As many as 7.5 million adolescents and 8.5 million American adults are becoming overweight, says a study.
Previous studies have found that Americans have been getting less and less exercise in recent years and are becoming increasingly overweight.
However the new nationwide study of over 5,300 Americans by Mercedes R. Carnethon of Northwestern University and other researchers found a significant proportion of US teenagers and young adults destined for heart diseases and other ailments unless they started exercising more.
The treadmill tests, conducted on people between the age 12 and 49, found that about one out of every five had poor cardiovascular fitness, including about one-third of teenagers and 14 percent of young adults, 'The Washington Post' reports.
"What we found is pretty worrisome. We have an obesity epidemic already. And it doesn't seem as though the trends are changing for the better. We see decreasing levels of physical activity across all age groups," Carnethon said.
"This is something that we thought was happening and that we have been concerned about. But no one had ever documented before how poor fitness actually is across the population in the US," Carnethon said in the study published in the latest issue of the Journal of the American Medical Association.