Next thing you know, we'll all be eating celery

CHICAGO (Reuters) – U.S. researchers estimate that an 18 percent tax on pizza and soda can push down U.S. adults' calorie intake enough to lower their average weight by 5 pounds (2 kg) per year.

With two-thirds of Americans either overweight or obese, policymakers are increasingly looking at taxing as a way to address obesity on a population level.

link: Tax soda, pizza to cut obesity, researchers say - Yahoo! News

And as soon as they figure out some theory of second-hand calories they can not only tax pizza, they can make you stand outside to eat it.