For children who are overweight, gaining a few extra pounds comes with sharper increases in blood pressure than those seen in kids of normal weight, a new study suggests.

For most kids, increases in body mass index, or BMI (a measure of a person's weight in proportion to her height), seem to cause an increase in blood pressure, though the exact cause of the link is not known.

But for kids with a BMI in the 85th percentile of kids their age (so 85 percent of their peers have a lower BMI), each 1-percentile increase in BMI brings a blood pressure increase nearly five times greater than the blood pressure increase seen in kids of normal weight. The 85th percentile is considered the threshold for being overweight.

The findings also show if an overweight child loses just a few pounds, the impact on blood pressure can be great, study researcher Wanzhu Tu, an associate professor of medicine at Indiana University, told MyHealthNewsDaily.

Prior to this finding, "one would be likely to overestimate the BMI effects on blood pressure in normal weight children, but underestimate effects in overweight and obese children," Tu said.

Researchers measured the height, weight and blood pressure of 1,113 children, ages 4 to 17 at the study's start, in Indiana. They followed the kids for up to 10 years, and then calculated how much children's blood pressure increased with every increase in BMI.

In children whose BMI placed them at the 85th percentile for kids of their age, researchers observed a 0.5 percent increase in blood pressure with every further 1-percentile increase in their BMI. For kids at or above the 90th percentile BMI, blood pressure increased by 0.7 to 0.8 percent with every 1-percentile increase in BMI, Tu said.

But in kids of a normal weight, each percentile increase in BMI brought only a 0.1-percent increase in blood pressure, he said.

The findings held true for girls and boys, he said, as well as for the diastolic and systolic blood pressure readings. Diastolic blood pressure measures the force of the blood against the inner walls of arteries when the heart contracts, whereas systolic blood pressure measures the force of the blood against the arteries when the heart relaxes.

High blood pressure in children is important to monitor and control, because it sets the stage for developing hypertension later in life, Tu said.

"In the adult population, hypertension is associated with chronic kidney disease and all these other diseases and also increase[s] cardiovascular risk ," he said. "These are the things that make us pay attention to the blood pressure in children."

Tu will next conduct additional research on the relationship between BMI and children who have hypertension.

The study was presented today (Oct. 15) at the American Heart Association's High Blood Pressure Research Scientific Sessions conference.