Why are some kids getting fatter, more slowly?

That, in essence, is the question raised by the latest tranche of statistics on obesity. The headline: The rate of increase in childhood obesity slowed by 43 percent over the past decade in the U.S., with declines all across the aboard, although less so among poorer, at-risk families. Researchers at the CDC found that just 8 percent of 2- to 5-year olds surveyed met the clinical definition for obesity, compared to an average of 14 percent in previous studies.

Overall, the number of adults and children who qualified as obese in the study was about the same. But the finding for kids is heartening because it means that, at least if this study is correct, obesity is not an overdetermined social problem, or one that is immune to attack and amelioration. Still, here is the actual conclusion of the study: "Overall, there have been no significant changes in obesity prevalence in youth or adults between 2003-2004 and 2011-2012. Obesity prevalence remains high and thus it is important to continue surveillance."

That's the bad news.

The class gap remains. And I suspect that, as more data is released, what you'll find is that families with better access to the health infrastructure and to health capital in general are able to use those resources to make better, more informed choices about what to eat. That's why aggressive interventions, even those that offend the libertarian sensibilities of some people, might be warranted. If the ability to fight obesity correlates with socioeconomic status, and if obesity is something that's worth fighting against, then we ought to use the resources we've collectively marshaled to fight obesity on interventions that will cause the most improvement to the populations most at risk. In truth, replacing logos is a very limited lesion to the free market, and something that I'm sure the makers of food products will have agreed to prophylactically.

In general, the recession has reduced the disposable income available to populations with the highest rates of obesity. Food, especially fast food, remains cheap. But food prices increased rather dramatically in 2007 and 2008, then plunged dramatically as the global recession hit, and is now back up to between 2.5 percent and 3.5 percent a year, still higher than the average increase from 1991-2006. The lowest quintile of U.S. households spend on average about 11 percent of their total family earnings on food; for the wealthiest quintile, the figure is about 6 percent. So poorer families may have had less money available to buy food, according to the Congressional Research Service.

For households with low disposable income levels where food expenditures are a large share of the budget, rising food prices result in diminished purchasing power and may force difficult budgetary tradeoffs. To help food-deficient households during periods of rising prices, many domestic food assistance programs are linked to price inflation through escalation clauses, in order to retain consumer purchasing power during periods of rising food prices. However, even for households with escalation clauses, a time lag usually occurs between the time the price inflation is measured and the time when the wage or program benefit is adjusted upward to compensate.

A decrease in the growth rate of obesity among 2- to 5-year-olds is very good news because, by 5, a predisposition to obesity seems to be set. This means that, before most kids experience the ability to choose what foods they eat, something (genetics, environment, hormones, stress) has already determined they'll be obese. Still, virtually all of the way that older kids interact with food is changing. Television advertising is changing. Public school cafeteria food is changing. Attitudes and awareness are changing. Restaurant experiences are changing. Science is changing. The study today suggests that the changes, collectively, are having no net effect just yet.