Did obesity in young kids really drop 43%?

A new study was published last week in the Journal of American Medicine using data from a National Health and Nutrition Examination survey taken between 2004 and 2012. They found that obesity in 2 to 5-year old kids declined from 13.9% in 2004 to 8.4% in 2012 (a 43% drop in the obesity rate for this age group). If accurate, this decrease is astonishing.

The study measured obesity in people of all ages. For adults, obesity was defined as a BMI greater than or equal to 30. For kids, obesity was defined as "weight for length at or above the 95th percentile of the sex-specific Centers for Disease Control and Prevention (CDC) growth charts" i.e. age, height, and sex adjusted weights.

The New York Times trumpeted the result with the following article headline: "Obesity Rate for Young Children Plummets 43% in a Decade."

After reading the original paper, my reaction was not so fast. One should not get overly excited about a single data point. It is important to put this particular finding in its proper context. First, the authors found that obesity in adults (ages greater or equal to 20) actually grew slightly from 32.2% (2004) to 34.9% (2012). Second, the rate of obesity in all kids (ages 2 to 19) barely budged from 17.1% (2004) to 16.9% (2012); obesity decreased significantly only in the 2-5 age group. Third and most importantly, there is a fair amount of variability in the 2-5 data if you look at the time progression and error bars (95% confidence interval in parentheses):

1. 2004: 13.9% (10.8-17.6%)
2. 2006: 10.7% (8.5-13.3%)
3. 2008: 10.1% (7.8-12.9%)
4. 2010: 12.1% (9.9-14.8%)
5. 2012: 8.4% (5.9-11.6%)

The trend is clearly downward but there is a noticeable fluctuation up in 2010 (12.1%), and the error bars suggest that the 2012 data point could be 10% or higher. The 6-11 age group also showed a (modest) decrease from 18.8% (2004) to 17.7% (2012) as presumably some 2-5 year-olds aged into this older bracket. Obesity in the 12-19 age group actually increased from 17.4% to 20.5%.

As noted in the New York Times piece, this drop is exciting because "2- to 5-year-olds are perhaps the most significant age group, as it is in those years that obesity — and all the disease risk that comes with it — becomes established, and it is later very difficult to shake." Indeed some estimate that if you are obese as a school-age kid, it increases your risk of adult obesity by a factor of 2 or more.

One possible explanation for this potential decline are education programs raising awareness about dietary choices. For example, the First Lady Michelle Obama has led a public campaign (Let's Move initiative) to change the eating and exercise habits of kids. This increased awareness may have led to the purchasing of healthier, lower calorie foods.

Although the jury is still out until there are more data points (i.e. we will see what happens in another 2 years), the apparent obesity decrease in this important age group is very encouraging. Let's hope that it holds up and the gains persist in the future as the kids become older.

Figure 1. Hopefully this is the start of a trend of decreasing obesity in young kids.