If you’ve always been a normal weight without having to try too hard, you may consider yourself lucky. But the mirror and the scale only tell part of the story: You can look great in a bikini or have a body-mass index (BMI) in the normal range, but if you don’t take care of yourself, you could be just as unhealthy as an obese person.

This phenomena—sometimes known as skinny-fat, or “normal-weight obesity”—may affect up to one-fourth of normal weight people, according to one 2008 study. “They look healthy, but when we check them out they have high levels of body fat and inflammation,” says Ishwarlal Jialal, MD, director of the Laboratory for Atherosclerosis and Metabolic Research at UC Davis Health System. “They’re at high risk for diabetes and cardiovascular problems, but you wouldn’t know it from their appearance.”

Getting your blood pressure, cholesterol, and blood sugar levels checked is the only way to know for sure how healthy you are metabolically. But there are some warning signs that may help you determine whether you’re at risk for normal-weight obesity. If these characteristics apply to you, talk to your doctor about how you can make sure you’re physically fit, both inside and out.

You have a muffin top

Even if you’re a healthy weight overall, sporting a spare tire can be dangerous. In fact, a recent study in the Annals of Internal Medicine found that normal-weight people with excess fat around their middle had an even higher risk of dying early than their overweight or obese peers.

“Fat around the middle is worse than fat anywhere else,” says Dr. Jialal. “It’s where the damage starts in terms of insulin resistance and inflammatory proteins.” And this kind of fat doesn’t appear all at once, either, so it’s easy to ignore. “It should be a warning sign if you’re slowly increasing your belt size or your pants start feeling tighter around the waist.”

RELATED: 10 Reasons You're Not Losing Belly Fat

You can’t do a push-up

Skinny people can still have high levels of body fat, especially if they lack lean muscle mass. And if you can’t remember the last time you broke a sweat, there’s a good chance this could be you. “When people aren’t overweight, they don’t have the incentive to work out and get in shape,” says Dr. Jialal. “But without regular exercise, they just get more and more unhealthy.”

Getting regular aerobic activity—like brisk walking, cycling, or running—is important for maintaining a healthy heart and lungs. But adding in regular strength training will also help you build muscle, which will rev your metabolism and burn toxic fat.

You have a family history

If a parent or sibling has developed diabetes, heart disease, high blood pressure, or high cholesterol (no matter what his or her size), you may be genetically predisposed to these conditions, as well. Maintaining a normal weight will certainly lower your risk, but it will be most effective if you do it in a healthy way—through exercise and a balanced diet.

Talk to your doctor about other ways you can avoid health problems that run in your family. If you still have risk factors (like high blood pressure or elevated blood sugar) even with a healthy lifestyle, medications may help you keep them under control.

RELATED: 57 Ways to Lose Weight Forever, According to Science

You don’t eat a healthy diet

Maybe you’re able to eat burgers and guzzle sodas without gaining a pound. Or maybe you count your calories, but you fill up on white bread and junk food rather than fruits and veggies. Either way, consuming too much sugar and fat—and not enough vitamins, fiber, and lean protein—can damage your organs and raise your risk for diabetes, heart disease, cancer, and more.

Yo-yo dieting, skipping meals, and extreme cleanses can also skinny-fat syndrome, says Cynthia Sass, RD, Health’s contributing nutrition editor. That’s because you pack on body fat when you get hungry and overeat, but you lose muscle mass when you drastically cut calories—a bad combination that wreaks havoc on your health.

You’re in an at-risk population.

BMI isn’t a perfect measurement for any group of people, but research suggests that it may be even less useful for certain ethnic groups as a measure of overall health. For example, a 2011 study found that people of South Asian descent are more likely to store excess fat around their internal organs, compared to Caucasian people of the same BMI.

This type of fat that surrounds organs, known as visceral fat, has been associated with metabolic problems and chronic disease. “Genetics are definitely involved in how people store fat, but culture and diet also probably play a role, too,” says Dr. Jialal. “And obviously eating healthy and getting exercise is good for everyone, no matter what your risk factors.

Another 2014 study found that older adults should pay less attention to their BMIs. Because we lose muscle as we age, it’s common for elderly people to have high body fat percentages, even at normal weights. Focusing on building muscle mass, rather than worrying about the number on the scale, can help older adults live longer and healthier lives, the study concluded.