The rise in obesity could lead to 7.8 million more cases of diabetes, 6.8 million cases of heart disease and stroke, and more than half a million extra cancer cases in the United States—all of which would balloon health care costs by $66 billion a year, the report says.

And our friends across the pond are following our lead: The number of obese people in the United Kingdom will increase from 15 million to 26 million, Lancet reports. But being obese isn't just a Western problem—researchers say people almost everywhere are putting on weight.

In 1980, about 5 percent of men and 8 percent of women worldwide were obese. By 2008, the rates were nearly 10 percent for men and 14 percent for women, meaning 205 million men and 297 million women were obese. An additional 1.5 billion adults were overweight.

Still, Wang urges, the predictions don't have to come true. Even the slightest weight reductions can have a dramatic (and positive) effect.

For instance, the researchers calculate that just a 1 percent reduction in body mass index at the population level would prevent as many as 2.4 million cases of diabetes and 1.7 million cases of heart disease and stroke.