Washington, Nov 3 (IANS): American children are at greater risk of becoming overweight or obese during summer vacation than during the school year, a new study has found.

Paul von Hippel of the University of Texas, Austin and colleagues followed more than 18,000 children from the start of kindergarten in 2010 through the end of second grade in 2013, Xinhua news agency reported.

Their results, published in the journal Obesity on Wednesday showed that the prevalence of obesity increased from 8.9 per cent to 11.5 per cent, and the prevalence of overweight children increased from 23.3 per cent to 28.7 per cent during the study period.

All of these increases occurred during the two summer vacations, it said. During the three school years, overweight prevalence did not change while obesity prevalence slightly declined.

"Educators have long worried that summer break leads to knowledge loss, and now we know that it is also a time of excessive weight gain for our youngest school children," von Hippel said in a statement.

"Our findings raise questions for parents and policymakers about how to help children adopt healthy behaviours during the long summer vacation to stop unhealthy weight gain. Our results also suggest that we cannot reverse the obesity epidemic if we focus only on what children are doing and eating while they are in school."

Amanda Staiano, spokesperson for the Obesity society, a non-profit US scientific organisation dedicated to the study of obesity, said the next step is for parents and educators to work together to shape out-of-school behaviours.

"Parents can take some simple steps to help their children like sticking to a school-year sleep schedule and reducing screen time," Staiano suggested.