— Charlie Tipton, Lincolnton, North Carolina
A: I believe they can, but the science is not decisive. The studies suggesting diet sodas, or anything containing sugar substitutes, can contribute to weight gain are based almost entirely on animal research. What scientists have found is that a rodent's brain relies on the link between taste and calories to keep track of just how much eating has occurred. Sugar substitutes—saccharin, sucralose, aspartame, neotame, and acesulfame-K—unbundle the taste of sweetness from calories: The taste buds tell the brain that food is coming in, but the body doesn't get the energy it's expecting. This, apparently, undermines the ability of rats to judge how much they've consumed, and, over time, they begin to overeat and gain weight.
The same mechanism may occur in people, too, but we don't yet know for sure. Though some human studies indicate sugar substitutes help with short-term weight loss, an equal number suggest they don't. My particular concern is that artificial sweeteners are 200 to 13,000 times as sweet as sugar, and that is a potent stimulus for turning a sweet tooth into a fang. Other research suggests that the taste of sweetness is mildly addictive—the more you eat, the more you need to feel fully satisfied. If artificially sweetened sodas increase your cravings, the calories they take out of your diet are apt to sneak back in later when you, for instance, need a larger or sweeter dessert to feel satisfied.
I do make one exception for diet drinks: They can be a great substitute for people who drink a lot of regular soda and are trying to cut down. Just be sure to take the next step—toward the water cooler.