CHICAGO — American children are getting fatter at an alarming rate, with the percentage of significantly overweight black and Hispanic youngsters more than doubling over 12 years and climbing 50 percent among whites, a study shows.

By 1998, nearly 22 percent of black children ages 4 to 12 were overweight, as were 22 percent of Hispanic youngsters and 12 percent of whites, according to researchers who analyzed data from a national survey.

In 1986, the same survey showed that about 8 percent of black children, 10 percent of Hispanic youngsters and 8 percent of whites were significantly overweight.

"Prior studies show it took 30 years for the overweight prevalence to double in American children," said Dr. Richard Strauss, a pediatrician at the University of Medicine and Dentistry of the New Jersey-Robert Wood Johnson Medical School. This study should be "a call to action," said Strauss, who conducted the research with Harold Pollack of the University of Michigan.

Among the reasons given for the increase: Children are spending much more time watching television, using computers and playing video games, and busy parents are relying more on fast food to feed their families.

Also, black and Hispanic youngsters are more likely to live in poor neighborhoods where outdoor exercise may be unsafe and where the quickest, easiest foods may not be the most nutritious, Strauss said.

The study was based on data from the National Longitudinal Survey of Youth, which followed a nationally representative sample of 8,270 youngsters from 1986 to 1998. The findings appear in Wednesday's Journal of the American Medical Association.

Overweight was defined as having a body-mass index higher than 95 percent of youngsters of the same age and sex, based on growth charts from the 1960s to 1980s. By some criteria, that would be considered obese. Body-mass index is a measurement of weight relative to height.

Disturbing trends also were seen in the number of children who had a body-mass index higher than 85 percent of their peers. In 1986, about 20 percent of blacks, Hispanics and whites alike were in that category. By 1998, those figures had risen to about 38 percent of blacks and Hispanics alike and nearly 29 percent of whites.

"These trends carry enormous public health implications because of the known effects of excess body weight on the risk for type 2 diabetes, heart disease and other complications," said Dr. David Ludwig, director of the obesity program at Children's Hospital in Boston.

Dr. Rebecca Unger, a pediatrician and nutrition specialist at Children's Memorial Hospital in Chicago, said small changes in children's diets can make a big difference.

"If we can catch a 3-year-old who's still on a bottle, drinks tons and tons of juice, and goes to McDonald's five times a week, we can stop the bottle, cut out the juice, eat at McDonald's only two times a week — and you will see a tremendous difference in growth pattern," Unger said.