It's not the school year when kids pack on the most pounds, says a study in the American Journal of Public Health. It's summertime.

A bunch of kindergartners and first-graders from across the nation were studied recently, and here's what researchers found: Out of 5,380 kids, body mass index (BMI) -- a not-so-accurate measure of body fat -- rose more than twice as fast when kids were on summer break than when they were in school. Overweight kids benefited most by being in school -- they gained weight no faster than other kids during this time -- but during the summer months, they gained more quickly.

What's the deal here? Probably that kids have less access to food while in school. And maybe teachers instruct kids on how to make healthier choices about diet and exercise. What about physical education classes? Yep, these can help. Even though limited -- my kids get only 40 minutes per week -- it may be more than some kids get at home.

The lesson for us all: To recognize that what our kids learn outside the classroom, and especially during the lazy days of summer, has a major effect on childhood obesity and to do something about it. What will you do?