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The risk of elevated blood pressure among children and teens increased by nearly a third during a thirteen-year period, according to new research.

A new study, published in the journal Hypertension, suggests that higher body mass, larger waistlines and excess sodium intake may be the reasons behind the elevated blood pressure readings.

The findings are troubling because high blood pressure increases the risk for stroke, heart disease and kidney failure.  Researchers said high blood pressure accounts for about 350,000 preventable deaths a year in the United States.

Researchers compared more than 3,200 children ages eight to 17 in the National Health and Nutrition Examination Survey (NHANES) III in 1988-1994 to more than 8,300 in NHANES in 1999-2008. The study accounted for differences between the two groups in age, sex, ethnicity, body mass, waistline and sodium intake.

The findings revealed that while boys were more likely to have higher blood pressure, blood pressure rates increased significantly more in girls from the first study to the second study. Researchers also found that more children were overweight, and both sexes, especially girls had bigger waistlines in the second study. 

Children whose body mass or waistline measurement were in the top 25 percent of the age group were about twice as likely to have elevated blood pressure as children with measurements in the bottom 25 percent. Children with the greatest sodium intake were also 36 percent more likely than those with the lowest intake to have elevated blood pressure.

The study revealed that African American children had a 28 percent higher risk of elevated blood pressure than non-Hispanic white children.

Researchers said that more than 80 percent of children in both studies had a sodium intake above 2,300 milligrams.  However, fewer children in the second study had an intake above 3,450 milligrams.

"Everyone expects sodium intake will continue to go up," lead author Bernard Rosner, Ph.D., and professor of medicine at Harvard Medical School in Boston, Mass said in a statement. "It seems there's been a little bit of listening to dietary recommendations, but not a lot."

Previous studies found that Americans eat an average 3,400 milligrams of sodium daily, which is more than twice the 1,500 or less that the American Heart Association recommends.  Research has also revealed that around two-thirds of sodium consumption is from store-bought foods and one-quarter from restaurant offerings.

Researchers said limiting sodium intake can lower blood pressure, and the latest study suggests that reducing sodium intake in children's and adolescents' can lower average systolic (top number) blood pressure by 1.2 mm Hg and average diastolic (bottom number) pressure 1.3 mm Hg.

"High blood pressure is dangerous in part because many people don't know they have it," said Rosner. "It's a very sneaky thing. Blood pressure has to be measured regularly to keep on top of it."

In adults older than 19, blood pressure should be less than 120/80 mm Hg.  However, blood pressure norms vary according to age, sex and height among children and teens.

While the latest research found "elevated" readings, researchers noted that the children could not be called hypertensive because blood pressure readings must be high three times in a row for an official diagnosis.