The dreaded bonk. Those who have experienced the physical shutdown of this energy-less state know how important carbs are to long distance running. It’s those stored carbohydrates, in the form of glycogen, that are primarily used by the body to fuel endurance exercise and because of that, ensuring adequate carb intake—i.e. carb loading—has become an important facet of training and a pre-race ritual for many athletes.
“Carb loading works and will continue to work. It works within a day,” says Dr. Asker Jeukendrup, one of the world’s experts in performance nutrition and author of Sport Nutrition: An Introduction to Energy Production and Performance. “We have known this since the 1960s that it is important to have full glycogen stores at the start of an important endurance event.”
But researchers wanted to go a step further: store carbs but also spare them by ramping up fat burning. Muscles then rely more on fat stores, a virtually inexhaustible source of energy for long-distance exercise, and save glycogen, lowering the chance of bonking. The body can only store enough glycogen for roughly two hours of exercise, but even the skinniest runner has enough fat to fuel over 100 hours of running.
Researchers began tinkering with carb availability, restricting carbs before exercise in an effort to force the body to “learn” how to burn fat as a fuel, a strategy they called “train low.” These attempts at using fat to stoke the fires of endurance exercise didn’t go too well, however. Despite the clear physiological changes found after training with low-carb availability, performance showed no improvement.
In fact, emphasizes Jeukendrup, most of the “train low” studies so far have been unable to show actual performance benefits. “Training low is something that does not work short-term (studies often show a reduced exercise capacity and lower quality of training when training low),” he says. It’s hard to train in a perpetually bonked state.
Realizing that the “train low” strategy lacked practical value, a team of researchers from France and Australia attempted to learn from past research and build a strategy that didn’t just work in the lab, but actually enhanced endurance performance. In a study in Medicine and Science in Sports and Exercise, an experimental group of trained triathletes, termed the “sleep low” group, was given the following strategy: a carbohydrate-fueled high intensity training session in the evening followed by a low-carb recovery and overnight fast, depleting muscle glycogen for the night. The following morning, the athletes had a prolonged, moderate intensity training session preceded by a light, low-carb breakfast, forcing their glycogen depleted muscles to adapt to burning fat.
The control group maintained their usual carbohydrate intake over the day and tackled each training session with normal or high carbohydrate availability.
After the three-week study period had concluded, the experimental group showed a 73-second improvement in a 10K running time trial. And not only were they faster at race pace, the “sleep low” group became more comfortable at a lower intensity, improving exercise efficiency and decreasing perceived effort.
The researchers concluded that the “sleep low” protocol used in the study allowed the runners the best of both worlds—high carbohydrate availability for intense training during the day and low carbohydrate availability at night to spur the body to use fat as a fuel. “Our study provides evidence that periodizing carbohydrate availability around these training sessions is an important determinant of the performance results,” the study concluded.
But significant improvement or not, experts advocate caution in implementing the strategy during an important training or race period without first trying it out.
“If done well, periodized well, dosed well and integrated well with training, the risks are minimal. But this is the challenge and this is where many make the mistake. These strategies are not intended for everyday use. Use them occasionally and strategically,” emphasizes Jeukendrup.
Dr. Stacy Sims, exercise physiologist and nutrition scientist, believes that while the results can be generalized to male runners similar to those studied, large scale testing needs to be performed before it can be recommended to all. “We can say that the men in the study most likely will have similar results on the road, and men similar to these dudes in the study may also have similar results, but we can’t generalize to older or younger men, and definitely not women,” Sims says.
Jeukendrup also believes that since this is the first study that has demonstrated a benefit from a “sleep low” strategy, the results should be interpreted with caution. “I would use strategies like this (and other “train low” strategies) in a preparation phase,” he says. “Once athletes enter a competition phase there must be an emphasis on recovery and quality of training while minimizing risk of injury, overtraining and getting ill.”
For those that are interested in squeezing out every last ounce of potential performance, the “sleep low” researchers may be on to something. Manipulating carbohydrate availability along with training intensity during training has the potential to improve performance—just use it wisely.
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