This is the VOA Special English Health Report.

Researchers say American children now eat an average of three snacks a day between meals. A study found that those snacks add up to almost one-third of all the daily calories eaten by children. And those extra calories could help explain the rise in overweight children in the United States.

The study was done at the University of North Carolina at Chapel Hill. The researchers studied the diets of 31,000 children ages two to 18 over a 30-year period.

They found that snacking has increased since the 1970s. And what kinds of snacks have increased the most? Salty, high-fat foods like chips.

The study also found greater snacking on cake, cookies and other treats that past generations might have saved for after dinner.

The study is in the journal Health Affairs. Nutrition professor Barry Popkin was the lead investigator. He says parents should limit snacks to one a day for children age six and older. He also advises parents and caregivers to provide healthy snacks like fruits and vegetables.

Professor Popkin says American schools also need to improve their nutrition. For example, schools may have vending machines that offer what many people would consider junk food.

There has been a push for schools to offer more healthful snacks and lunch choices and fewer sugary drinks.

Earlier this month Coca-Cola said it would stop selling sugary drinks in American schools unless parents requested them. Its competitors at Pepsi just announced that they will stop sales of sugared drinks to schools worldwide.

America's top public health officer wants to see more changes like this. Surgeon General Regina Benjamin recently spoke to lawmakers about making healthy foods more available.

REGINA BENJAMIN: "There is a growing consensus that we as a nation need to recreate our communities and our environments where healthy choices are easy choices and affordable choices."

Agriculture Secretary Tom Vilsack -- who has talked about his own childhood struggles with weight -- agrees about the need.

TOM VILSACK: "We need to do a much better job of making sure that what's in those vending machines is very consistent. We think that the time has come for standards."

First lady Michelle Obama is leading a campaign to fight childhood obesity. Public health officials reported in January that 17 percent of American children are severely overweight.

And that's the VOA Special English Health Report, written by Caty Weaver. For more health news, and to comment on our stories, visit us at voaspecialenglish.com. I'm Steve Ember.

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（来源：VOA 编辑：陈丹妮）