Everyone knows someone who eats cookies for breakfast, chips for lunch, and has never actually seen the inside of a gym. Yet she still doesn’t seem to gain a pound. Maybe it’s a good friend. Maybe it’s you.

While your gut reaction might be envy (or guarded pride) recent research has exposed a dangerous reality underneath lithe frames.

It’s called being skinny fat. Technically called “normal weight obesity,” skinny fatness occurs in anyone who has a normal range BMI (body mass index), but high body fat. “They are skinny by weight, but actually have a high percentage of body fat,” explains Beth Ricanati, M.D., YouBeauty Wellness Advisor.

In a 2014 report, those with normal weight obesity were found to have a significantly higher risk of developing metabolic conditions, such as type 2 diabetes, high blood pressure and high cholesterol, than those without. When you’re eating a diet high in sugar and processed foods, it causes visceral fat storage, and can lead to the same risk factors as being overweight.

Jeff O’Connell, a fitness journalist and writer of “Sugar Nation,” learned the hard way that just because you don’t gain weight doesn’t mean you can eat whatever you want. Despite being tall and lean and regularly lifting weights, O’Connell learned at age 43 that he was prediabetic, a metabolic condition one step before type 2 diabetes. Reflecting on his lifestyle habits, he realized he was eating a lot of junk food, and didn’t bother with cardio since he looked thin and healthy.

Looks, it turns out, can be decieving. “This is not obvious to diagnose just by looking at yourself,” Ricanati notes.So how can you tell if you’re skinny fat?

Don’t Trust Your BMI

BMI takes into account your height and weight, and is an awesome general measure of health. But, it does not factor in an important aspect of overall health: body fat percentage, which is the key determanent of skinny fatness. While BMI can be a good jumping off point to assess health, it isn’t foolproof. For example, if you’re super muscular or big boned, you might fall into a higher BMI category even though your body fat percentage is normal.

BMI’s shortcomings can pose some serious health concerns if the numbers (and mirror) tell you you’re at a healthy weight, or even underweight, when your body fat percentage is higher than it should be—putting you at risk for metabolic diseases.

Get Tested

“Measurements of body fat, as well as [blood tests] such as c-reactive protein, lipid (cholesterol) levels and glucose (sugar) levels can determine if you are actually thin on the outside, but fat on the inside,” notes Ricanati. If you have a family history of any metabolic conditions, such as high blood pressure, high cholesterol, diabetes or fatty liver, Ricanati says, you should be getting these tests regularly, no matter what you weigh.

Look at Your Lifestyle

Before you even make an appointment, it’s pretty simple to decide if you need a lifestyle makeover to avoid health problems down the road. Take a look at your habits and give yourself an honest assessment. Do you eat way too much sugar? Do you avoid physical activity like the plague? Do you smoke? When’s the last time you thought about the nutritional value of your food?

If you’re realizing you’ve tossed your healthy habits to the wayside, it might be time to make some changes.