(Health.com) -- Children are considered overweight if their body mass index (BMI) -- simple ratio of height to weight -- is in the 85th percentile or above for their age. That imaginary line may seem arbitrary to some, but a child's risk of having high blood pressure nearly triples if he or she crosses it, a new study has found.
Fourteen percent of the overweight and obese children in the study had high blood pressure (hypertension) or the milder condition prehypertension, compared to just 5% of normal-weight kids. What's more, rates of hypertension and prehypertension appeared to stay relatively flat as BMI rose, before spiking at the 85th percentile and continuing upward from there.
"If you have a child [who] is already overweight and there is a small increase in BMI percentile, then the risk for hypertension actually increases pretty dramatically," says the study's lead author, Wanzhu Tu, Ph.D., a professor of biostatistics at the Indiana University School of Medicine, in Indianapolis. "At the same time, if you were able to reduce the BMI percentile even by a little bit, you might actually see much benefit in blood pressure."
The connection between blood pressure and excess weight is well established in both children and adults, but this study is the first in children to investigate how blood pressure varies across the full spectrum of body sizes.
The sharp increase in hypertension and prehypertension risk at the 85th percentile is "striking," says Bonita Falkner, M.D., a professor of medicine and pediatrics at Thomas Jefferson University, in Philadelphia.
Doctors have generally considered being overweight -- as opposed to obesity (defined as the 95th percentile of BMI or above) -- to be relatively harmless in children, but the new findings suggest that being overweight should be considered a serious problem in and of itself, says Falkner, who cowrote an editorial accompanying the study.
Recent studies have demonstrated that damage to the body from high blood pressure -- such as enlargement of the heart, increased blood vessel stiffness, and altered mental function -- can begin in childhood and adolescence, she adds.
"There is some pretty good evidence that [if] overweight children with higher blood pressure [are] able to get their weight down, their blood pressure is better," Falkner says. "The problem is it's so difficult to accomplish that."
The study included 1,111 socioeconomically diverse Indianapolis children between the ages of 4 and 17. The researchers followed the kids for an average of four and a half years, during which time they measured their blood pressure and BMI about twice a year.
The spike in hypertension and prehypertension risk at the 85th percentile was seen in all age groups, and across boys and girls as well as both black and white children.
Hypertension rates have been rising in children, partly as a result of the obesity epidemic, says Falkner's coauthor, Samuel Gidding, M.D., the head of cardiology at the A.I. DuPont Hospital for Children in Wilmington, Del.
"Over a long period of time, children who are overweight are much more likely to have their high blood pressure persist and develop into much more severe hypertension," Gidding says. "Because kids are so overweight now, it's a problem which is starting to emerge in childhood or adolescence."
The study appears in the November issue of the American Heart Association journal Hypertension, but was released online this week.
Copyright Health Magazine 2011