Many endurance athletes see “bonking” or “hitting the wall” as a rite of passage, but what if it was avoidable? In fact, you may be able to train your body to utilize available fuel more efficiently, thereby skirting the dreaded bonk. While the physical training piece plays a role in this process, it is largely driven by a nutritional practice known as “fasting runs.”
As most triathletes know, the body relies heavily on carbohydrates to produce energy during training and racing. Put simply, when you eat carbohydrates, they are stored in the body as glycogen, which is then used as fuel. But you’re only capable of storing so much glycogen. Once you burn through it, you are forced to rely on utilizing fat, which is a less efficient fuel source. This is the point at which you “hit the wall.” Your pace slows as pain and fatigue set in and there’s little you can do about it.
The idea behind fasting before the occasional run is to teach your body to burn fat more efficiently. “At any given time, we have a lot more available fuel in our bodies in the form of fat, so training the body to use that fuel helps immensely with endurance and aerobic performance,” explains Caleb Masland, a coach and 2:34 marathoner.
The easiest way to run in the fasted state is to go in the morning before the hunger pangs set in, drinking only a bit of water. After an overnight fast, your glycogen stores will be low, forcing your body to burn fat. By doing this sporadically, your body becomes better at utilizing fat as a source of fuel. What’s more, a study out of New Zealand that compared a group of cyclists’ glycogen stores in the fasted state versus the carbohydrate-loaded state discovered that workouts done in the fasted state helped increase their glycogen storage capacity. So not only do fasted runs potentially help you burn fat for longer, thereby pushing back the point at which you switch to burning carbs, they also increase your carb stores for when that point is reached.
The best time to do these fasted runs is on slow, easy recovery days and sometimes longer, relaxed runs. “Workouts need to be chosen carefully,” explains Katie Davis, a Chicago-based sports dietitian and nutritionist. “Run fasted during low- to moderate-intensity training, when the body’s reliance on glycogen is low and the ability to use fat as fuel is high.” Masland agrees, cautioning athletes not to overdo this type of training. “Just like any other training variable, more repetition yields more efficiency, but overuse can lead to burnout.”
To be sure, it’s also important to do glycogen-loaded runs because that’s what you’ll do on race day. Fasted runs should be reserved for some of the slower efforts, while high-intensity workouts should be approached with plenty of fuel storage. “Training at high intensity will not be effective in a fasted state,” Davis says. “The body relies on carbohydrate in this type of aerobic training, so any training undertaken without proper carbohydrate ingestion will sacrifice overall performance during the workout.”
Davis says there’s no one-size-fits-all prescription when it comes to this type of training. “Not every runner will respond well to fasted runs,” she says. “Therefore, each athlete should listen to his or her own body.”