(HealthDay)—As the school year ends, many children feel they're gaining two months of freedom. But new research suggests many will gain something else: unwanted weight.
Between June and August, many U.S. kids pack on excess pounds, particularly if they're overweight to begin with, according to a Harvard-led review of previous research.
"The majority of the studies we looked at demonstrated that, for at least some of the children, there was a risk for accelerated weight gain during the summer months," study lead author Rebecca Franckle said. "And the risk was particularly high among those who were already overweight, and also among blacks and Hispanics."
The researchers can't approximate how much weight these school vacationers gain "because we were looking at different study designs and different age groups and, of course, we expect children to gain weight as they grow," said Franckle, a doctoral student and research assistant in the department of nutrition at the Harvard School of Public Health. "It becomes more a question of weight gain acceleration than just gain itself."
More than one-third of American children and teens are overweight or obese. And, generally speaking, most children spend roughly half the year out of school, the study authors noted.
The study was published online June 12 by the U.S. Centers for Disease Control and Prevention in its journal Preventing Chronic Disease.
The researchers analyzed the findings of seven studies that focused on the question of summer weight gain among children 5 to 17 years old.
The studies, published between 2005 and 2013, recorded body composition measurements before and after each child's summer vacation.
Crunching the numbers, the Harvard team found that six of the seven studies unearthed clear evidence of a faster rate of weight gain among at least some children during the summer months compared to the school year.
This dynamic was particularly notable among children who had already reached adolescence; among black and Hispanic kids; and among those who had struggled with excess weight during the prior academic year.
"The seasonal pattern is certainly striking," said Franckle. "Now, the important question is, why is this summer acceleration happening?"
There is no single answer, she said. One explanation is that school environments exert a positive influence that holds back unhealthy gains. If that's the case, she noted, "we need to look at what's happening outside the school setting that's causing this pattern."
The various studies suggest many potential reasons for summer's expanding waistlines. It's possible that children have more opportunities to be sedentary or snack during the summer. Or maybe they have less access to healthy meals during the summer, when in-school meals aren't available.
"Do they have fewer opportunities to be active outside, perhaps because they have fewer safe parks to play in?" Franckle wondered.
"We will definitely need more research to examine these patterns in order to figure out exactly why this is happening," Franckle concluded.
One specialist said the findings highlight a need for youth-oriented summer nutrition and activity programs that tackle obesity risk head-on.
"It is disappointing that progress made during the school year is lost during the summer for many of these kids," said Lona Sandon, an assistant professor of clinical nutrition at the University of Texas Southwestern Medical Center at Dallas.
"This suggests that the lack of the structure of school and perhaps regular meals and scheduled activity during the summer months could be driving the accelerated weight gain," Sandon said.
Prevention is critical, she added. "Once a child or adolescent is overweight or obese, it is challenging to return to a normal weight," she explained.
By identifying times in children's lives that increase their risk for becoming overweight, "then we can identify solutions to help prevent unnecessary weight gain during that time period," Sandon said.
Explore further: Women who gain too much or too little weight during pregnancy at risk for having an overweight child
For more information on childhood obesity, visit the American Heart Association.