The bonk run (or ride) is a workout both feared and revered in some elite endurance training circles. The general idea behind it is this: you work out sans carbs, like going for a morning run without eating anything first or during the run. That way, your muscles don’t have much glycogen (sugar) available to power your effort so they learn to use fat as fuel instead.
This adaptation would be a boon to endurance athletes, who have limited glycogen but plentiful fat reserves. Because when glycogen runs out, you bonk. Hit the wall. Blow up. You get super tired and weak and can’t maintain your pace or you stop moving altogether.
So training yourself to avoid an in-competition bonk by running low on glycogen once in a while might seem like a decent idea. But the ultimate performance benefits have already been questioned, and a new study published in the journal Frontiers in Nutrition brings up two more glaring reasons to avoid the bonk, or to at least approach it carefully.
First, there’s the issue of severe inflammation. Glycogen depletion can cause markers of inflammation called cytokines to skyrocket, along with the possibility of getting sick or prolonging recovery. “Whenever carbohydrate stores are knocked low, it’s a red flag—a huge physiologic stressor—and the entire body feels the effects,” says the study’s lead author, Dr. David Nieman, a pioneer in the field of exercise immunology and professor of health and exercise science at Appalachian State University.
If you’ve caught a cold post-marathon, that finding may not be surprising. This one shouldn’t be either, but it’s the second reason to question the sanity of the bonk run: not every athlete’s body utilizes their glycogen stores similarly.
“Typically when you’re at 70 percent of your VO2 max, which is about marathon pace, most people are relying heavily on glycogen to make it through,” Nieman says. But some well-trained runners going at that intensity for more than 75 minutes will experience very little glycogen depletion whereas other equally well-trained runners use it all up. In other wrods, Neiman's research suggests you can't train your way into being a highly-efficient glycogen-sparing athlete; it's an attribute that may come more from nature than nurture.
“In general, the more you train, the better you get at glycogen sparing,” Nieman says. “But there’s still this uncanny variance that we’ve seen where some runners are much better at preserving their glycogen than others.”
The thing is, there’s no way to tell what type of athlete you are; at the end of prolonged, intense endurance exercise, everyone feels like crud. (Though it’s worth mentioning that poor glycogen-sparing may be one reason some athletes develop chronic fatigue doing the same endurance workouts as other athletes who recover well.)
That’s why athletes should skip the bonk workout and keep carb stores topped off before and during endurance workouts lasting longer than 75 to 90 minutes. A half a banana every 15 to 30 minutes, or 30 to 60 grams of carbs an hour should help keep potentially harmful inflammation in check.
You could also do interval workouts, with these same fuel guidelines, to help attenuate the inflammatory response. “Intermittent activity, the immune system handles that in great fashion—there’s no ill effects,” Nieman says. “Run up a hill, jog down, rest a bit and do it again. That’s going to be a lot better to your body’s physiology and immune system than running 90 minutes or longer at high intensity.”