CHICAGO – About a third of U.S. teens would flunk a treadmill fitness test, a new study shows, meaning that more than 7 million youngsters could face higher risks for heart disease later in life.

While that finding is not surprising – given previous research showing that about 16 percent of U.S. schoolchildren are seriously overweight – it’s “very concerning,” said Dr. David Ludwig, director of the obesity program at Children’s Hospital Boston.

Ludwig, who was not involved in the study, said the results show that “at a time in life when adolescents and young adults should be at peak levels of fitness, there’s in fact a very high prevalence … of very low fitness.”

The analysis of nationally representative data from government health surveys by Northwestern University researchers found that 34 percent of girls and boys aged 12 to 19 showed a poor level of cardiovascular fitness on an 8-minute treadmill test.

The study included 2,205 adolescents and 3,110 adults aged 20 to 49 who participated in the 1999-2002 surveys.