Jessica Heidari is a life-long athlete who used to run marathons. She said at 5 feet, 8 inches tall, she weighed 116 pounds.

"So just because I was real skinny doesn't mean I was healthy at all. I was actually underweight and nutritionally that's just not good for you," Jessica said.

While Jessica calls it "skinny fat," the Mayo Clinic researchers call it 'normal weight obesity.' Scientists studied more than 6,000 Americans with normal body mass index, or BMI, and found that people can be thin and have too much fat. Those with higher BMIs were at risk of heart problems.

Dr. Riva Rahl is the medical director of the Cooper Clinic Wellness Program. She said the bathroom scale tells only half the story.

"Now, while the scale may tell the total number of pounds, it doesn't tell what your muscle mass is, what the fat mass is and where the fat is located," Dr. Rahl said.

Dr. Rahl see patients all the time who say they're underweight and don't need to exercise. She said that many times, it's not true.

"So if you have a bunch of fat concentrated around the mid-section, your risk for heart disease could be high even if you have skinny little legs and you're under a hundred pounds," Dr. Rahl said.

Back at the Energy Fitness in Dallas, nutritionist-trainer Sagi Kalev said many people underestimate the importance of weight lifting to build lean muscles.

"Looks can be deceiving. Don't judge a book by it's cover," Said Kalev.

As for Jessica? She started lifting weights and said she is no longer skinny fat.

"My body fat was actually higher and my weight was actually lower. Now I'm at 142 and my body fat percentage is only at 11 or 12 percent. Many girls you see today in Dallas, they just want to be skinny and they really don't think about just because they're skinny, do they have muscle?" Said Jessica.

So the next time you pull out that bathroom scale, Dr. Rahl said, don't believe everything you read.

"I think as a general group, Americans need to exercise more, not focus on the number on the scale," she said.

A BMI of 15.5 is considered underweight while 30 or more is considered obese.