Consumption | Klick and Helland write, “First, an increase of one serving per day is a large change in consumption. In fact, it is more than four times the average change in consumption observed in the data.” In stating this, they confuse a secular trends analysis (i.e., changes in a population) with a cohort design such as ours that examines individuals over time. It is true that average consumption of sugar-sweetened beverages among youth in our cohort increased by only a few ounces during the two-year study. This average includes many youth with more substantial increases and many with more substantial decreases. A better measure of this variation is the standard deviation, which was 1.14 servings of sugar-sweetened beverages per day. It is typical and appropriate to examine the effect of a one-standard-deviation change upon outcomes, as we did.
In any event, secular trends have little bearing on the understanding of how changes in individual consumption may affect body weight. By their argument, one could not, for example, make any inferences about how changing cigarette smoking by one pack a day affects lung cancer risk if, during the observation interval, overall cigarette smoking changed by less than that amount. In actuality, a change in one serving of sugar-sweetened beverages per day is not only well within the observed variation in our cohort, but also feasible from a public health perspective.
Excluding incidents of obesity | Klick and Helland write, “Perhaps more troubling, the Ludwig results themselves appear to either purposely or negligently ignore an important part of the data: almost as many children transitioned out of obesity from the start of the study to the follow-up period (35) as entered obesity (37)…. More generally, excluding the already-obese children (more than a quarter of the entire sample) when analyzing the determinants of obesity demands some explanation.” In noting this, they overlook a basic concept in statistics, that analysis of incidence must exclude any individual who has already developed the condition of interest. For the same reason, an analysis of cigarette smoking and incident lung cancer must exclude all existing cases of lung cancer at the start of the observation period. Thus, our analysis of obesity incidence excludes these initial cases. Of note, all 548 children (without exclusion for obesity status) were included in our analysis of change in body mass index (BMI), which also showed a statistically significant, positive association with increasing sugar-sweetened beverage consumption.
Sodas and obesity | Klick and Helland write, “Another problem … is the authors’ failure to recognize that the consumption of sugar-sweetened beverages explains a very tiny fraction of the BMI increase observed in their study.” In fact, we have never argued that sugar-sweetened beverage consumption alone explains most of the obesity epidemic. Even so, an increase in BMI of 0.24 units with an increase in one serving per day has major potential significance from a population perspective. Excessive sodium intake is not the only cause of stroke, but that does not provide justification to ignore the public health significance of salt.
Ad hominem | Finally, Klick and Helland write, “While their presentation [pertaining to use of the one-serving-per-day unit] is not wrong, it is certainly a more provocative way to frame the results — something that would be expected from activists, but not from scholarly researchers.” For reasons indicated above, we believe that our choice of units is scientifically appropriate. Furthermore, we have no conflicts of interest related to food or beverage companies, and understand that this is not the case for Klick and Helland who received at least indirect support from the American Beverage Association.
Klick and Helland assert “the trivial importance of soda consumption in the growing obesity epidemic.” This belief has no foundation in science.
