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On Wednesday, Obama sounded an alarm at a Healthy Kids Fair on the White House lawn. "Believe it or not, medical experts are now warning that for the first time in the history of this nation, we're headed for the next generation being on track to have a shorter lifespan than us," she said.

Dozens of children from area schools joined Obama for exercise and some healthy snacking on zucchini quesadillas and baked apples. At such events, the first lady often sounds like "first mom," as she admonishes kids and their parents to make better choices.

"So when vegetables are on your plate, we don't want to hear the whining. We want you to eat it. Just eat it. All right?" she said.

Beyond the photo-ops, Obama promises to roll up her sleeves and fight for policy changes to promote healthier eating. Those shifts include raising federal school lunch standards — and convincing schools that good eating should extend beyond the cafeteria doors to school snack bars and vending machines.

Jocelyn Frye, director of policy and projects for the first lady, says that for now, vending machines in schools are safe.

"I think the push that we've taken is really just to focus on nutritional standards. I mean, you can have a vending machine with healthy food in it," Frye tells NPR's Michele Norris. "It's more about the standards of the food that you have in the school, and less about the vehicle that you use to disseminate the food."

Some groups in the food industry have expressed trepidation about the first lady's message. Their concern is that the White House is telling Americans what they should and should not eat. But Frye disputes that.

"Nobody is telling people what to eat, or saying you can't eat certain things. It's more that we really want to have a conversation about children's health," Frye says.

But if children start eating less salt and sugar and fat, won't those companies be heading into a less profitable future? "I don't think that's necessarily true," Frye says. "I think there are certainly companies that are exploring all sorts of ways to make foods healthier and to address these concerns about healthy eating and still be profitable in what they do. And we're relying on them to do it."

So how does the White House gauge whether it's making a difference?

"I think to a certain extent, the first lady has already made a difference in the increased awareness," says White House chef Sam Kass. "I think ultimately you know you've made a difference when you see health outcomes of kids improving — more kids reaching adulthood at a healthy weight, less childhood diabetes. Ultimately, that's when you've really had a major impact."