29.5% — Percentage of population 20 and over who are obese

America’s spare tire keeps growing. This year, 29.5% of the U.S. population over 20 was obese, according to the Centers for Disease Control and Prevention, up from 21.8% in 2000. That’s creating not just a big health problem for the country, but a big economics one.

Economists John Cawley at Cornell University and Chad Meyerhoefer at the Lehigh University have estimated that annual cost of treating obesity in the U.S. accounts for as 16.5% of national spending on health care.

Higher health costs are just the start. Research at Shell Oil Co., for example, found that obese employees missed 3.7 days of work compared to normal-weight coworkers. Shanjun Li of Cornell, Yanyan Liu of the World Bank and Junjie Zhang the University of California San Diego found that higher obesity rates increase people’s predilection for bigger, less fuel efficient cars, increasing the country’s energy tab.

If current trends continue, nearly half of the U.S. adults will be obese by 2020, calculate Harvard University economists David Cutler and Susan Stewart with University of Michigan medical researcher Allison Rosen. That would overrun the positive health effects of the continued decline in smoking.

But even though obesity is a growing problem, it doesn’t seem to be registering with many Americans. In a Gallup poll conducted in November, 29% of respondents said they weighed more than 200 pounds. That compared to 15% in a similar poll from 1990. But the share of people who considered themselves very overweight or somewhat overweight fell from 48% in 1990 to 39% this year.