Surely a beer so dark and creamy must be thicker and more caloric than the bright yellow beers on offer at the bar. In reality Guinness is usually the lowest calorie offer on tap. First, the dark colors come from the kinds of roasted malt, not the amount; and the creaminess is caused by nitrogen injected into the beer when on tap or released by a widget in the can or bottle. Once you get past the dark complexion and creamy mouthfeel, what you're left with is a roasty, lightly chocolaty low alcohol session beer at 125 calories and 4 percent alcohol. Take care when choosing Guinness though, most consumers don't realize they offer a few varieties, and the Extra Stout brand tacks on an extra 50 calories along with it's bolder flavor and alcohol boost.