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Could making your kid play outside help prevent nearsightedness later? A Chinese study published Tuesday suggests it can.

With 30 percent of Americans suffering from myopia, and closer to 40 percent of the population in Asia, it’s a question worth exploring.

People tend to believe that too much “close work,” such as sewing or reading in poor light, might be the culprit. Other studies suggest time outdoors can reduce the rates of myopia. Scientists know that some combination of genetic risks and childhood activities are playing a role.

So a team in China decided to try their own version of studies showing that time spent outside might help.

They found 12 schools willing to take part in the experiment. Half the schools assigned their first-graders to an extra period of outside recess for every day of the school year. Half didn’t. In all, 1,900 first graders, aged 6 and 7, took part in the three-year-long experiment.

“Our study achieved an absolute difference of 9.1 percent in the incidence rate of myopia."

Three years later, nearly 40 percent of the kids who did nothing extra had developed myopia, compared to 30 percent of the kids who got the extra outdoor activity.

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But the effect wasn’t as big as the researchers had hoped.

The team, headed by Dr. Mingguang He of the Zhongshan Ophthalmic Center in Guangzhou, had hoped for something like the 50 percent reduction that researchers in Taiwan got by locking kids outside for as long as 80 minutes a day. Their study, reported in the Journal of the American Medical Association, gave the kids an extra 40 minutes a day for the nine-month school year.

“Our study achieved an absolute difference of 9.1 percent in the incidence rate of myopia, representing a 23 percent relative reduction in incident myopia after three years, which was less than the anticipated reduction,” they wrote.

“However, this is clinically important because small children who develop myopia early are most likely to progress to high myopia, which increases the risk of pathological myopia,” they added. So they could have prevented severe myopia in at least some of the kids.

"Small children who develop myopia early are most likely to progress to high myopia, which increases the risk of pathological myopia."

They also noted that parents did not take their advice to encourage kids to spend more time outdoors playing. It might be if children spend even more time outside, the combination of sunlight and activity will help protect their vision.

The researchers don’t think it’s exercise alone, or time spent away from books and computers alone, that’s making the difference. In a study in which kids were assigned to exercise indoors, rates of myopia were not affected, they said.

It's not clear what it is about being outdoors that might affect vision -- exposure to sunlight, forcing the eye to focus on objects a varying distance away, exercise, or a combination of these factors.

Dr. Michael Repka of Johns Hopkins University School of Medicine said it would be important to see how long the effects of the experiment lasted.

“In some studies evaluating myopia treatment, there has been catch-up in the progression of myopia in the treatment group after the intervention was discontinued,” he wrote in a commentary.

A team that came up with a formula for predicting myopia found that children tend to be slightly farsighted in first grade, and that kids who are less farsighted at this age tend to become myopic later.