What is the science behind expanding waistlines?

Shedding pounds is undoubtedly one of the most common New Year's resolutions. Yet many people have already lost their resolve. Dr. Barry Popkin, professor of global nutrition at the University of North Carolina at Chapel Hill and the author of "The World is Fat," highlights the key causes of the obesity epidemic and what can be done about it. Questions and answers have been edited.

Q: You have spent the last 25 years studying trends in food, nutrition and obesity. Over that time, obesity has come to outweigh hunger as the most pressing public health threat, even in developing countries. What is driving this phenomenon?

Clearly, over the last half century we have reduced activity in all modes of life, and it is very hard to bring back much of that activity, which comes from our jobs. However, at the same time we have consumed greater numbers of calories. The major cause of that has been caloric beverages: soft drinks, juices for children, and those beverages - plus alcohol - for adults. Consuming three to six snacks a day - or just constantly eating - is the other major cause.

Q: That all makes sense when looking at the United States, but why are residents of previously impoverished countries becoming overweight?

Within a decade or two, these countries experienced a leap from an 18th- or 19th-century lifestyle to one more similar to ours. They got TV and watch hours a day, got modern transportation technology, refrigerators, rice cookers and all the home technologies we have.

At work, their jobs got very modernized relative to decades earlier. The result was and remains an enormous decline in energy expenditures.

At the same time, their diets changed greatly. They got access to cheap vegetable oils and added an awful lot of this to their dishes, turned to fried foods rather than grilling, baking and steaming, started consuming lots of sugar in their diet.

In China, they went in 1990 from no-overweight to close to a third of adults overweight today.

Q: What can individuals do?

Cut out juices and sugary beverages. Shift to water, tea and coffee with minimal added sugar; shift to reduced-fat milk. Shift snacks to only vegetables and fruits

Q: What about from a public health standpoint?

The most effective first step would be to put a very simple label on foods that are healthy and not label other foods. The second simple option is to tax each added gram of sugar 1 to 2 cents when it is in any beverage. Several countries are effectively doing this.