Preventive Medicine Column

Dr. David L. Katz

The media headlines attached to the major medical news story of the past week might have been: “Shocking but True: nearly 1 in 10 US children UNDER THE AGE OF 5 is obese!”  Had that been the headline, it would indeed have been every bit as true as it is shocking.  And the really shocking part is: that’s the good news.

The headline we did get, in no less august a source than the New York Times, was: “Obesity rate for young children plummets 43% in a decade.”  This would seem to invite the question: have we, in fact, turned the tide?  Is the mission accomplished?

A good place to start generating an answer is, obviously, the headline-spawning study itself, conducted by scientists at the CDC, and reported in JAMA.  The first reality check is the study conclusion in the authors’ own words: “Overall, there have been no significant changes in obesity prevalence in youth or adults between 2003-2004 and 2011-2012.” Dr. David Allison at the University of Alabama publishes a weekly overview of important studies related to obesity, including a feature on discrepancies between what headlines say, and what the corresponding studies actually say.  This one topped that list.

What the investigators actually reported, tracking data over a decade, is, as noted, no overall change in the population rate of obesity.  In the survey sample of just over nine thousand people, representing the population of 300 million, obesity rates were stable in most age groups, including infants under the age of 2.  Rates rose significantly in women over age 60, and fell significantly-although barely so- in children between the ages of 2 and 5.

Even so, a decline of 43% as reported in the New York Times and elsewhere still seems pretty impressive.  But what does 43% mean? You might think it means something like: obesity rates were 60% and are now 17%.  If so, you would be wrong.  The 43% reported is not an absolute percentage; it’s a relative percentage.  I’ll explain the difference.

Imagine a population of exactly 100 children, ages 2 to 5; and imagine that, indeed, 60 of them are obese.  Compare this group to another group the same age a decade later in which only 17 of the kids are obese.  The absolute difference in the obesity prevalence here would be 43%.

Now, imagine instead that in the original group a decade ago, about 15 of the kids were obese; and currently, in a comparable group, about 9 of the kids are obese.  The absolute decline in obesity prevalence is obviously only 6% (i.e., 15%-9%).  But what about the RELATIVE decline in obesity?  That would be 40%.  The formula for it is

[(15%-9%) / 15%] = 40%.  The 6% decline is 40% of the baseline number. Relative percentages are often reported in the medical literature, and routinely reported in the popular press, for the most obvious of reasons: they tend to sound a whole lot more dramatic than the much smaller, absolute numbers.

What were the actual, absolute numbers in this case?  Obesity rates in the 2 to 5 age group fell from 13.9% to 8.4%, an absolute difference of 5.5%.  I trust you can see why the relative change made the headlines.

In setting the record straight, I don’t want to go too far.  A whole lot of attention has been directed at the problem of childhood obesity over the past decade, including the signature efforts of the First Lady.  We recently saw study data indicating that obesity by age 5 is a potent predictor of lifelong weight struggles to follow, implying the converse: weight control prior to age 5 could confer lifelong benefit.  That being the case, any decline in obesity prevalence in this age group is of particular importance.

So, the new study offers some potentially quite encouraging and important news.  But I say “potentially” advisedly.

First, as noted at the start, the “good” news here is that nearly 1 in 10 of our kids under age 5 is obese.  That such a statistic is cause for celebration says more about how bad we let things get, than about how good they are.

Second, looking at the detailed data tables in the article, I see that obesity rates did not fall steadily over the past decade in 2 to 5 years olds; they fell, then rose, then fell again.  This suggests we may not yet have a reliable trend established-and the most recent numbers are just part of a fluctuating baseline.

No, the tide has not yet turned, and we should recall that floods crest long after the precipitation stops.  But maybe- just maybe- the rain has at last stopped falling.




Dr. David L. Katz