WASHINGTON (AP) — Michelle Obama had doubts about making a campaign against childhood obesity one of her signature issues.

“I wondered to myself whether we could really make a difference, because when you take on a problem this big and this complicated, at times it can be a little overwhelming,” she said in a recent speech.

WATCH COVERAGE OF MICHELLE’S CAMPAIGN HERE

The anti-obesity campaign Mrs. Obama calls “Let’s Move!” celebrates its first anniversary Wednesday. Is it making a difference?

In some ways, yes. In others, it’s much too soon to tell.

Advocates who have worked on the issue for a long time say the first lady’s involvement is raising awareness about the potential future of the U.S. as a nation of fat, unhealthy people unless the trend is reversed, and that Mrs. Obama has been doing it in ways that they can’t. “She has been a spark plug,” said Nancy Brown, chief executive of the American Heart Association.

Mrs. Obama has addressed governors, mayors, school groups, food makers and other constituencies, urging them to build more bike paths and playgrounds, to serve healthier school lunches and to make and sell more food that’s better for you.

She has visited schools across the country to see what changes they are making, from planting fruit and vegetable gardens modeled after her own celebrated White House plot to opening salad bars in their lunchrooms. And she’s worked herself into a sweat at exercise clinics with kids, including on the White House South Lawn.

Her year of effort has led to promises of change from beverage makers, food manufacturers and most recently, and perhaps notably, Walmart, the country’s largest retailer, to cut the levels of salt, fat and sugar in their products.

Lasting change will take years of effort, though, and some doubt it will happen at all.

“I’ve been through so many of these enormous announcements by food companies about how they’re going to profoundly change the way they’re doing business and they don’t,’ said food expert and New York University nutrition professor Marion Nestle. “So it remains to be seen.”

Mrs. Obama said when she launched the campaign that it will benefit future generations by helping children born today become adults at a healthy weight. The issue is picking up momentum, she said.

“We are seeing a fundamental shift in our national conversation about how we make and sell food,” the first lady said at an appearance in Washington with Walmart executives for their announcement last month. “That’s something that wasn’t happening just a year ago.”

Walmart promised to reformulate thousands of its store-brand products to reduce sodium, sugar and fat, and push its suppliers to do the same. The company also pledged to cut fresh fruit and vegetable prices, build stores in areas without grocers and develop a logo for products that meet its health criteria. Walmart’s grocery business accounts for about 15 percent of the U.S. grocery industry.

“All this will take some time,” Leslie Dach, an executive vice president at Walmart, said in an interview. He said Walmart worked with Mrs. Obama’s office for about a year to win her approval.

A new child nutrition law aims to make all school food more nutritious by letting Washington decide what kinds of foods may be sold during the school day, including in vending machines and at fundraisers. The law also increases by 6 cents the amount of money the government reimburses schools for providing free lunches, although some advocates say that’s hardly enough.

Agriculture Secretary Tom Vilsack recently unveiled guidelines for putting the changes into place. In the case of sodium, the amount in meals would be gradually reduced over 10 years, with an eventual goal of cutting levels by more than half.

Mrs. Obama called for “clear, consistent” nutrition labels on the front of packaged foods to help harried shoppers make informed choices. The Grocery Manufacturers Association, which represents large food and beverage companies, is working on a label it hopes to begin rolling out later this year. The Food and Drug Administration is also exploring “front-of-pack” labeling.

The beverage industry has started putting labels with calorie counts on the front of its bottles, cans and packages of soft drinks, juices, teas and waters. The goal is to have a label on all non-alcoholic drinks by next year, said Kevin Keane, a senior vice president of the American Beverage Association. The group is following up on the commitment it made when Mrs. Obama launched “Let’s Move!” last February.

Keane did not say how much companies are spending, except that it’s “a great deal.”

The new health care law also requires restaurants with 20 or more locations operating under the same name to list nutrition and calorie information on their menus or menu boards. FDA guidance on that is due soon, said Sue Hensley, a spokeswoman for the National Restaurant Association.

Copyright 2011 The Associated Press.