Editor's note: David Frum is a contributing editor at Newsweek and The Daily Beast and a CNN contributor. He is the author of seven books, including a new novel, "Patriots."
Washington (CNN) -- Nobody seems to have a positive word for Mayor Michael Bloomberg's proposal to ban oversized servings of sugary drinks in New York's food-service establishments.
The mayor has been decried as a nanny. He has been accused of selective enforcement. (A Starbucks 20 ounce drink can have more than 500 calories, but will be exempt from the ban because it contains more than 50% milk.) The beverage industry complains that solutions to the obesity problem ought to be more "comprehensive." One important conservative magazine called the mayor's actions a form of "fascism."
So let's defy the trend here and say: Good for Bloomberg. Obesity is America's most important public health problem, and the mayor has led the way against it. This latest idea may or may not yield results. But it is already raising awareness. Even if it fails to become law, it ought to prod the beverage industry into acting as more responsible corporate citizens.
Sugary drinks now provide 7% of the calories in the American diet, the largest single national source of calories. Teen boys average more than a quart of sugary soda per day. Even adults who say they are trying to lose weight still drink more two 12-ounces cans per day, on average.
There is little doubt about the serious health effects of sugary soda. Just one soda a day doubles a woman's risk of diabetes, according to the Harvard Journals of Public Health. Two sodas raises her risk of heart disease by 40%.
Americans drink more soda for the very simple reason: it's getting cheaper. The inflation-adjusted price of soda has declined by an estimated 48% over the past 20 years. Improvements in packaging account for much of this price decline. It costs barely anything more to manufacture a 64-ounce "double gulp" container than to produce the former standard sizes.
Some object that the mayor's proposal to restrict serving sizes will restrict liberty. But the liberty restricted is not the liberty of the soda-drinker. If they wish, soda drinkers can buy a 2-liter bottle of soda at the grocery for about $1.70 and pour as much of it down their throats as they wish. The liberty that is being restricted is the liberty of the soda seller to manipulate known human weaknesses to the seller's advantage and the buyer's detriment.
Human beings are not reasoning machines. We are animals who have inherited certain propensities not always well-adapted to modern urban life. We evolved in conditions of food scarcity. Our bodies have adapted to store food energy against famine; our subrational minds crave sweetness. The sugary beverage industry has invested massively to understand better how to use our very human natures against us.
The ever-increasing size of the standard soda serving has changed our understanding of what is and what is not an appropriate amount of sugar to consume at a sitting. The beverage industry works on Americans before they have learned to read; even before they have learned to speak. In 2010, children and teens were exposed to twice as many full-calorie soda TV ads as they were in 2008, according to a study by the Yale Rudd Center for Food Policy and Obesity.
The beverage industry is correct that a soda crackdown alone will not on its own address the obesity problem. Americans spend twice as much time in cars as in the 1970s and average more than 26 hours per week of sedentary entertainment. Some 80% of Americans do no physical exercise at all. As I've written before in this space:
"The campaign against obesity will have to look a lot less like the campaign against smoking (which involves just one decision, to smoke or not to smoke) and much more like the generation-long campaign against highway fatalities, which required the redesign of cars, the redesign of highways, and changes in personal behavior like seat-belt use and drunk driving."
But if a restraint on soda serving size will not do everything, it may still do something. Or possibly not. The idea may fail. The idea is an experiment, and most experiments fail. We learn from failure how to design a better effort next time. And when we do at least succeed in this difficult struggle for public health, we will all owe New York's visionary mayor our thanks for leading the way.
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The opinions expressed in this commentary are solely those of David Frum.