Obesity rates among elementary and middle school students in New York City public schools have dropped significantly over the last five years, federal health officials reported Thursday.

In fact, the decreases are the largest declines in childhood obesity reported by any large city in the country, according to the federal Centers for Disease Control and Prevention.

“The trend toward reduced prevalence of obesity is encouraging,” researchers wrote in this week’s issue of the Morbidity and Mortality Weekly Report.

Dramatic increases in obesity among Americans, especially among children, has generated intense concern. Obesity increases the risk for many health problems, including diabetes, heart disease and cancer.

Obesity rates among New York City public school students in kindergarten through eighth grade—meaning those ages 5 through 14— decreased from 21.9 percent in 2006-2007 to 20.7 percent in 2010-2011, a 5.5 percent drop, according to the report. The biggest decline was among children ages five and six, whose rate dropped from 20.2 percent to 18.2 percent in the same period—nearly a 10 percent decline, according to the report.

The decreases occurred among children of ages and in all socioeconomic and racial and ethnic groups. But the decrease was smallest among blacks and Hispanics. And despite the encouraging trends, one in five children in grades kindergarten through eighth grade were still obese.

“Childhood obesity rates are still high and more needs to be done to reduce them further,” the researchers said.

The report did not analyze the reason for the decline. But researchers noted that between 2003 and 2009, New York City implemented a number of measures aimed at fighting childhood obesity, including routinely measuring body mass index (BMI) and taking steps to improve school lunches and nutrition at day care centers and increase physical activity time, such as by limited time sitting in front of computer and video screens.

“A causal relationship cannot be inferred between BMI and fitness interventions implemented by New York City in schools and the decrease in prevalence of child obesity described in this report,” the report states.

“The larger decreases in obesity prevalence among children aged 5-6 years suggest that changes in the preschool or home environment might have been particularly important. The smaller reductions among older children might indicate that changes in school-based nutrition and physical activity programs also helped reduce the prevalence of obesity.”