In a speech to a meeting of the U.S. Conference of Mayors, Mrs. Obama said locally elected leaders are among the first to see what's happening to the people in their communities.
Her remarks, coming on the anniversary of her first year as first lady, marked the beginning of what Mrs. Obama has said will be a major initiative on her part to raise awareness about childhood obesity.
A formal rollout of her program is planned for next month. Mrs. Obama has said she will look to businesses and nonprofits, community and health centers, educators, religious leaders and government to help.
Childhood obesity rates have tripled in the past 30 years, and the latest figures show that about one in three children are overweight. Nearly one in two, or 17 percent, are obese, or dangerously overweight.
"The statistics still never fail to take my breath away," Mrs. Obama told the standing-room only audience.