A new study co-authored by a University of Iowa researcher suggest that extra sales taxes on soda do nothing to improve people’s health or reduce obesity.

The researchers said that while some older studies suggest taxes on sugar-sweetened beverages will reduce obesity by 20 percent, they rely on household data instead of individual consumption patterns, and they assume that individuals don’t replace the calories in the soda with calories from another source. The new research finds that while soda taxes do correlate to reduced soda consumption, they do not reduce calorie intake.

The study, published in the journal Health Economics, was co-authored by David Frisvold, assistant professor of economics of the Tippie College of Business; Jason Fletcher, of the University of Wisconsin’s LaFollette School of Public Affairs; and Nathan Tefft, of the University of Washington’s School of Public Health.

In their new research, the co-authors conducted two studies. One uses data from the National Health and Nutrition Examination Surveys to estimate effects on reported consumption and caloric intake of soda and other beverages as well as measured height and weight for a nationally representative sample of adults between 1989 and 2006. The evidence demonstrated that large increases in soft drink taxes are unlikely to reduce total caloric intake, as the impact of soft drink taxes on the body mass index is small in magnitude and not statistically significant.

The second study looks at Ohio and Arkansas in the early 1990s, a time when both states substantially increased soda taxes. The co-authors compare weight outcomes in those two states to outcomes in control groups drawn from other states. The apparent effects of a soda tax depend on which control group they used. Additionally, although the tax appears to reduce body mass index and obesity prevalence in Arkansas when compared with states with no tax change, the tax increased body mass index in Ohio, Fletcher says.

“Our results cast serious doubt on the assumptions that proponents of large soda taxes make about the effects on population weight,” the study says. “Given that people substitute other calories when they give up soda, these new results suggest we need fundamental changes to policies that make large soda taxes a key element in the fight to reduce overall obesity rates.”