Think you're thin? Think again.

That bathroom scale has never been your best friend, now it turns out it's also a liar. Mayo Clinic researchers studied more than 6,000 Americans and found that even people who had normal Body Mass Index number, many of them still had an unhealthy amount of fat.

Doctors say that lead to heart disease. We talked to a girl who has been an athlete her entire life. She's 5' 8" tall and used to run marathons. At the time she weighed 116 pounds. She calls it 'skinny fat." Mayo Clinic researchers call it 'normal weight obesity." She's now lifting weights and weighs 142 pounds and her BMI is somewhere between 11 and 14, actually lower than when she weighed 116 pounds.

Bottom line? Never believe what you read, especially the bathroom scale.