Weighing in on a recent New York Times Magazine article about the “obesity virus” hypothesis, Yale public health professor David Katz wrote a letter complaining that the article overlooked the “obvious” causes of obesity, claiming that “[o]besity rates have skyrocketed during the past half-century in lock step with increasing portion sizes, fast food, processed food.” In reality, however, the only thing that’s obvious is Katz’s single-minded willingness to blame American obesity ills on food and nothing else. This is the same academic, remember, who could not back up the claim that life expectancies will shorten due to obesity, and who urged the medical profession to paper over the serious and ongoing internal debate over controversies like the Body Mass Index and fitness versus fatness.

Speaking of fitness, it’s also obvious that physical activity rates have dropped, especially among the young. Among children, the weight of evidence indicates that caloric intake among kids hasn’t changed, but physical activity levels have plummeted. As researchers published in the Lancet medical journal in 2005:

These results suggest that habitual activity plays an important role in weight gain, with no parallel evidence that energy intake had a similar role … the drastic decline in habitual activity during adolescence might be a major factor in the doubling of the rate of obesity development in the USA in the past two decades, since no concomitant increase in energy intake was apparent.