Do CDC’s Obesity Stats Tell the Whole Story?

October 18th, 2013 by MorganDowney Leave a reply »

Nanci Hellmich reported in USA Today on a just-released update from the Centers for Disease Control and Prevention their data on the prevalence of adult obesity in the United States. They found that 78 million adults are obese or 34% of the adult population in 2012 and that this has not significantly changed since 2010. But does this the whole picture? Maybe not.

Three studies draw a more complex picture. Razak and colleagues in Canada analyzed surveys from 37 low- and middle-income countries looking at BMI distribution among women. Their study found a “consistent pattern of unequal weight gain across percentiles of the BMI distribution as mean BMI increases, with increased weight gain at high percentiles of the BMI distribution and little change at low percentages.” They conclude, “Mean changes in BMI, or in single parameters such as percent overweight do not capture the divergence in the degree of weight gain occurring between BMI at low and high percentiles. Population weight gain is occurring disproportionately among groups with already high baseline BMI levels. Studies that characterize population change should examine patterns of change across the entire distribution and not just average trends or single parameters.”

In the second study, Sperrin and colleagues looked at rates of adult obesity in England from 1992 to 2010. Like the CDC picture, they found a less steep increase in the rate of obesity in recent years. But they found two different cohorts. They found about 23.5% of men and 33.7% of women were in a hi-BMI subpopulation. The rest appeared to be an obesity “resistant” normal-BMI population. The high-BMI group continued to increase in body weight while the “resistant” group stayed the same.

A 2002 study out of Finland by Kautianinen et al of Finnish adolescents from 1977-1999 found that the BMI distribution changes were not equal. Those in the lower percentiles had little increase while those at the upper end of the BMI scale had significant increases. They even found the 95th percentile increase was more marked than the 85th percentile increase.

The problem with the CDC approach is that they treat obesity like a light switch. One is either above a BMI of 30 or not. The problem is that this approach may make the current numbers look flat, while, in fact, the fat are getting fatter and the ‘obesity-resistant’ population is staying the same. It may look like progress but it isn’t. But of course, we won’t know for sure until the studies are done in the United States. Until then, our public health authorities should avoid indicating that the obesity epidemic is leveling off and creating a false sense that the obesity problem is receding.