Training for a personal best? Get your carbs at breakfast, then at lunch, but stop there. Credit: Alex Hayden/Getty Images

Do carbohydrates really supply the best fuel for athletic performance? Or will you race faster if you restrict carbs, thereby forcing your body to tap into its fat stores for energy? This question has sparked heated debate lately. Some coaches and athletes are convinced that low-carb, higher-fat diets boost performance by training the body to burn through fat. Others maintain that lots of carbs are necessary to ensure you have enough in the tank to go hard, long, and fast.

A team of exercise scientists from Australia and France have been analyzing the perks and pitfalls of both philosophies over the past few years. Their many different studies have turned out disparate results, some making the case for old-school carb-loading, while others have found merit in the low-carb plan.


The results of the latest trial, published in Medicine & Science in Sports & Exercise, provides perhaps the clearest answers yet. It turns out, rather than choosing one program or the other, both carb-loading and carbohydrate restriction can have a place in your training regimen. The combination approach, which incorporates "sleeping low" — cutting out carbs the evening before a moderate-intensity morning workout — may indeed increase performance. But you have to go about this the right way or else sleeping low can backfire.

The proof comes from 21 competitive triathletes who test-drove these philosophies over a three-week period. After they completed a simulated triathlon and had their body fat and other measures taken, the researchers commandeered their diet and exercise plans. Half were put on a typical training diet — plenty of carbohydrates at every meal and after each workout — four days a week. They could eat whatever they wanted on the remaining three days. The rest of the triathletes followed a sleep-low plan four days a week. That menu included the same amount of total daily carbohydrates as the other group's, except that almost all of those carbs were consumed for breakfast and lunch, leaving hardly any for dinner.

On the same days the triathletes followed their prescribed diets, they all knocked out an intense interval training session in the afternoon, which sucked their carbohydrate stores dry. The control group got to re-up on carbs that evening, while the sleep-low volunteers were left depleted. Then, before breakfast the next day, everyone cycled at a moderate pace for one hour. While the control athletes had a fresh cache of carbs to draw from for this workout, the sleep-low group was forced to dip into their fat stores for energy. Next, both groups ate a carb-rich lunch to power up for the afternoon's brutal sweat session.

By the end of the three-week study, the sleep-low group was cranky. No surprise there, since their bodies were screaming for carbs. However, their total fat mass had decreased, while their lean mass stayed the same — proof that they'd been shredding fat for fuel. The control group's body fat didn't change. And when all volunteers completed a second simulated triathlon, those on the sleep-low program improved their cycling power by about 12 percent and ran roughly 3 percent faster than in their first trial. The control group ran about the same as they did in round one.

According to these results, restricting your carbohydrate intake in the evenings and then working out lightly on low carb stores the next day may indeed help you knock time off your race. By forcing your body to burn fat in the absence of carbs, you melt more fat and, as a result, go faster.

But the key thing to remember here is that the sleep-low group did only moderate exercise without a full stockpile of carbs. If they'd attempted super-intense workouts while depleted of carbs, they'd have run their bodies ragged, as has been shown in past studies. These triathletes also didn't follow this meal plan every single day or keep it up for a prolonged period of time. Sleeping low may not be effective — or even safe — to do long-term. But if you want to incorporate it into your training program for a few weeks leading up to your race — and if you can handle the plan's rigidity and a few hunger pangs at night — you might just be stoked about your times on race day.