Spend time on the sidelines of any marathon, and you may spot a perplexing phenomenon: Some otherwise svelte runners—people with toothpick arms and legs—may have oddly protuberant bellies bulging from beneath their race bibs.
Call it “runner’s paunch.” It’s not exactly widespread, but it shows up often enough to be a topic of conversation among some regular runners. Experts say there are two likely explanations—one of which may pose a significant health hazard.
“I think one possibility is fairly common, especially among older individuals, and that’s where you lose a lot of weight and the belly skin sags,” says Tim Church, a professor at LSU’s Pennington Biomedical Research Center.
Church explains that skin is “really thick” and that, as we grow older, it loses elasticity and the ability to reabsorb into our bodies. If someone in her 40s or 50s (or beyond) adopts a healthier lifestyle and loses a lot of weight, she may have a lot of sagging belly skin—the kind that collects low down on her abdomen, and just above her groin—even if she’s otherwise slim. That’s doubly possible if a person used to carry a lot of weight in her gut.
“A buddy of mine in jujitsu has this—where you can see his six-pack, but he still has loose skin below it,” Church says. “The only way to get rid of that [excess skin] is to have it cut off.”
The second and potentially more insidious cause of runner’s paunch may have to do with a runner’s diet. Time and time again, studies have shown that aerobic exercise alone isn’t a great way to lose a lot of weight. That’s not to say you can’t drop a few pounds training for a race; you can expect to shed 3 to 5 pounds by adopting a regular running routine, research suggests. And regular aerobic training is undoubtedly good for your health.
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But the only real way to lose a lot of weight is to make changes to your diet. “Most people can’t run enough to lose weight,” says Michael Jensen, an endocrinologist at Mayo Clinic in Minnesota.
Jensen says a person who runs but also eats a poor diet “could indeed keep their paunch” if their genes predispose them to carry excess fat in their midsection, and Church agrees. “In general, if you eat poorly, you tend to put weight on in your belly,” he says. (Ditto if you drink a lot of alcohol, he adds.) And study after study has linked weight around a person’s middle with a higher risk for metabolic disease and mortality—even compared to people who have high BMIs.
So if a person runs regularly but doesn’t adopt a healthy diet to complement his exercise habit, he could end up with a gut or paunch.
This may be especially true for people of Asian ancestry. “You see a lot of cases of normal-weight diabetes in people of Asian descent,” Church says. Thanks in large part to their DNA, many people of Asian descent are naturally slim, but they also tend to carry any weight they accrue in their tummies, he explains. “You’ll see people who look skinny," he says, "but their abdomen looks like there’s a basketball in there."
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This article originally appeared on VICE US.