When it comes to childhood obesity, the raging debate over soda being sold in schools has about as much substance as the time-worn question: How many angels can dance on the head of a pin?

According to a 2002 study, the average kid gets one half of 1 percent of his or her calories from vending machines. While that is admittedly up from one third of 1 percent in the 1970s, it seems that limiting vending machines to water won’t make a big difference.

And the more political energy expended on vending machines, the less there will be left to address the real cause of childhood obesity: physical inactivity.

Former Food and Drug Administration Commissioner Mark McClellan observes, “Actual levels of caloric intake among the young haven’t appreciably changed over the last 20 years.”

Study after study corroborates McClellan’s point.

A study published in the Journal of Clinical Endocrinology & Metabolism pointed out: “It is often assumed that the increase in pediatric obesity has occurred because of an increase in caloric intake. However, the data do not substantiate this.”

Translation: The problem won’t be found in vending machines. It’s in the gym. It’s in the recess yard. And it’s in our neighborhoods, where kids now spend far, far more time with their Xboxes than they do running around outside or biking with friends.

Walking and biking trips by children have dropped more than 60 percent since the late 1970s. A full quarter of American children get no physical activity whatsoever.

So why are we so quick to blame vending machines? Public attitudes have been skillfully manipulated by interest groups, whose greatest concern is that someone, somewhere may be enjoying what they eat and drink.

Anti-soda activists-who also seek extra taxes and warning labels on soft drinks, as well as tobacco-style class-action lawsuits-have an insatiable thirst for regulating our diets. They allege soda makers’ new school distribution policy doesn’t go far enough. They want a complete ban on soda in all schools.

In other words, a young man or woman old enough to carry a gun in Iraq won’t always be able to choose his or her own beverage.

America’s dedicated diet scolds also want diet soda out of schools. Pop may have unfairly drawn the short stick in the obesity blame game. But does any rational person think that replacing a zero-calorie beverage with milk or juice will do anything to prevent weight gain?