Philadelphia City Council's decision to include diet drinks in a proposed beverage tax took public-health advocates by surprise and would move the city into unstudied territory.
Advocates of a soda tax for health reasons say they have never pushed to include artificially sweetened beverages, because the scientific evidence linking sugar with obesity and diabetes is so much stronger.
Far from being unhappy about the development, however, some see the move as an unexpected gift.
Diet beverages "are filled with artificial sweeteners and chemicals. Do we really want people to drink these?" said nutritionist Mary Story, a professor at Duke University's Global Health Institute. She did say that researchers had not found any proof of harm caused by the products.
Diet beverages were included in a last-minute amendment that led to Council's preliminary approval of a 1.5-cent-per-ounce beverage tax Wednesday night. A final vote on the compromise measure - Mayor Kenney had proposed 3 cents an ounce, solely on drinks with added sugar - is expected next week.
If distributors passed the full amount on to consumers, the price of a six-pack of 16-ounce bottles would go up by $1.44. That's 50 percent more than the only similar excise on soft drinks in the United States, a penny-per-ounce levy passed by referendum two years ago in Berkeley, Calif.
Kenney sold the tax as a revenue-raiser for prekindergarten and badly needed upgrades to recreation centers. Public health was not emphasized in his campaign.
Virtually every other beverage tax proposed in the United States has been premised on health benefits. A half-dozen are under consideration; scores have been defeated.
Proponents could not remember any that included diet drinks.
Public-health campaigns "are really focused on reducing calories," Story said.
She was a member of the advisory committee that drew up the 2015 Dietary Guidelines, an influential set of federal recommendations that are updated every five years.
The group found "very strong evidence that sugar-sweetened beverages are associated with weight gain both in adults and children," she said, and clear links to diabetes and cardiovascular disease.
It was enough for the committee to recommend that governments "explore" higher taxes to reduce consumption. Its report did not suggest including sugar-free drinks, but "we recommended that low-calorie should not be replacing sugar in the diet," Story said.
A tax on diet drinks was barely on anyone's radar before this week. A 12-page guide that beverage tax proponents released three months ago, "Best Practices in Designing Local Taxes on Sugary Drinks," mentions them in passing.
They were never seriously considered several years ago when New York City Mayor Michael Bloomberg tried to ban jumbo sodas and the state legislature considered a beverage tax, said Thomas Farley, who then was New York's health commissioner and now leads Philadelphia's health department. Both New York attempts failed.
By calorie count alone, it seems artificially sweetened beverages would not cause weight gain. Some evidence suggests otherwise, but it is far from clear whether that's true.
Farley emphasized that the idea of including noncaloric sweeteners in Kenney's beverage tax had come from Council. But he had no objections.
"People will be less likely to switch from sugary drinks to diet drinks, but they may be more likely to switch from sugary drinks to water, and that is what we want," he said.
But economists have never projected how consumers are likely to respond to a diet drink tax.
The beverage industry has argued for three months that a tax on sugary drinks would be ineffective and unfairly target those who could least afford to pay. On average, low-income residents consume more sugary drinks than higher-income residents.
While some Council members said that adding diet drinks would help spread the burden, the industry says the opposite.
"These beverages have no sugar or calories and serve as an option for those looking to reduce their sugar or calorie intake," Lauren Kane, a spokeswoman for the American Beverage Association, said in an email.
Philadelphia's move to include artificially sweetened drinks could affect the industry's lobbying in other jurisdictions, she said.
"This helps us fight these taxes, because it unveils the true motivations behind them - money for more government spending - and reinforces that these discriminatory taxes won't make a dent on health issues," Kane said.
Shiriki Kumanyika, an emeritus professor of epidemiology at the University of Pennsylvania, said approval of a tax would be "a step forward, because it shows it is not impossible to challenge the industry on some of these issues."
But she cautioned against any predictions about how the diet-beverage issue would play out. She said hydration - unlike, say, tobacco - is essential, so consumers who can no longer afford beverages because of the tax would have to find alternatives. Tap water would be a good one, and could be carried in reusable bottles, but no one knows what people would choose if both regular and diet soft drinks are taxed, Kumanyika said.
"What it does," she added, "is really create a natural experiment as to what this particular type of tax will do in terms of revenue and [human] behavior and industry behavior."