When Skinny People are Really Fat

Think you can tell whether people are fat just by looking at them? Guess again.

New research from the Mayo Clinic finds as many as 30 million Americans considered to be of normal weight actually have a high percentage of body fat that puts them at risk for health problems. It's called "normal weight obesity."

CBS News medical correspondent Dr. Jennifer Ashton explained how it's possible that you could be obese but look slim.

"You cannot confuse being slim with being healthy," she said. "In fact, what this Mayo Clinic study found is that you can have internal fat -- we call it visceral fat -- that actually surrounds your internal organs, even if externally your body looks slim."

So if you look thin, how do you know if inside you have dangerous fat?

"This is where it gets a little tricky. There are many ways to check your body fat percentage. Those ways range from a water test to caliper or skin-fold testing to things that are much more expensive," Ashton said. "The bottom line is -- if you have that testing done and find that your numbers are above a certain level, it does indicate that even though externally you could look healthy, internally you could have the same exact risks as someone who's obese."

Those at high risk, Ashton said, are men with over 23.2 percent of body fat, and women with 33.3 percent of body fat.

To offer a comparison, Ashton sale male athletes have as low as eight percent of body fat and female athletes have around 14 to 15 percent body fat.

Risks of high body fat include:
• Metabolic syndrome
• Diabetes
• High cholesterol
• High blood pressure
• Heart disease

To help get rid of that excess fat, Ashton suggested emphasizing the importance of exercise for cardiovascular health. She said weight-lifting and other resistance training will also help build lean muscle.

In addition, Ashton said eating a healthy diet is an important part of losing the fat.

She said, "It's not just about restricting calories, but eating the right foods to build lean muscles."