The Skinny on 'Hidden Fat': Why Being Thin Is Not Equal to Being Healthy

— -- In the ongoing war on obesity, health officials have consistently focused on Body Mass Index, or BMI, as a measure of weight appropriate to a certain height.

The bad news is that more than a third of Americans, 34.9 percent, are obese, with a BMI of over 30, according to 2014 study by the U.S. Centers for Disease Control and Prevention published in the Journal of the American Medical Association. Another third of Americans are overweight, according to the CDC, with a BMI of between 25 and 30.

But that's not where the bad news ends. Many health experts have long been concerned that BMI does not properly account for people who look svelte but have fat hidden away, making them "normal weight obese."

Those people can still store away reservoirs of fat in the body or even in the organs or muscles, leading to serious health consequences similar to those of a person whose BMI indicates they're overweight, experts note.

A 2010 study published in the European Heart Journal found that as many as 30 million Americans are suspected of having normal weight obesity.

"It’s absolutely true there are some people who seem like no matter what they’re doing, they look really good but looks can be deceiving," Carol Garber, a professor of Movement Science at the Teacher College at Columbia University, told ABC News.

Garber said she has seen first hand how even skinny patients can be at risk for heart disease.

"We would regularly see people who had heart attacks come to [our] rehab program and look perfectly fine," Garber said. But "if you measured their body fat, they had a greater proportion of fat than they would have thought."

It's key to be clear that apparent thinness does not always equal health and that even a skinny person with a low BMI can be unhealthy if fat has built up around their organs, Garber said.

"[Fat] affects different kinds of inflammatory substances that have been implicated in heart disease and diabetes," Garber said. "They can cause damage to blood vessel walls and affect how your blood vessel works."

Some body fat is essential to stay healthy, Garber emphasized, with a range stretching up to 25 percent of body weight for women and around 15 percent for men. People who are thin and active likely don't need to be afraid that they have normal weight obesity, she noted.

"The bottom line is think about your lifestyle ... no matter what your weight is," said Garber. "Irrespective of your weight, everyone is going to benefit to shift your diet and eat more fruits and vegetables, and not smoking and not over-drinking."

People who focus on losing weight are often frustrated when the scale refuses to budge, she said, but a healthy lifestyle overall will give tremendous benefit, even if it's not reflected on the scale.

"You can be smug and healthy and a little overweight," she said.