First lady Michelle Obama will formally announce her childhood obesity initiative Tuesday at the White House, an effort to reverse an epidemic in which almost one-third of American children are either overweight or obese.

But in making the fight against childhood obesity her signature cause, Obama has chosen to tackle a seemingly intractable problem that has thwarted experts for the past two decades.

Health professionals are optimistic that Obama’s personal appeal and the power of the White House may provide the necessary impetus to change the situation regarding child nutrition in this country, but they say she faces formidable barriers.

The first lady is undeterred and describes childhood obesity as an “imminently solvable” problem. Her ambitious plan is designed to improve the nutritional quality of school meals, get children to exercise more, provide healthier, affordable food to rural areas and the inner city and help people make healthier choices.

Obama is bringing together members of the president’s Cabinet to help, as well as media, sports, entertainment and business leaders, alongside mayors, the medical community, foundations and nonprofit organizations. The White House announced that she’ll be joined Tuesday by NBC correspondent and former NFL player Tiki Barber; Dr. Judith Palfrey, president of the American Academy of Pediatrics; and Will Allen from Milwaukee, who is considered a leader in the effort to bring fresh produce to inner cities and urban areas.

“The importance of the White House really can’t be underestimated, from an inspirational point of view,” said Marion Nestle, a professor at New York University and author of “What to Eat.” “Mrs. Obama’s White House garden is enormously inspiring.

“You need an environment that supports healthier choices, and that’s the challenge. You are fighting a trillion-dollar food industry whose job is to sell more food, not less,” Nestle said.

Whether Obama would actually testify before Congress is not clear. And it’s unlikely she would go head to head with the food industry, whether it’s the Grocery Manufacturers of America or the American Beverage Association, which represents the nonalcoholic beverage industry. Both have pledged to work with her.

“What we’ve heard from Mrs. Obama’s team is they want us to come to the table and do something meaningful,” said Susan Neely, president of the beverage group.

But those who have dealt with the industry in the past don’t trust it. Dr. Kelly Brownell, director of the Rudd Center for Food Policy and Obesity at Yale University, said Americans don’t believe Big Business anymore. “Industry arguments that it can be trusted to act in the public interest are falling on deaf ears.”

Experts believe Obama is likely to be most successful in getting more healthful food into schools along with more time for physical activity because there is already momentum across the country for those changes. But the administration’s 2011 budget increase for school lunch programs is small. Many school food advocates are shocked at how small it is.

According to Ann Cooper, director of nutrition services for Boulder, Colo., school districts, known as the Renegade Lunch Lady because of her efforts to improve the quality of school lunches, the budget would add between 11 cents and 20 cents per child per day — the equivalent, she said, of half an apple.

But Cooper is still very supportive of the first lady’s efforts. “I am so optimistic because someone is caring about the next generation and working to make it better,” she said. “Conversely, it’s a huge uphill battle and struggle because I don’t believe the first lady’s campaign is going to mesh easily with the president’s budget and Agriculture Department priorities.”

What confounds many who have worked so long in the field is the amount of resistance to improving diet and health, even though in the long term it can actually have an impact on the economy and productivity.

Obama understands that, calling “obesity one of the greatest threats to the American economy.” But she also knows making changes will not be easy. To reverse the obesity curve, she said, may take a generation.

By MARIAN BURROS