Why Exercise Can Actually Cause Weight Gain

Feeling the burn can, counterintuitively, pack on the pounds

Diet and Exercise
illustration by darren gygi

After finishing my first (and only) full marathon, I saw a picture that a friend had snapped of me on the course as I neared the end. My face conveyed a mix of agony and ecstasy from having  put my body through brutal punishment for the sake of fleeting athletic glory. But my eye was immediately drawn down lower: to my mid-section, which had somehow—mysteriously, maddeningly—burgeoned during my months of relentless, shoe-shredding training.

I Googled “marathon and weight gain,” and it quickly became clear that my experience wasn’t unique. Sure, there were the optimistic articles that attributed weight gain from marathon training to water retention and increased muscle mass. But many more acknowledged  the reality that I and countless other runners have experienced: The number on the scale is up, your clothes are fitting more tightly—and it isn’t due to your newly rippling quads. 

It recently emerged that Coca Cola is funding a new nonprofit, the Global Energy Balance Network, which promotes the notion that Americans have become overly focused on diet and calorie consumption, while under-appreciating the role of exercise in achieving and maintaining a healthy weight. But if it’s true that exercise offsets questionable dietary choices, what explains the pounds I packed on during what was inarguably the most intensely exercise-laden period of my life?

Perhaps the most common pitfall for those trying to lose weight is overestimating the number of calories burned during exercise. In one study, participants exercised on a treadmill and were then taken to a buffet, where they were instructed to eat the caloric equivalent of what they estimated they had burned. The participants ended up eating two to three times the number of calories they had actually expended.

“We feel like we’re burning more calories than we actually are, and it increases appetite.”

Dr. Jennifer Linde, an associate professor in the School of Public Health at the University of Minnesota, breaks it down to simple math. “We are much more
efficient at taking in calories than we are at spending them,” she says. “Think about how many calories you can consume in 30 minutes versus how many you can burn in the same amount of time.” 

It’s no contest. One Chipotle burrito can deliver more than 1,000 calories. For most people, even the most vigorous 30-minute cardio session burns only 200 to 400 calories. “We’re really good at finding time to eat, but people have difficulty finding time to exercise,” Linde adds, and even when we do allot time to work out, it can lead to an inflated sense of caloric entitlement.

Kelly Scheller, a registered dietitian at the University of Minnesota Medical Center, explains the exerciser’s physiology and psychology. “We feel like we’re burning more calories than we actually are, and it increases the appetite. You might burn 300 calories, then eat 600 because you feel like you earned it.”

Scheller points out that exercise is important, though, in many ways that go beyond weight loss. “Exercise has so many health benefits. It improves heart health, muscle strength, and bone strength,” she says. But when it comes to weight loss, “Diet trumps exercise. … You need to nail down those calories.”

Weight management, Linde says, is about making fundamental changes to how you approach eating and moving. “Increase your level of exercise enough so you feel comfortable and confident that you can maintain it, and do the same with reducing overall calories,” she suggests. Scheller agrees: “It’s about matching your caloric intake with your overall energy expenditure.” In the case of a single 20-ounce Coke, she points out, that would mean walking about five miles—a fact that seems unlikely to become part of the company’s public-relations pitch any time soon.

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