If you want to wage a war, you have to have an enemy. By almost any measure—scientific reports, documentaries, government announcements, and nearly thirty million Google searches—we have been waging a War on Obesity for many years. It is not going well. The most comprehensive recent data, from the 2012 National Health and Nutrition Examination Survey, found that sixty-eight million Americans qualified as obese, and sixty-five million more were overweight. Smaller but more recent studies show no easing of these numbers, which clearly amount to a public-health crisis.

In most wars, the enemy stays the enemy. In this one, however, our allies and demons keep swapping places. When I was young, fat was the principal nemesis, and any other form of calorie was considered acceptable. My mother, keenly interested in the health of her children, insisted that we put margarine on our Wonder Bread. I am not sure I knew what butter was until I read about it in a Hardy Boys book.

Along with our margarine, we drank skim milk, ate ice milk, […]