Everyone knows obesity carries the risk of a worrying number of diseases, from heart disease to type 2 diabetes. But a slim physique doesn’t offer a free pass from chronic conditions, and growing evidence suggests being so-called skinny fat could be just as damaging as being obese.

The misconception that those who are slender are less likely to suffer from potentially life-threatening illnesses partly lies in how society often values thinness above health, and confusion over how the physiologically ideal body type is measured, experts told Newsweek.

Unlike obesity, which is identified as a body mass index (BMI) of 30 or more and excessive fat accumulation, “skinny fat” is a looser term.

“Skinny fat refers to people who fall into the normal category of weight for their height, a term called Body Mass Index, yet have a disproportionately high proportion of body fat,” Dr. Eva Tseng, assistant professor medicine at the Johns Hopkins University School of Medicine, told Newsweek. 

Professor Jimmy Bell, an expert in obesity and metabolism at the University of Westminster, U.K., puts it another way. Referring to the Oscar Wilde novel about a man who appears ageless, but whose sins are marked by painting unseen for the rest of the world, he dubbed it “Dorian Gray syndrome.” Skinny fat is also known as normal weight obesity and TOFI: thin on the outside, fat on the inside.

Keeping our bodies on their toes, so to speak, is another trick for tackling not only the physical but the potential mental effects of a skinny fat body, such as dementia, said Bell. Scientists believe frequent bursts of intense exercise are more beneficial than sporadic physical activity. Go to the gym for three short sessions a week rather than once a week for two hours, he said.

“Move in as many ways as possible,” he said. “Dance, skate, play sports. Because then you use your brain rather than sitting on a bike in the gym. Walk up the stairs instead of getting the lift. Walk to work if you can. Park far away [from your destination] and walk.”

The take-home message? “People think that beauty is more important than health,” said Bell. “It’s not until you’re 60s and 70s you realize beauty isn’t important. The most important thing is health.”