"High blood pressure is the predominant risk factor for stroke, and stroke rates have been rising in children in the U.S. over recent years," said Dr. David Katz, director of the Yale University Prevention Research Center. He was not involved with the study.
Harvard researchers collected data on more than 3,200 children aged 8 to 17 who took part in the National Health and Nutrition Examination Survey from 1988 to 1994, comparing them to more than 8,300 kids in the same survey from 1999 to 2008.
Although children in the study had elevated blood pressure, they could not be classified as hypertensive, because readings must be high three times in a row to make that diagnosis, the researchers noted.
As the obesity epidemic continues, doctors are seeing more children with high blood pressure, an expert said.
"Today alone I will see 10 to 15 [patients], mostly teenagers, that are overweight with hypertension," said Dr. Ana Paredes, a pediatric nephrologist at Miami Children's Hospital.
The first step in treating these children is to change their diet and increase the amount of exercise they do, Paredes said. "I give them a plan they can follow," she said. "I tell them to try to lose a pound a week."
Paredes also counsels her patients to reduce the salt in their diet. Much of the salt that children consume comes from processed foods and drinks like sodas, she said. "If you are drinking Gatorade while watching TV or working on the computer, you're just intoxicating yourself with salt," she said.
Katz said that the new study adds to the weight of evidence that sodium intake affects blood pressure in children as well as adults.
"The pathway from increased sodium intake to elevated blood pressure to rising incidence of stroke is cause for both concern and corrective action," Katz said.
Americans eat an average 3,400 milligrams (mg) of salt every day -- that's more than twice the American Heart Association's recommendation of 1,500 mg or less. Two-thirds of the salt is from processed foods and restaurant meals.
The association between body-mass index -- a measure of body fat -- and blood pressure is even stronger than the association with sodium, Katz noted. "As obesity rates rise in our kids, so, too, does their blood pressure," Katz said.
The high salt content of processed food is correctable as are the influences making children obese, he said. "If we needed another reason to take action, this study provides one. As our kids' blood pressure rises, so does the pressure on us all to do what's necessary to put a stop to these trends."
For adults, normal blood pressure is considered to be less than 120/80 mm Hg. Among children and teens, however, normal blood pressure varies by age, sex and height, the researchers noted.
In calculating risk for high blood pressure, they took into account differences between the two groups of kids in terms of age, sex, race/ethnicity, body mass, waistline size and salt intake.
Highlights of the study, published online July 15 and in the August print issue of the journal Hypertension, included the following:
- Although boys were more likely to have elevated blood pressure, the rate increased more rapidly in girls.
- Over time, more children were overweight and had bigger waistlines -- especially girls.
- Children with the biggest waistlines were about twice as likely to have high blood pressure, compared to the children with the thinnest waistlines.
- Black children had a 28 percent higher risk of high blood pressure than white children.
- Children who consumed the most salt were 36 percent more likely to have high blood pressure, compared to children who consumed the least.
- In both studies, more than 80 percent of children had a daily salt intake of more than 2,300 mg. However, fewer children in the later study had an intake above 3,450 mg.
"Everyone expects sodium intake will continue to go up," lead researcher Bernard Rosner, a professor of medicine at Harvard Medical School, said in a statement. "It seems there's been a little bit of listening to dietary recommendations, but not a lot."
"High blood pressure is dangerous in part because many people don't know they have it," he added. "It's a very sneaky thing. Blood pressure has to be measured regularly to keep on top of it."
High blood pressure accounts for some 350,000 preventable deaths each year in the United States, the researchers noted.
To find out more about hypertension in children, visit the Nemours Foundation.