From 2003 to 2011, obesity rates for poor children dropped in New York to around 16 percent. But they rose in Los Angeles and ended at about 20 percent.
The researchers focused on children ages 3 and 4 enrolled in a government program that provides food and other services to women and their young children.
The Centers for Disease Control and Prevention released the study Thursday.
The authors noted that the Los Angeles program has many more Mexican-American kids. Obesity is more common in Mexican-American boys than in white or black kids.
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