Weight loss has been shown to reduce or even cure sleep apnea in people who are overweight, but it’s not clear why.

A new study led by University of Pennsylvania researchers suggests the main reason is that dropping pounds causes the tongue to shrink, mostly by losing fat.

“Now that we know tongue fat is a risk factor and that sleep apnea improves when tongue fat is reduced, we have established a unique therapeutic target that we’ve never had before,” said senior author Richard Schwab, chief of sleep medicine at Penn’s Perelman School of Medicine.

About 15 million Americans suffer from sleep apnea — harmful pauses in breathing during sleep, caused when the airway collapses or becomes blocked.

Being overweight or obese is a huge risk factor for sleep apnea, which is usually marked by loud snoring. Other risk factors include a small jaw and large tonsils. Besides daytime sleepiness, the disorder can contribute to high blood pressure, heart disease, stroke, diabetes, and depression.

Devices that provide a constant, gentle flow of pressurized air during sleep, commonly known as a CPAP machine, help about 75% of patients, but many people can’t tolerate the headgear or don’t wear it enough to get relief.

For decades, researchers have studied the anatomy of the upper airway, throat, and mouth to try to understand apnea. Six years ago, Schwab’s team focused on the tongue, intrigued by a California autopsy study that found the size and fattiness of the human tongue increased in relation to obesity.

Using specialized MRI scans, the Penn scientists discovered that obese adults with apnea had bulkier tongues than obese people without apnea. On average, apneics’ tongues were a cubic inch bigger and had 17% more fat.

The new study, published Friday in the American Journal of Respiratory and Critical Care Medicine, set out to see whether weight loss reduced soft-tissue volumes in the throat and mouth, and if those changes would correspond with reductions in apnea. The 67 patients all had at least 10 apnea episodes per hour, and had an average body mass index of 43 — 220 pounds for a 5-foot-tall person. (A BMI of 40 or higher is characterized as severe obesity.)

Through dieting or weight-loss surgery, the patients lost an average of 10% of their weight over six months. MRI imaging and statistical analyses revealed that several structures — including a jaw muscle and muscles on the sides of the airway — had smaller volumes with weight loss and this correlated with slight apnea improvements. But the effect was most striking with reductions in tongue fat.

For example, a patient who went from a BMI of about 45 to 30 reduced his tongue fat by half. He saw his apnea improve from 121 to 25 episodes per hour. (Thirty per hour is severe.)

Because some people who aren’t obese have apnea, Schwab’s team is looking at whether fatty tongues play a role and whether that fat can be targeted. Tongues, of course, can’t run laps or do push-ups to get slim and trim, but Schwab believes potential interventions exist. Among the possibilities are a low-fat diet, upper-airway exercises such as singing or playing an instrument, and a noninvasive, FDA-approved cooling technique that has been shown to freeze and kill fat cells.

“It’s not been studied, so no one knows,” Schwab said. “We don’t understand why people have tongue fat. Why is there any fat there? Maybe it’s genetic. And we understand — even less about how to reduce it.”