Katie Couric has reported countless stories on the rise of obesity among Americans, particularly children, in her 35 years on TV, she said, but she never saw attention paid to why Americans were gaining weight.

"Once I started looking into it and hearing things like this generation of children is the first expected to live shorter life spans than their parents … I said, 'You know, we have to do something about this,'" Couric said today on " Good Morning America."

WATCH: Katie Couric Voices 'Fed Up' Film on Obesity Epidemic

What Couric did was executive produce and narrate a film, "Fed Up," that takes an up-close look at sugar in the American diet. The film bills itself as the "film the food industry doesn't want you to see" because of its no-holds-barred look at what the hidden sugars in foods do to your body.

"It's incredibly alarming," said Couric, who co-produced the documentary with Laurie David, the Academy Award-winning producer of "An Inconvenient Truth."

READ MORE: Fed Up' Film Examines Food Industry

"You're supposed to eat six to nine teaspoons a day - or that's the safe threshold, according to the American Heart Association, but there's hidden sugar in everything," Couric said. "Of the 60,000 products in the grocery store, 80 percent have added sugar."

The film, which had its premiere at this year's Sundance Film Festival, opens in theaters May 9. It follows the lives of a group of children over the course of two years as they diet and exercise in an effort to become healthier.

"Fed Up" also includes interviews with experts, doctors and newsmakers, and examines the relationship between the food industry and the U.S. government.

"The USDA promotes agriculture and establishes dietary guidelines, which is an inherent conflict of interest," Couric said on "GMA." "Fifty percent of school districts serve junk food for lunch, fast food for lunch. The kids are getting terrible choices."

Couric is calling on people not just to watch the film when it opens Friday, but to take action in the form of a "Fed Up challenge" that begins next Monday, May 12.

WATCH: Fed Up' Tackles Food Industry, Health

"We're going to try to get as many people as possible to give up added sugar for 10 days," Couric said. "We're trying to say you can't have added sugar or artificial sweeteners, which really have the same impact on your brain as added sugar."

Those who want to do the challenge can click HERE for more information. Gone on the 10-day challenge are all artificial sugars and sugar substitutes, all sweetened beverages, foods that have added sugars, including honey, molasses and agave, and the foods that may have hidden sugars, like sweetened yogurts, canned foods, spaghetti sauce and ketchup.

"You have to kind of really wean yourself from some of these foods," Couric said. "Just try it and see what it's like."

Couric is practicing what she preaches and starting the challenge on Monday, herself.

"Game on," she said. "We think it's a great way to start out and start a healthier lifestyle."