Nine million American children -- nearly one in five -- are overweight. New evidence indicates those children are more likely to develop, or even die, of heart disease as adults.

Being overweight is about more than just size.

"I used to feel terrible all the time. I wasn't happy,” sat Athena, 12.

Athena was concerned about her image. Her mother was concerned about her health.

"If she can't run around the block, or go for a 5-minute walk with me without panting and crying and saying she wants to go home, there is a problem that has nothing to do with beauty, but a lot to do with fitness,” said Sarah Diligenti.

By watching her diet, getting active and seeking professional help, Athena lost 28 pounds in a year and, according to researchers, significantly reduced her risk of developing heart disease. University of California researchers predict heart disease will rise as much as 16 percent – 100,000 new cases -- and deaths could climb 19 percent by 2035 if today's overweight children become overweight adults.

"We were really struck by the magnitude of this increase because we don't normally think of 35-50 year olds as being at high risk for heart disease,” said Kirsten Bibbins-Domingo, the lead author on the study.

Dr. Tania Heller, a pediatrician and children’s obesity specialist, sees the warning signs every day.

"We see diabetes in younger kids these days. Many patients come to us with lipid abnormalities, high cholesterol and some have hypertension,” said Heller.

Doctors say kids can improve their odds by exercising and avoiding junk food. A Danish study also in this week's New England Journal of Medicine found every little bit can help or hurt. For instance, a 13-year old boy who's 25 pounds overweight increases his risk of developing heart disease by a third.