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Berkeley put a tiny tax on soda. Consumption plummeted by 21 percent.

Soda consumption is on the decline in Berkeley — the first city in the US to get a soda tax.

It’s increasingly clear that soda is one of the most worrisome food products on the shelf — it’s a major source of added sugar in the American diet and it’s consumed at perilous volumes that are contributing to obesity and diabetes. So it makes sense that, in recent years, health officials have been experimenting with taxes to try to curb the guzzling.

In March 2015, Berkeley, California, became the first US jurisdiction to implement a 1 cent per ounce tax on soda. Other US cities have tried to pass similar measures, too, but in vain. Only Philadelphia has managed to overcome the stiff opposition of politicians, soda companies, and other constituents to enact a 1.5 cent levy per ounce on both sugary and diet drinks this past June.

Now, research is mounting that suggests these taxes do seem to work.

Before we get into the findings, let’s be clear that this research is very preliminary. Most taxes haven’t been around long enough to get a clear sense of how they impact purchases and, importantly, health in the long term.

But researchers are trying to learn what they can. In the latest study, released Tuesday in the American Journal of Public Health, a group at the University of California, Berkeley, looked at the extent to which low-income Berkeley residents cut down on sugary drinks four months after the implementation of the penny-per-ounce tax on sugary sodas and juices, energy drinks, and syrups that go into sugary drinks at cafes (think Starbucks Frappuccinos). They also compared the trend in Berkeley to Oakland and San Francisco — two neighboring cities that don’t have soda taxes.

The findings are pretty striking: While the consumption of sugary drinks decreased by 21 percent in Berkeley, it increased by 4 percent in the other cities. They also discovered a massive uptick (63 percent) in water consumption in Berkeley during the study period. So low-income Berkeley residents were drinking less soda and more water — a very healthy change.

From Mexico to Hungary to Berkeley, California — soda taxes seem to work

These conclusions echo the research from other parts of the world. Several countries have introduced soda taxes, including the UK, France, Hungary, Chile, and Mexico. After Mexico enacted its soda tax, public health researchers found that every socioeconomic group drank fewer sugary drinks. The biggest reduction (17 percent) was among low-income households — which also happen to be the groups most affected by obesity. Hungary has seen similar success with its "junk food tax" on unhealthy food products including soda.

University of North Carolina researcher Barry Popkin, who studies soda taxes and other nutrition policies, said, "The big picture is that, thus far, particularly among low-income populations, the sugar-sweetened beverage taxes do work — they do reduce consumption of sugary beverages."

Popkin says he anticipates even steeper drops in soda consumption in Berkeley over the longer term based on trends he’s seen in Mexico. As well, cities poorer than Berkeley — a relatively wealthy area, which had lower soda consumption relative to the rest of America to begin with — will see even greater gains. These ideas will be tested out in Philadelphia, the first major US city to get a soda tax, where per capita income is only $21,000.

But the really urgent question is whether these taxes will make a dent in obesity over the longer term. There's also the question of how much public-health campaigns that often come with the introduction of soda taxes contribute to getting people off the sugary stuff.

It’s still too early to tell and we need more and better data.

"It will be interesting to see how sugary drinks tax plays out in other places," said Jennifer Falbe, lead author on the Berkeley study. For now, she added, "The Berkeley results are encouraging — that a sugary drink tax may be one of the many tools that can be used in public health to address obesity and diabetes."