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 per year.

The researchers suggested taxing could be used as a weapon in the fight against obesity.  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.

California and Philadelphia have introduced legislation to tax soft drinks to try to limit consumption.