories a day should consist of between 225 and 325 grams of carbohydrates a day. For perspective, one cup of cooked brown rice contains 45 grams of carbs.)

Moderate-intensity exercise (such as easy cycling or running) up to one hour per day: five to seven grams/kilograms per day.

Endurance exercise of moderate to high intensity (such as cardio exercise that includes bouts of high intensity) for one to three hours per day: six to 10 grams/kilograms per day.

Moderate- to high-intensity exercise for more than four hours per day: eight to 12 grams/kilograms per day.

When it does, and doesn’t, make sense to “carbo-load”

If your event won’t last longer than about 90 minutes, gorging on carbs before a big event could even backfire, because jamming glycogen into your muscles also increases your water weight.

For every gram of glycogen you store, you also take in another 2.7 grams or so of water, Stellingwerff says, and that means that carbo-loading could result in as much as a 2% to 3% increase in body weight. That extra weight could spell trouble. Stellingwerff points to an old Swedish paper from 1971 that looked at carbo-loading versus non-loading in cross-country runners who were racing on a 30-kilometer course. After carbo-loading, several of the participants experienced a 4% or more increase in body weight, which reduced performance, Stellingwerff says.

On the other hand, for an hours-long event like a marathon, it’s a good idea to make sure you’re topped off on carbs. “If you feel a little bloated or heavy on race morning, that’s normal, it’s okay, don’t panic,” Stellingwerff tells his athletes in this situation. “You’ll burn off that fluid weight in 20K, and you’ll have more body water which will help with your overall hydration status. You’ll feel a heck of a lot better at 30K than that little bit of sluggishness you feel at 5 and 10K.”

When you’ve got a big day of strenuous and long exercise ahead, it’s also a good idea to eat breakfast, because it’s possible to use up 50% to 75% of your liver glycogen overnight while sleeping, Stellingwerff says. No breakfast means you’re missing a chance to replenish those liver stores you lost overnight. Ideally you should eat four hours before the event so your body has time to digest the food. “I don’t care if that means you have to get up at 4 in the morning,” Stellingwerff says. You can go back to sleep or rest if you need to.

Instead of trying to pre-fuel a marathon or other long endurance event, the current thinking is that the best strategy is to make carbohydrates available during an event and to take them in while you exercise. It takes practice to figure out the best way for you to ingest these carbs and train your gut to process them while you’re on the move, but a popular way to do this is with sugar-containing sports drinks or gels. But real food works too. My personal favorite is bacon and egg rice cakes. But don’t assume my go-to food will suit you too. Experiment to figure out what works for you.