Can you enjoy drinking booze while staying health-conscious? Personally, when I'm sipping a great cocktail, I'm already thinking of it as a splurge and a few more drops of sugar ("Whoa, can you use Splenda syrup instead of simple syrup, please? Mmkay thanks") aren't going to bother me.
But the growing trend in products marketed around low-cal drinking (like MGD64, and the entire Skinny Girl lineup) suggests that some folks are looking for lower-calorie booze.
But do the calories in alcoholic drinks work the same way as the calories in food we eat? Any time you talk about food or drink and its effects on the body, it's almost impossible to make bulletproof conclusions. But consider this article a short primer on what little we do know.
Here's how the industry calculates calories:
Imagine a reinforced steel oven. Inside, pressures reach levels 30 times atmospheric pressure. Instead of propane, an electric spark ignites special fuels that burn food so completely, not even ashes are left. A device like this is called a "bomb calorimeter."
On a nearby computer, a display shows the exact amount of energy released, be the sample potato chips, coca cola, or a shot of bourbon.
When ethanol is burned in a device like this, about 7 kilocalories of energy per gram of booze gets released.
So assuming that the majority of calories in beer come from alcohol, to find the calorie count of a beer, you can simply multiply the amount of alcohol (in grams) in that beer by seven.
The calculation is simple, precise, and wrong.
*What we normally call "calories" are actually kilocalories, or 1,000 calories.
Here's how you should be calculating calories:
Imagine what would happen if you ran the calorie test above on, say, a hunk of pinewood. You'd get a pretty high reading, right? I mean, wood does burn pretty effectively.
Let's say, for argument's sake, that the sample registered 100 calories.
Does that mean you can eat a chunk of wood and get as much energy from it as 25 grams of chicken breast?* Of course not.
The primary carbohydrate found in wood is lignin, an extremely tough starch that humans can't digest (though apparently termites find it delicious).
On the flip side, if you were to drink 100 calories of sugary soda, that sugar in the soda would be almost 100% digested, so you would actually be getting close to 100 calories from the soda. How many calories we get from a food or drink all depends on digestion.
So how much energy do we get from drinking alcohol?
*Chicken breast has about 4 cal/gram.
How humans digest alcohol
Machines burn alcohol efficiently. Human beings do not digest alcohol efficiently.
As I'm sure we've all heard at some point in high school health class, alcohol is a toxin. And our bodies work actively to excrete toxins. As Wikipedia helpfully explains, the metabolism of alcohol is a complex, multi-stage process that takes place mostly in the liver and kidneys, not in the intestines, where normal digestion occurs.*
More significant to the current discussion, the Wikipedia article explains that alcohol is almost never fully metabolized, but rather excreted as acetic acid (because it's a toxin and we want to get rid of it, remember?). This implies that we are getting far less than the theoretical maximum of 7 calories/gram from alcohol.
However, the prevailing knowledge in health and nutrition circles seems to contradict this assessment: nutritionists are taught to assume that the energy stored in alcohol is almost 100% available to the body.1
*The references for the Wikipedia article aren't great, but the info found in this book on nutrition and alcohol seems to back up what Wikipedia says.  Frary, Carol D. and Johnson, Rachel K. "Chapter 2: Energy" in Krause's Food and Nutrition Therapy, 12th ed. 2008. pp. 35-36.
Why the difference?
The answer is complex and remains the subject of some debate, but I'll try to summarize the facts here.
Remember that the body wants to get rid of alcohol because it sees it as a toxin. One way to get rid of alcohol is to use it for energy. But, your liver can only process so much alcohol as energy before the body has to start dumping the excess as urine.
Basically, from a physiological standpoint, if you have one drink slowly, you'll probably digest most of the alcohol as energy. Drink heavily, and the calories don't "count" as much, because you'll end up excreting them.2
 Alcohol, Overweight and Obesity (2013)
So which drinks should I drink if I'm watching my weight?
As far as published research is concerned, it doesn't really matter what you drink if you drink in moderation: a few drinks has no long-term impact on weight. In addition to the work cited in , other studies have shown that moderate consumption of alcohol (1-2 drinks per day) results in insignificant weight gain.3 These studies tested both traditional drinks, like beer4 and wine, as well as controlled doses of straight ethanol.5
Since the published research doesn't offer any clear link between moderate alcohol consumption and weight gain, the next culprit to consider is sugar. Sugar contributes the most calories to drinks (after alcohol) and most nutritionists do agree that excess sugar intake results in long-term increased risk of overweight and obesity.
How do different drinks stack up, sugar-wise?
Sweetened liqueurs (and cocktails that use them) contain the most sugar. Sweet wines come next. Dry wines, on the other hand, can contain very little residual sugar.
Beer is tricky. Whereas it's relatively easy to tell if a wine is low in sugar by simply tasting it, there's no easy way to avoid sugary beers. That's because many beers contain non-sweet sugars like maltose or low-sweet sugars like glucose and those sugars can be hidden by beer's bitterness while still packing a significant caloric punch.
Finally, most distilled spirits contain almost no residual sugar, so there's always that option, as long as you don't mind sipping them neat.
Do these differences in sugar levels between drinks matter? That's up to you. Personally, I'd always choose the tastier drink, even if it means doing a few extra situps the next day.
Does calorie count factor in to your imbibing choices?