This summer, Mayor Michael Bloomberg proposed banning the sale of large sodas in New York City. This new policy, intended to limit access to products that are widely thought to contribute to obesity, sparked a national conversation about the ban’s effectiveness. The News sat down with Dr. David Katz, a professor of preventive medicine at Yale Medical School and an expert on obesity, to discuss the ban and other ways to fight the obesity epidemic.

Q. You are a doctor in the field of preventive medicine. Can you define “preventive medicine” for our readers?

A. As the name suggests, preventive medicine is about identifying and changing vulnerability so disease is avoided rather than treated whenever possible. The serious chronic diseases that plague our society, such as heart disease, diabetes and cancer, have known risk factors, and long timelines. Fully 80 percent of all chronic disease could be prevented just by avoiding tobacco, eating well and being physically active. Preventive medicine is a field dedicated to turning what we know about disease prevention into what we do at both the individual and societal levels.

Q. You wrote in a 2006 paper published in the Harvard Health Policy Review, “In the United States, obesity is not only an epidemic, but arguably the gravest and most poorly controlled public health threat of our time.” What made you come to that conclusion?

A. The epidemiology showing the relentless advance of obesity prevalence over recent years and decades is perfectly clear. As for the effects, obesity is a major risk factor for the diseases that are, in turn, the leading causes of death in our country: heart disease, cancer, stroke and diabetes. So it’s a pretty straightforward tale.

Q. Why are Americans so susceptible to obesity? Does it have more to do with the food industry or our exercise habits?

A. It’s both. We eat too much of all the wrong foods and do too little. If we had to pick one, diet is the more important factor for weight control per se, although both are comparably important to health. Everything about “modern” living that makes it modern — from highly processed food to labor-saving technology — contributes to the perfect storm of “obesogenic” factors in our society.

Q. Are there ways that we think about obesity that perpetuate the problem?

A. Obesity in the United States is a genuine crisis, taking years from life, and life from years. We don’t take it seriously enough because it is a calamity in slow motion. If a natural disaster caused this much damage, it would rank among the worst of all time. We bog down in debates about whether this is a matter of personal responsibility or public policy, instead of accepting it is both and doing all we can to put an end to the crisis. We also act as if obesity is complex and needs endless research into causes. While it will not be easy to fix, it is simple: we eat too much, and do too little. When the food supply was more wholesome and close to nature and calories not so abundant, and when everybody did physical work every day, obesity was almost nonexistent. This is not rocket science.

Q. What is your take on Mayor Michael Bloomberg’s large soda ban proposal in New York?

A. I am ambivalent. I have not had a soda in 35 years, and consider such beverages chemistry experiments in a cup. Nobody needs them! And the sizes are ridiculous. But people do not like being told what to do, and that strategy can backfire badly. It empowers those who rail against the “nanny state,” and who contend that the government wants to tell us all how to live our lives and what we can have for breakfast. If our proposals inspire strong enough resistance, we can actually go backward instead of forward. So, while I support what Mayor Bloomberg is trying to do, I am not convinced this is the best way to get it done.

Q. Do you think a tax would be more effective than a ban?

A. No one knows, since there has been no direct comparison of the two strategies. In my opinion, we should regulate food marketing to children; provide clear, unambiguous, at-a-glance nutrition information about all products to empower consumers; and provide financial incentives to choose more nutritious foods. I favor these approaches over either a tax or a ban.

Q. Critics say that the soda ban will adversely affect people of lower socioeconomic status because healthier foods are more expensive. How would you respond to this criticism?

A. First, anyone who is thirsty and can’t afford soda is at liberty to drink water. There is no inalienable right to drink sugar-sweetened beverages. And people who can’t afford soda certainly can’t afford the endocrinologist to treat their diabetes. As for more nutritious food being more expensive, it is in part an urban legend. It’s true sometimes, but often the problem is that people don’t know how to identify the more nutritious food that isn’t more expensive.

Q. What are some solutions you propose?

A. We need physical activity to populate our daily routines once again. We need healthful foods in reasonable quantities to constitute our diets once again. We need time for food preparation. We need to sleep better and manage stress so that we have the energy and equanimity to eat well and exercise. But there are problems with [this] “fix everything” platform, no matter how defensible it may be.

First, no one is in charge of everything: food marketing, farm subsidies, sin taxes, school programming, school food, nutrition labeling, work-site wellness, transportation, and so on. And since no one is in charge of everything, calls to fix everything have the potential to lead nowhere, because everyone assumes someone else should get it all done.

And second, even if we can figure out where the buck should stop, fixing everything can be quite overwhelming. And when we feel overwhelmed, we may not even bother to try. That tends to happen at the individual level, where people who don’t feel they can succeed at eating well or being active simply abandon the effort. But it can also play out at the level of policy, where if doing it all is out of the question, the alternative all too often is doing nothing.