CHICAGO — The average cholesterol levels in U.S. children and adolescents have decreased during the past two decades, according to a study published in the Aug. 8 Journal of the American Medical Association.

Despite the improvement, almost 1 in 10 youths still have elevated cholesterol, and obesity rates have not declined.

The study, led in part by Brian K. Kit of the Centers for Disease Control and Prevention, looked at more than 16,000 youth ages 6 to 19 who participated in a national Health and Nutrition Examination Survey during three time periods between 1988 to 1994, 1999 and 2002 and 2007 to 2010.

“Between 1988-1994 and 2007-2010, a favorable trend in serum lipid concentrations was observed among youths in the United States, but adverse lipid profiles continue to be observed among youths,” the researchers said. “For example, in 2007-2010, slightly more than 20% of children aged 9 to 11 years had either a low HDL-C or high non-HDL-C concentration, which according to the most recent cardiovascular health guidelines for children and adolescents indicates a need for additional clinical evaluation.”

Some believe the removal of trans fats from many processed foods has contributed to the lower cholesterol levels in youths.

“Further research is needed to investigate the hypothesized contributors to change in childhood and adolescent lipid levels,” said Sarah D. de Ferranti of the Harvard Medical School and Boston Children’s Hospital. “Better understanding is needed about dietary trends and physical activity during childhood.”