Editor’s Note: Every Wednesday, James Hamblin takes questions from readers about health-related curiosities, concerns, and obsessions. Have one? Email him at email@example.com.
The greatest fear of many distance runners is a devastating fate known as hitting the wall. In marathon running, that often happens about two hours into a race. The body says, Okay, we’re done here. It is no longer capable of motion. You want to collapse on the ground and weep, but instead you fall sideways and lie rigid in the road like a horrible work of taxidermy. Other runners step over you until someone can drag you off the course.
I know because this happened to me (slightly less extremely). When I ran the Chicago marathon in 2013, around the 23rd mile, my legs turned to tree trunks. I could feel them digging roots into the ground. By the time I reached the home stretch along Michigan Avenue, I could move only by swinging my hips like a marionette as crowds cheered and people passed me.
This phenomenon happens when the muscles run out of glycogen. Normally, the liver and muscles stockpile this carbohydrate, which breaks down into glucose and powers motion. Humans typically carry enough glycogen to last for 90 to 120 minutes of pretty intense exercise before running out—so depletion isn’t an issue during a typical jog or CrossFit session. But in longer events, even the athletes who swear by a low-carb “keto” or “paleo” lifestyle reach for sugar.
Last month, when the Kenyan runner Eliud Kipchoge ran the fastest marathon ever, he had cyclists alongside him feed him a sugary drink. Likewise, on Sunday, the New York City Marathon will litter the streets with empty packets previously filled with goos and gels of simple sugars. I will be among the runners, and this time I am determined to hit no walls. I’ve started training by eating Sour Patch Kids on long runs. I had some left over from my birthday party, and I found that, unlike the commercial formulas that somehow manage to make sugar taste bad, candy makes training much more enjoyable.
Then I saw a new study that seemed to suggest I should be eating potatoes instead. When ingested, the starch in potatoes breaks down to become glucose, so at least the concept is sound. In the Journal of Applied Physiology, researchers at the University of Illinois compared the performance of elite cyclists when they were fed three different substances: a sugary goo, an identical amount of carbohydrate as potato puree, and a placebo. Twelve endurance athletes each did a two-hour time trial under each condition, on separate days. All were fed through a syringe, their physiology tracked minute by minute. The performances suffered in the placebo condition, as expected. But overall, the distances were comparable when the athletes got sugar and when they got potato puree.
The study’s authors, led by Nicholas Burd, a kinesiology researcher at the University of Illinois, were curious about potatoes because some athletes already eat them during competitions—especially ultramarathoners, who sometimes run for days at a time. The researchers also knew that the panoply of gels for sale at a pre-race expo can be overwhelming, and that the decision of how to properly fuel oneself is a source of anxiety. “I told my research team, just go buy potatoes, bake them, and puree them,” Burd told me.
The research was funded by a grant from the potato industry, but he says the point isn’t that everyone should eat more potatoes. The results simply show, according to Burd, that “you don’t have to be too fancy with your fueling strategy. At the end of the day, you just need to increase carbohydrate availability—whether it’s from a goo or from a potato.”
He sees this study as basic proof of concept that rice or comparable starchy foods might be effectively interspersed during lengthy events. Both have the additional benefit of being cost-effective, plus Bird posits that a savory option can avoid the “flavor fatigue” of using the pure-sugar concoctions for hours upon hours of exercise.
Intrigued, I asked him if I should bring a potato with me on Sunday. But he was definitely opposed. “Switching up your dietary strategy on race day is one of the worst things you can do,” he said. This was a relief to hear, because I was already deterred by a caveat in the study: When the cyclists ate the potatoes, they experienced higher levels of abdominal pain, flatulence, and bloating. (The paper contains a bar graph whose vertical axis is “Flatulence %”). The effect might be due to the fact that to get enough sugar, the cyclists had to eat the pureed equivalent of four potatoes per hour.
Amadeo Salvador, a researcher at the University of Illinois who helped design the study, told me that, with time, it’s theoretically possible for people to get used to running with different things in their stomach and intestines. “It’s called gut training,” he says. But he, too, implied that I should not carry potatoes on Sunday. “First try it in training situations, and then, if it goes well, during a race.”
I presented all this to Daniel Baur, who studies physical performance at Virginia Military Institute. He was not involved in the study, but he has done other work on carbohydrate optimization. While the research is well done, he said, he expressed significant skepticism about the practicality: “There’s something to the idea of getting sick of sweetness, but I don’t think that would drive me to mash up a kilogram of potatoes. The fact that the subjects had worse gastrointestinal symptoms is not surprising, because potatoes are going to have fiber in them.” That simply amounts to added bulk to rumble around inside you.
Though new carb-fueling solutions are constantly being proposed, Baur said that it seems the gold standard has already been found. To max out sugar absorption, he explained, you want a mix of glucose and fructose—and as little else as possible. The starch in a potato breaks down only to glucose, and Baur and others have found that glucose alone can cause more gastrointestinal issues than a glucose-fructose mix. The ideal is about 60 grams per hour of the former and 30 of the latter. This is the formula Kipchoge and other superhumans use.
Fortunately, the sugar in most candy breaks down to half glucose and half fructose. For all but elite, professional runners, Baur says, this ratio should suffice. “In the real world, the average person probably isn’t going to notice a difference,” he said. Like the other researchers, he agreed that I shouldn’t change up my fuel source at this point. So if I do indeed cross the finish line in Central Park, my teeth and pockets will be coated with the residual sugar of many Sour Patch Kids.
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