One-third of all U.S. teens and 15 percent of young adults show signs that they could develop heart disease. Northwestern University associate professor Mercedes Carnethon was lead author of the study. She says the numbers underestimate the levels of low fitness in the U.S. population. That's because the highest-risk people were excluded because it was actually too risky in this population study to put them on a treadmill, because she adds, They were already at such high risk.

The study reviewed health and fitness records of 5,000 participants over 15 years. Ms. Carnethon says, that some individuals are in worse shape than others in this country, namely African Americans, Mexicans and females as opposed to males, she says.

Heart disease and stroke are the first and third leading causes of death for Americans.

Mercedes Carnethon says the development of risk factors for heart disease and stroke is not just the natural result of aging. Physical fitness, she says, will help maintain a healthy weight and protect the heart as you get older. The study was published in the Journal of the American Medical Association.