That is up from 7 percent in 1994, according to the heart association's annual statistical report on heart disease and stroke.
The 10 percent number comes from 2002, the most recent year for which figures are available, and the situation is probably even worse now, said Dr. Robert H. Eckel (search), president-elect of the heart association and professor of medicine at the University of Colorado.
"These statistics are not anything but alarming," Eckel said.
The prevalence of obesity among adults is well-known, with an increase of 75 percent since 1991. So is the problem with school-age children, reaffirmed by new statistics showing that nearly 4 million children ages 6 to 11 and 5.3 million young people ages 12 to 19 were overweight or obese in 2002.
But the findings among preschoolers are a strong indication that kids' weight problems are beginning even earlier.
"I think that what we're seeing is that obesity is increasing across the board in adults, adolescents and children," Dr. Christopher O'Donnell, chairman of the heart association's statistics committee and associate director of the Framingham Heart Study, which has been following the health of generations of Massachusetts residents.
Experts blame the prevalence of junk food marketed to children, too much TV, and the decline in the number of families who sit down together to eat.
Dr. Sarah Blumenschein, an assistant professor of pediatric cardiology at the University of Texas Southwestern Medical Center at Dallas, said doctors and parents need to watch the weight of even very young children.
"We have a lot of people that think that their kids look cute plump: `Look at her — she has all those bracelets of fat,'" she said.
Dr. William Cochran, a pediatric gastroenterologist and nutritionist for the Geisinger Clinic in Danville, Pa., said he sees many youngsters in his weight management clinic who weigh 300 to 400 pounds. He is also seeing more and more children with diabetes, high blood pressure, even liver disease.
"Some kids are drinking a liter or two liters of soda a day," said Cochran, a member of the task force on obesity for the American Academy of Pediatrics. "In 10 to 30 years, the incidence of heart disease and stroke and diabetes are just going to be astronomical."
Other highlights of the report:
— About 1 million youths ages 12 to 19 in the United States — or 4.2 percent of the age group — have metabolic syndrome, defined as three or more of the following five factors: high triglycerides; low "good" cholesterol; high blood sugar; high blood pressure; and a big waistline. These factors raise the risk of heart disease.
— In 2002, heart disease killed 927,448 Americans, keeping its place as the nation's No. 1 killer.
— The Framingham study found that being overweight or obese can take years off your life. For example, a 40-year-old woman who does not smoke could lose 3.3 years of life because she is overweight and 7.1 years for being obese.