Igor Dutina / Featurepics.com

So beautiful, so bad for the waistline. A cocktail of cranberry juice plus vodka can easily total 300 calories.

You’ve switched to diet soda and scrupulously have been counting calories at every meal, but you’re still not losing weight. What’s up? You might be missing a big diet buster: that glass of wine you like to have with dinner.

As it turns out, on average Americans are consuming 100 calories a day in alcoholic beverages, according to a new report from the National Center for Health Statistics. Broken down, that’s 50 calories a day for women and 150 for men -  which, by the way, is the equivalent of a 12 ounce can of sugary soda, researchers from the Centers for Disease Control and Prevention report.

And while 50 calories a day may not seem like much, over the course of a year, that adds up to 18,250 extra calories. At 3,500 calories per pound, this could potentially translate into a weight gain of about 5.2 pounds in a year. And that might explain why you can’t drop that pesky last five pounds, no matter how well you stick to your diet.

“I just don’t think this is on people’s radar screen,” says the study’s lead author Samara Joy Neilsen, a nutritional epidemiologist at center. “It’s not even highlighted in the scientific literature. Researchers usually focus on sugary drinks or sodas or fruit juices or energy drinks.”

Neilsen and her colleagues surveyed 11,000 Americans in a nationally representative sample.  Each was asked to detail what they’d eaten over the last 24 hours. To make sure the findings weren’t biased, some people were surveyed during the week and others on weekends.

While the average may be 100 calories per day, a full 19 percent of men and 6 percent of women are consuming more than 300 calories per day through drinking, the researchers found.  You can see where that ends up in terms of weight gain.

The new findings are “not surprising, but they are concerning,” says David Sarwer, a professor of psychology and director of clinical services at the Center for Weight and Eating Disorders at the Perelman School of Medicine at the University of Pennsylvania.  “For every extra 100 calories per day you’re taking in over and above what you are burning there’s a potential for gaining one pound per month.”

Sarwer suspects that the numbers in the new study may actually underestimate just how many calories people are taking in from alcohol.  “We know from other areas of alcohol research that most people tend to underestimate how much they are consuming,” he says.

Weight loss experts like Sarwer say that their patients often forget about alcohol when they’re totaling their calories for the day. And that’s especially true for those consuming wine or mixed drinks, says Susan Bowerman, assistant director of the Center for Human Nutrition at the University of California, Los Angeles.

“I think people are aware of beer,” Bowerman says. “Everyone talks about beer bellies. But when it comes to wine, I don’t think people account for the calories. And I don’t think people are aware of how many calories they can get in a mixed drink.”

For example, Bowerman says, if you add the 100 to 120 calories for a shot of any alcohol to the 160 calories in a glass of cranberry juice, you’re getting mighty close to 300 calories from just one drink.

And then there are those extra-large glasses that some restaurants like to use for wine. A 5 ounce glass contains about 120 calories. “One of those huge balloon wine glasses can contain 8 to 10 ounces,” Bowerman says. “That’s a lot of calories.”

The average person in the study isn’t a problem drinker in the traditional sense, Sarwer says. But the calories they are taking in with that glass or two of wine over dinner, or the beers on the weekend, may be wreaking havoc with waistlines.

Neither Sarwer nor Bowerman suggest people go on the wagon. But cutting back by half could make a big difference, they said.

“We don’t want to send the message that you can’t have any fun with food at all,” Sarwer says. “We tell our patients in response to a study like this that it’s a reminder that they might want to stop at one glass of wine, instead of two, or maybe drink every other day. It’s a good investment in their weight and in their health.”

Spirits manufacturers said most Americans aren't problem drinkers.

"This research shows that the overwhelming majority of adults drink moderately as defined by the federal Dietary Guidelines," Lisa Hawkins, Vice President of the Distilled Spirits Council said in a statement.

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