Among American 15-year-olds, 15 percent of girls and nearly 14 percent of boys were obese, and 31 percent of girls and 28 percent of boys were more modestly overweight, according to a study led by Inge Lissau, a researcher at the National Institute of Public Health in Copenhagen. See report below; CHICAGO — Teenagers in the United States have higher rates of obesity than those in 14 other industrialized countries, including France and Germany, a study of nearly 30,000 youngsters ages 13 and 15 found. Among American 15-year-olds, 15 percent of girls and nearly 14 percent of boys were obese, and 31 percent of girls and 28 percent of boys were more modestly overweight, according to a study led by Inge Lissau, a researcher at the National Institute of Public Health in Copenhagen. The heaviest countries included Greece, Portugal, Israel, Ireland and Denmark. Weight calculations were based on teens' body-mass index, a height-weight ratio. U.S. teens were more likely than those in other countries to eat fast food, snacks and sugary sodas and were more likely to be driven to school and other activities, contributing to a more sedentary lifestyle, said co-author Mary Overpeck of the U.S. Maternal and Child Health Bureau. "The rest of the world may be catching up, but we're still in first place in a race that, unfortunately, we shouldn't want to be winning," said Dr. David Ludwig, an obesity researcher at Children's Hospital Boston, who was not involved in the study. Dr. Ludwig led a separate study published yesterday that found that nearly one-third of U.S. youngsters eat fast food on any given day. It was in the journal Pediatrics. Lithuania had the lowest obesity rates in the study. Among Lithuanian 15-year-olds, about 2 percent of girls and 0.8 percent of boys were obese, and 8 percent of girls and 5 percent of boys were overweight. That is probably because Lithuania has fewer fast-food restaurants and its teens have less money to buy snacks and fast food, Miss Overpeck said. The findings are based on school questionnaires given to youngsters in the 15 countries in 1997 and 1998. The study was published in the January issue of Archives of Pediatrics & Adolescent Medicine. In some countries, such as Ireland, Portugal and Sweden, 13-year-old girls were more likely than 15-year-old girls to be obese. Among French 15-year-olds, 4 percent of girls and almost 3 percent of boys were obese, and nearly 13 percent of girls and 10 percent of boys were overweight. Among German 15-year-olds, about 5 percent of girls and boys were obese, and nearly 15 percent of girls and 14 percent of boys were overweight. The other countries studied were Austria, Czech Republic, Denmark, Belgium, Finland, Greece, Ireland, Portugal, Slovenia and Sweden. Miss Overpeck said preliminary data from more recent surveys show little if any change in rates among the countries studied. The World Health Organization last year said obesity is no longer mostly an American problem, but is an increasing concern in Europe and other developed nations because people are abandoning traditional dietary habits and adopting more sedentary lifestyles. Countries with some of the heaviest youngsters after the United States, based on data from 15-year-olds, were: •Greece: 5.5 percent of girls were obese and about 16 percent were overweight; nearly 11 percent of boys were obese and almost 29 percent were overweight. c Portugal: nearly 7 percent of girls were obese and almost 21 percent were overweight; about 5 percent of boys were obese and 14 percent were overweight. •Israel: about 6 percent of girls were obese and 16 percent were overweight; nearly 7 percent of boys were obese and 20 percent were overweight.