WEDNESDAY, Nov. 2, 2016 (HealthDay News) -- Summer vacation may bring unexpected weight gain for young children, a new study finds.

Researchers tracked more than 18,000 children from the start of kindergarten through the end of second grade. They found that the kids' rate of overweight increased from 23 percent to 29 percent during that time. The researchers also saw that the rate of obesity rose from 9 percent to 11.5 percent.

All of these increases occurred over the two summer periods during the study -- and not during the school years, the researchers said.

The findings were published in the journal Obesity and released Wednesday at the annual meeting of the Obesity Society, in New Orleans.

"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," said study co-author Paul von Hippel, from the LBJ School of Public Affairs at the University of Texas, Austin.

"Our findings raise questions for parents and policymakers about how to help children adopt healthy behaviors 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," von Hippel said in a society news release.

Obesity Society spokeswoman Amanda Staiano said, "Now that we have solid data pointing to summer vacation as a time for potential weight gain in young children, the next step is to work together to shape out-of-school behaviors."

She suggested that parents help their children stick to a school-year sleep schedule and reduce the amount of time with electronic screens -- TVs, computers, tablets and the like.

"We hope these findings galvanize efforts by parents, educators, public health advocates and officials to make sure that summer does not set back efforts made during the school year to not just teach our children, but to keep them healthy," Staiano said.

More information

The U.S. Centers for Disease Control and Prevention offers tips for keeping children at a healthy weight.