Tipping point, inspiration, harbinger. Those were the words that public health advocates are using to describe Philadelphia City Council's vote to advance the first big-city levy on soda.

"The arc of history is bending toward public health," said Jim O'Hara, health promotion director for the Center for Science in the Public Interest, which has been pushing for years for taxes on sugar-sweetened beverages as a way to attack epidemics of obesity and diabetes.

Philadelphia's 1.5-cent-per-ounce tax is half of Mayor Kenney's original proposal, but would include diet drinks - a compromise intended to raise more money even as artificial sweeteners' safety and benefits are still being scrutinized.

"I think a big city moving forward could really be the tipping point," said Eric Rimm, a nutrition and epidemiology professor at Harvard's T.H. Chan School of Public Health. That it would be Philadelphia - the most impoverished of the nation's 10 largest cities - is noteworthy, he said, because obesity tends to increase disparities between high- and low-income populations.

Rimm compared the potential of Philadelphia's action to New York's bitterly contested restrictions on trans fats: "Five years later, 90 percent of the trans fat is out of the food supply." Like most other soda-tax proponents, he was interviewed in anticipation of the vote.

Philadelphia's excise tax, which won key approval by a Council committee Wednesday evening, appeared likely to be adopted next week and could take effect next year if not held up by legal challenges. It would add 30 cents to the cost of a 20-ounce bottle.

Despite multiple proposals, only one other U.S. jurisdiction - Berkeley, Calif. - has passed a beverage tax.

Philadelphia "takes away the industry's argument that Berkeley is a fringe community," said Jim Krieger, executive director of Healthy Food America, an advocacy group. He predicted that the Council vote also would send a message to legislators around the country that soda taxes are politically possible.

"We saw this exactly with tobacco taxes" decades ago, he said.

Berkeley's year-old penny-per-ounce tax on sugar-added drinks passed by public referendum.

Advocates there, as in most attempts around the country - including two by former Mayor Michael Nutter - focused on health benefits.

Kenney, by contrast, pitched his tax largely as a way to raise revenue to fund universal prekindergarten and improve recreation centers. Krieger said that offered a broader rationale.

Critics, including the beverage industry, have argued that evidence is lacking for direct public health benefits.

There is some truth to that, Krieger said, but he added that economic modeling provided stronger evidence for the health benefits of soda taxes than was available when the first indoor smoking bans passed.

The models are based largely on standard economic theory predicting how consumers respond to price, combined with what Krieger called the "crystal-clear scientific evidence" that excess calories in liquid form increase the risks for obesity, heart disease, diabetes, liver disease, and cavities.

His nonprofit advocacy group helped fund research that projected the health impacts of a Philadelphia tax. The analysis published six weeks ago examined Kenney's proposed 3-cent-per-ounce tax only on sugary beverages. But Steven Gortmaker, the health-sociology professor at Harvard who led the analysis, said his team looked at other possibilities.

Gortmaker said that a 2-cent tax would prevent an estimated 1,556 cases of diabetes (a 1-cent tax would prevent 834 cases) within a few years of implementation. Eventually, he said, 24,000 cases of obesity a year would be avoided by a 2-cent tax (12,300 with a 1-cent tax). Over a decade, 500 deaths would be averted by a 2-cent tax (250 by a 1-cent tax), he said.

Most projections of the impact of soda taxes assume that many consumers will be driven to bottled water and diet sodas. Including diet beverages "would lower the health impact" somewhat, Gortmaker said.

Krieger downplayed that difference, saying any kind of beverage tax would "inspire people around the country to follow the example of Philadelphia and will send a clear message to the public about soda and health."

A tax limited to sugary drinks implemented in Mexico two years ago has cut consumption by 12 percent, but the impact on health is still unknown.

Multiple jurisdictions aren't waiting. San Francisco has approved health warnings on labels, with similar proposals being considered in Baltimore and New York state.

Soda taxes are under consideration in four Northern California cities and Boulder, Colo.; the Illinois legislature is looking at a statewide levy to help close a budget gap.

Public attitudes have already shifted. Per capita consumption dropped for the 11th consecutive year in 2015.

But it took years to get to that point, after soda consumption rose along with obesity rates. Health advocates began looking for ways to reverse both trends in the 1990s.

Among the early supporters of a tax was obesity researcher Kelly D. Brownell, now dean of Duke University's Sanford School of Public Policy.

Reached Wednesday night, he called the vote "historic" for three reasons: The tax is 50 percent higher than any other in the United States; it was successfully pitched as a revenue-raiser, providing a guide for other jurisdictions; and it includes diet soda.

He said it was unclear whether including diet would help or hurt. But it will give researchers plenty to study.

215-854-2617@DonSapatkin