In the science leaflets of Le Monde this weekend, the highlighted number was 64 (along a short tribune by Cédric Villani reminding the readers of the 3% error rate in the opinion polls inundating the news these days…) Sixty-four (64 kcal/day) for the number of calories a U.S. youth should reduce his/her daily intake to prevent obesity. This stroke me as ridiculously small and I thus went to check further.
First, 64 calories is small in that a (boiled) egg brings over 64 calories, an apple reaches 75 calories (not that there is any call for reducing apple consumption!), a plain bagel or a muffin is 190 calories, a pint of beer amounts to 200 calories, the smallest French fries portion at MacDonald is three times this amount, &tc. And, in terms of exercising, running (ok, faster than 7 miles per hour!) or active rock-climbing for a mere 4 minutes burns more than 64 calories. (If I can put some trust in this calculator!)
The reference paper containing this figure is “Reaching the Healthy People Goals for Reducing Childhood Obesity” by Wang, Orleans, and Gortmaker, from Columbia, Harvard, and Princeton, resp., published in the American Journal of Preventive Medicine. (Incidentally, an Elsevier journal, making the open access to the paper the most surprising!) The first thing in the paper is that the 64 figure is the average reduction to stop the nation’s increase in the weight of U.S. youth, not to eliminate obesity. The second one I read from the small prints is “obesity was defıned as having a BMI greater than or equal to the age- and gender-specifıc 95th percentile on the CDC growth charts“. This means, according to the above chart, a weight above 95 kg or 210 pounds at the age of 20… I first thought this chart was based on the current population, which would have been non-sensical as the proportion would have remained at 5%! It seems to be connected to the 2000 figures, as the paper only links with the CDC website. However, this definition of obesity is problematic, as it sems to be non-stationary, i.e. to evolve with the population and hence under-estimate the problem (unless one uses a 1970 or 1950 reference). Third, and connected to the above, the goal is not expressed in terms of an ideal CDC chart but in terms of a percentage of obesity. For instance, to reach the figure of 5% of obeses, a reduction of 177 calories per day is necessary among teenagers (still less than a fries portion!), reaching 286 for non-hispanic black teenagers.
The (very limited) statistical analysis is summarized by “Average annual changes in obesity prevalence and body weight in the U.S. youth population were estimated by fıtting regression models to the compiled NHANES1971–2008 data (…) controlling for age, gender, and race/ethnicity (..) using SUDAAN, version 10.0.1”. The detail of which regressors were used does not appear in the paper, as far as I read, although the note at the bottom of Table 1 seems to hint at the calendar time as being the only regressor. This regression is only used to determine a year in the past corresponding to the goals set by the federal government (Healthy People 2020 and… Healthy People 2010, although the paper just appeared. In 2012, indeed!) of an “acceptable” percentage of obesity in the population. The “daily energy requirement [corresponding to those target weights] was estimated using published equations of basal metabolic rate and activity-related energy expenditure”, nothing more sophisticated than that…
There are similarly surprising figures in the paper: eliminating sweetened beverages from schools would only mean a 12 kcal/day gain, introducing after school physical education another mere 25 kcal/day… Overall, I am slightly puzzled by the amount of publicity this paper received, considering its limited methodological input, from Le Monde to Science Daily (which confuses the published figure with the “64-calorie difference between consumption and expenditure“).