Virtually everything you can buy at a grocery store comes with a nutrition label. Except one thing — alcoholic beverages.

Why is alcohol exempt? The short answer is that, mainly as a legacy of Prohibition, alcoholic beverages aren't regulated by the FDA, but a different federal agency called the Alcohol and Tobacco Tax and Trade Bureau (TTB) — and this agency doesn't require nutritional labeling.

consumer advocates have pushed for labeling several times — but it's never happened

But consumer advocates have also pressured the agency to require labeling several times — and it never seems to happen. Alcohol manufacturers have managed to fend off the push for years. Finally, in 2013, the TTB made nutrition labels optional for booze, but not required.

This might seem trivial, but some experts think it's a real public health issue. "Many adults take in a tremendous amount of calories from alcohol, and they have no idea," says Sara Bleich, a public-health researcher at Johns Hopkins. She has found that the average American who drinks regularly takes in 400 calories daily from alcohol — not a huge surprise, given that average beer or glass of wine has about 150 calories.

The historical reason why alcohol isn't fully labeled

The roots of this strange situation can be traced to 1935. Shortly after the repeal of Prohibition, congress passed the Alcohol Administration Act, which established what would eventually become the TTB (in order to generate tax revenue from newly-legal alcohol) and gave it the responsibility of regulating the labels on alcoholic beverages.

As a result, in 1990, when modern nutrition labels on all packaged foods became required by the FDA, alcohol wasn't affected.

after prohibition, alcohol became regulated by a new agency, instead of the fda

Instead, over the years, a hodgepodge of different labeling rules for different sorts of alcohol were put into place. Bottles of distilled liquor must have a label indicating the alcohol percentage, and the same goes for bottles of wine with more than 14 percent alcohol. On the other hand, these labels are optional for wines with less than 14 percent alcohol, as well as for all beers. (It used to be the case that beers couldn't list their alcohol content, for fear that they'd advertise on that basis and get into a boozy arms race, but a 1995 Supreme Court ruling said that ban violated the First Amendment.)

Meanwhile, when it comes to calories and nutrients (i.e. carbs, fat, and protein), the rules are even more convoluted. Wines with less than 7 percent alcohol and beers that don't have malted barley actually fall under FDA rules, which specify that they need to list standard nutrition facts and ingredients — but labels about the alcohol content are optional.

Calorie counts are optional for every other sort of beverage, but if they are listed, the amounts of carbohydrates, protein, and fat must be listed as well. Pretty much the only types of beverages that do this are low-calorie light beers.

Finally, listing ingredients (grapes, barley, rice, etc.) is entirely optional for all alcoholic beverages. Manufacturers do have to label beverages that have specific substances to which people might be sensitive (sulfites and yellow no. 5 dye), but labeling of other sorts of allergens (like eggs or nuts) is optional.

The many attempts to put nutrition labels on alcohol

At least six times since the 1970s, consumer advocate groups — most notably, the Center for Science in the Public Interest — have tried persuading the federal government to require comprehensive labels on all alcohol. They have been repeatedly thwarted by alcohol manufacturers, who have made a number of different arguments as to why it would be a bad idea.

The most recent wave of activity began in late 2003, when the CSPI and other groups lobbied the TTB to require nutrition labels. In response, manufacturers asked for voluntary labels. One of their arguments was that putting nutrition facts on all bottles of alcohol would make consumers erroneously think that alcohol was nutritious.

In 2004, the agency basically sided with manufacturers, issuing guidelines that allowed them to list calories, carbs, protein, and fat — if they wanted. Only light beers that were advertised as "low carb" were required to show this information.

The CSPI continued lobbying for mandatory and more comprehensive labels, and in 2007, the TTB proposed a new rule that would have required them. The new labels would have also included alcohol content and serving size, and would have looked a lot like the nutrition facts on other foods.

For the next few years, the TTB considered input it had received during a subsequent public comment period. Some alcohol industry groups, such as Diageo (which owns Guinness and Smirnoff) actually supported mandatory labeling, while others did not.

Beer manufacturers, sensitive to the high amounts of calories and carbs in beer, argued it was unfair to define a serving size as 12 ounces for a beer and 1.5 ounces for liquor, since many mixed drinks end up containing much more. Wine industry groups expressed concern about the difficulty and cost of testing every vintage.

In response to all this, in May 2013, the TTB issued a new rule that kept labeling optional, but added serving size, alcohol content, and servings per container to what companies were allowed to display.

The health consequences of poor labeling

Even though these labels were legalized more than a year ago, it's more likely than not that you've never seen one on a bottle of alcohol. The reason is pretty obvious: wine, beer, and liquor all have lots of calories, something that manufacturers don't want you thinking about when you're buying them.

Even though this information isn't on bottles, it's easily available online. A bottle of Budweiser has 145 calories and 10.6 grams of carbs, and even Bud Light has 110 calories and 6.6 carbs. Wines vary widely, but an average glass of red has 130 to 190 calories. A shot of Bacardi rum has 96 calories.

And there's good reason to believe that putting this information on a label could affect people's drinking habits.

"people have no idea that when they drink, they're taking in hundreds of calories"

"We generally know that people don't have a good sense of nutritional information," says Bleich, the Johns Hopkins public health researcher. "Americans are really bad at mental math."

She and colleagues have previously found that the particular information on nutrition labels can have a dramatic impact when it comes to consumption of soda and other sugary drinks. Though she hasn't yet looked at the potential impact of labeling alcohol specifically, she imagines the effect would be similar.

"I really think that people have no idea that when they drink, they're taking in hundreds and hundreds of calories," she says. "With alcohol, people just don't have any information available."

There's something pretty wild about a situation where bottled water is required to have nutrition facts, but you have to go online to figure out how much alcohol and how many calories are in a beer. Given the huge, national problem that is obesity, just labeling alcohol like everything else could prove a welcome first step.