The statistics are grim: Roughly one in six U.S. children are obese, and, at last count, nearly one in three are overweight, putting them at increased risk of health problems ranging from diabetes to being bullied at school.
There is a glimmer of good news. After two decades of dramatic growth, childhood obesity rates are showing signs of leveling off.
Government researchers have even reported slight declines among certain subgroups (such as younger children and girls), leading some experts to speculate that the epidemic may have reached the high-water mark.
But there are also signs that this turnaround applies to some kids more than others.
Obesity generally has been a bigger problem among children from low-income families, and a trickle of recent evidence suggests rates may be falling more slowly, or not at all, in this population.
Federal surveys of predominantly low-income children have not found the same declines among 2- to 5-year-olds seen in more comprehensive national surveys, for instance.
"Certainly, the burden of the obesity epidemic is carried by kids in low-income communities," says Shakira Suglia, Sc.D., an assistant professor of epidemiology at Columbia University's Mailman School of Public Health, in New York City.
A new study appearing in the May issue of the journal Pediatrics offers the latest evidence that less affluent children are faring worse when it comes to obesity. The study, which included a diverse group of nearly 37,000 Massachusetts children under age six, found that between 2004 and 2008 the obesity rate fell by 1.6 and 2.6 percentage points among boys and girls, respectively.
As the researchers expected, however, the falloff was more pronounced among children with non-Medicaid health insurance than among those on Medicaid, the government-funded health plan for low-income families.
"Unfortunately there seems to be some socioeconomic disparity in this decline," says lead researcher Xiaozhong Wen, M.D., a postdoctoral fellow at Harvard Medical School, in Boston.
A nationwide trend?
It's not yet clear whether this pattern is occurring elsewhere. Although the children in the study broadly resemble their peers nationwide (the ones with health insurance, at least), the study was limited to just 14 health centers in and around Boston, all of which are part of the same health-care network.
"We should be cautious about assuming that this trend found in one practice in eastern Massachusetts generalizes to the entire U.S.," says John Cawley, Ph.D., a professor of economics and co-director of the Institute on Health Economics, Health Behaviors and Disparities at Cornell University, in Ithaca, New York.
With that caveat, Wen's study does echo reports from elsewhere in the country.
In a 2010 study in the American Journal of Public Health, for instance, UCLA researchers tracked obesity rates among a representative sample of California teenagers. The overall rate was unchanged from 2001 to 2007, they found, but only because the declines among teens from middle- and high-income families were canceled out by a sharp increase among those living in poverty, especially the boys.
"When we looked at [obesity] rates broken down by income, we saw that for adolescents whose family incomes are below the poverty line, those rates have gone up, and not just slightly, but rather dramatically," says the lead author of that study, Susan Babey, Ph.D., a senior research scientist at the UCLA Center for Health Policy Research, in Los Angeles.
Wen and his colleagues don't draw any firm conclusions from their findings, but Wen has some theories about why the declines in obesity were smaller among children on Medicaid. "The health insurance is a proxy, or indicator, for some underlying... reasons for this disparity," he says. "I think they may be the family environment, how the parents feed the children, how do they control or monitor the child's eating or physical activity."
Compared to more affluent children, Wen says, kids on Medicaid may be less likely to live in neighborhoods where they can play and exercise safely outdoors, and their caretakers are less likely to have access to supermarkets selling fresh, healthy foods.
Babey cites several of the same possible reasons. Lower-income parents who are working full-time or at more than one job may have less time to be physically active and model this sort of healthy behavior for their kids, she says. And when time is tight, she adds, it's easier -- and sometimes cheaper -- to rely on fast-food meals than to buy and prepare fresh food.
No easy answers
A lack of exercise opportunities and access to healthy foods are almost certainly part of the story, Suglia says. She emphasizes, though, that behaviors that contribute to obesity often occur against a backdrop of physical and psychological challenges that can make weight watching an afterthought.
In a recent study, also published in Pediatrics, Suglia found that preschool-age girls in big cities were more likely to be obese if they'd undergone stressful experiences such as witnessing household violence, having a mother who was depressed or abusing alcohol or drugs, or living in a tenuous housing situation.
"When you talk about all these risk factors that we looked at -- in terms of violence, and moving around a lot, and depression -- certainly childhood obesity goes down the importance rank," she says. "We're talking about families that are worried about where they're going to live, or is there a safe place to be. That just makes it harder."
These factors, however, were not associated with higher obesity rates in preschool-age boys, which highlights the difficulty of making sweeping conclusions about obesity and socioeconomic status.
Children from low-income families are hardly a monolithic population, and research suggests the relationship between obesity and family income varies widely by gender, age, ethnicity, and geographical area. In Babey's study of California teens, for instance, boys accounted for almost the entire increase in obesity rates among teens living in poverty.
A 2006 study in the American Journal of Clinical Nutrition that looked at several decades of national data found that obesity was clearly tied to socioeconomic status only among white girls. No consistent relationship was found among Mexican-American children, and black adolescent girls were actually more likely to be obese if they were from affluent families.
Nutrition tips may not be enough
Experts agree that some headway has been made in the fight against childhood obesity. The encouraging signs among preschoolers reported in national surveys and in Wen's study may reflect the widespread attention paid to obesity in recent years, says Kristine Madsen, M.D., an assistant professor of pediatrics at the University of California, San Francisco.
"One of the most important things that's happened in the last decade is a tremendous increase in public awareness of the problem," says Madsen, who specializes in interventions targeting obese kids. "I think environments are changing. Day care centers, I think, are actually more aware than they used to be of some of these issues."
Likewise, Madsen says, school-based campaigns -- such as those urging children and parents to be more active, cut their consumption of sugary drinks, and limit portion sizes -- seem to be getting through, although more work remains to be done.
As part of their ongoing research, she and her colleagues recently found that when lower-income kids cut down their soda consumption, they tended to switch to juice -- which can itself be high in calories -- rather than water. The disparities seen in Wen's study "may reflect parents making efforts, but not always making efforts in the right direction," Madsen says.
Efforts to fight obesity in low-income families will need to take into account the extra challenges these families face, Suglia says. "I think more and more, there's an awareness that you can't just tell someone, 'Eat healthy.'"