Philadelphia's mayor recently proposed a 3-cent tax on every ounce of sugary drinks (sodas, iced teas, and sports drinks are the major categories). If it passes, Philadelphia will be the first major American city to enact what's often informally referred to as a "soda tax," though similar plans elsewhere have generally exempted diet sodas.

That would be a big deal — but is Philadelphia thinking big enough? Instead of just taxing soda, why not tax all sugar in the name of reducing obesity?

We asked this question on this week's episode of The Weeds — the podcast I co-host with Ezra Klein and Matt Yglesias — and couldn't come up with an answer.

If Mountain Dew is falling under the heavy hammer of big government, after all, it seems fair that the likes of Twix and birthday cake should join it. Neither of them is exactly health food.

Would a sugar tax even work? Why don't we hear about it? I called Barry Popkin, a leading expert on soda taxes at the University of North Carolina Chapel Hill to find out.

Popkin told me that, theoretically, a sugar tax might work. He's actually been talking to one country exploring that approach to regulation. But he sees good reasons — one logistical and one nutritional — to support a soda tax here in the United States, at least for the time being.

First, the nutritional rational: Reducing soda consumption seems to be a better way to tackle obesity than reducing the consumption of sugar in foods. That's because studies tend to find that reductions in beverage consumption don't lead to increases in food consumption. We don't feel the need to replace a coke, for example, with a donut or even a piece of fruit.

But restricting the consumption of sugar in food just doesn't seem to be as powerful. "We have a small effect on obesity when we limit sugar in food," says Popkin. That might be because we just replace sugary foods with other, less-sugar-based calories.

Then, there are the logistical challenges.

If Mountain Dew is falling under the heavy hammer of big government, after all, it seems fair that the likes of Twix and birthday cake should join

If we taxed all sugar, then fruits — which have a natural sweetener, fructose — would fall under the fine. Public health advocates don't want to tax apples and oranges. The thing that they probably want to tax is added sugar — sweetener that gets added in the process of creating food.

Right now, food manufacturers aren't required to differentiate between natural sugars (the kind in apples) and added sugars (the kind on top of Sour Patch Kids). And there are some products that have both (think of chocolate milk, which has a natural sugar — lactose — and added sugars from chocolate syrup). All of this would create the logistical nightmare of trying to sort out which items should and shouldn't fall under a sugar tax.

"Without the labeling it would be way too complex to set levels of natural sugar by product group," Popkin says.

Sodas make this task a bit easier. Nearly all sugar in sugary drinks is added; soda doesn't have any natural sugar at all. The number of borderline cases that come up under a soda tax (the treatment of milk and juice, for example) will be much more manageable than those that arise with a food-wide sugar tax.

Some states have, however, tried to get more aggressive in regulating sugar. Seventeen states have a special tax they levy just on candy or snacks (the rationale for these fees actually began in the World War I era — not as a public health campaign, but as a way to raise war revenue by taxing consumption of a "luxury good.")

This approach ultimately runs into the same issue as a soda tax in that it misses a lot of the sugar in the world. A since-repealed "snack tax" in California, for example, applied to Milky Way candy bars — but not Milky Way ice cream.

If there is any hope for a sugar tax, though, it probably rests with the Food and Drug Administration. The agency will soon require nutritional labels to quantify how much "added sugar" any food has. And that, Popkin says, could create a regulatory framework for a sugar tax. Your Twix bars, in other words, aren't quite safe yet.