Which alcoholic beverages are more diet-friendly?

(CNN) — I often tell people to steer clear of alcoholic beverages when trying to lose weight. After all, they don’t exactly provide nutritious calories, and consuming them can make it increasingly more challenging to lose weight. But that doesn’t stop people from asking me, “what is the best drink to have on a diet?”

Clearly, some alcoholic beverages are more waistline-friendly than others. So the short answer is: If you’re looking to shed pounds, some of your lowest-calorie bets are a shot of spirits (for example, a 1.5-ounce shot of vodka, gin, rum, whiskey or tequila contains an average of 97 calories), a glass of champagne (about 84 calories per 4 ounces); a glass of dry wine (approximately 120 to 125 calories per 5 ounces) or a traditional martini, with an average of 124 calories for a 2.5-ounce serving. A light beer (approximately 100 calories) or a glass of reduced-calorie wine (about 90 to 100 calories) are other lower-calorie options.

But if you want to know why some alcoholic beverages contain more calories than others, read on.

Pure alcohol contains 7 calories per gram — that’s less than a gram of fat, which has 9 calories, but more than protein and carbs, which have 4 calories per gram each — but other variables of your drink influence the calorie count. The amount of alcohol, the total volume of a beverage, the amount of carbohydrates and sugars and mixers all play a role.

Generally speaking, the biggest difference in calories in beverages comes from the alcohol content, but the presence of carbohydrates in alcoholic beverages also contributes largely to its calories, according to Dwayne Bershaw, who teaches winemaking classes in the Department of Food Science at Cornell University.

Any carbohydrates in wine consist of small amounts of sugar, which may either be left over from the original grape sugar after most of it ferments into alcohol or added from grape juice or some other source, in order to balance the acidity of the wine. Unless we’re talking about a dessert wine, this amount of sugar is generally small and does not contribute a significant amount of calories to wine.

Beer is made from grain, which stores carbohydrates in the form of starch — specifically large, branched sugar molecules, which are chopped into pieces by naturally occurring enzymes in malted barley during the brewing process. The smallest sugar pieces are converted to alcohol by yeast, but some larger pieces remain that cannot be broken down by yeast, according to Bershaw. These remaining carbohydrates contribute to the overall calorie count for most beers.

Generally, beer has more calories than wine, but the calorie difference in the two primarily comes from the leftover carbohydrates in beer, as the sugar content for most wines is fairly low.

Low-calorie beers have an additional enzyme added during the brewing or fermentation process, and it breaks down all of the starch molecules into simple sugars so there are no remaining carbohydrates. These beers also have a relatively low alcohol content to keep the total calorie count quite low, according to Bershaw.

And generally, spirits — including vodka, tequila, rum and gin — do not contain any carbohydrates or sugars. Some producers may add small amounts of sugar to combat any perceived bitterness, according to Bershaw, but like wine, this small amount would not bump the calorie count much. Though they have higher alcohol by volume (up to 40% or more), the volume of a standard shot is small (1.5 ounces), making them a relatively lower-calorie option — that is, as long as you can stick to one shot or mix it with a zero-calorie beverage, like a diet cola, soda water or seltzer.

Other ingredients — including mixers, tonic water, juice, soda, syrups, cream and coconut — all pack sugar and fat calories on top of alcohol and should be consumed with caution, explained Ginger Hultin, a registered dietitian, spokeswoman for the Academy of Nutrition and Dietetics and author of the blog ChampagneNutrition.

“Margaritas and pina coladas can have close to 500 calories per drink, depending on the size and how it’s made, and could be a real challenge for anyone with a weight loss goal,” Hultin added.