It seems to contradict the laws of physics. Regular sodas are full of calories, 140 per can and up. Diet sodas have zero calories. So it seems logical that replacing one with the other should help you lose weight, or at least stay the same weight. But no--several studies have proved conclusively that drinking diet soda is associated with weight gain. In one study, participants who started out normal weight and drank three diet sodas a day were twice as likely to be overweight or obese eight years later as their non-diet-soda drinking peers.
Some skeptical scientists point out that association is not the same as causation. Maybe not, but researchers have developed several theories that could very well explain why drinking diet soda causes weight gain. One or more of them are likely enough to be true that everyone who drinks diet soda should consider stopping now.
1. It makes our bodies produce insulin.
Insulin, secreted by the pancreas, is how the human body stores sugar. When the taste of artificial sweeteners (in soda, yogurt, or anything else) hits your brain, it automatically sends a signal to your pancreas to begin producing insulin. Insulin is what tells our cells to either use sugar as food or store it as fat--without it, our bodies can't process the sugar that lands in our bloodstreams. When your pancreas produces insulin to deal with anticipated sugar, but then no sugar arrives, it confuses your body and disrupts its metabolic process. This may explain why several studies have shown a link between regularly drinking diet soda and metabolic syndrome, a collection of symptoms that includes larger waist circumference, higher blood pressure, and higher blood sugar.
2. It conditions our taste buds for sweetness.
You probably know or have observed that the more regularly you taste something (sweetness, saltiness, etc.) the more inured to it you become. This is why people who stop eating sugar or salt suddenly find many commercially available foods extremely salty (potato chips, for instance) or extremely sweet (candy bars).
So it's worth considering that artificial sweeteners are dramatically sweeter than sugar, and although it may not register that way on your tongue, diet soda is in fact much sweeter than regular soda. All that sweetness accompanied by zero calories confuses your brain as well as your metabolic processes, and tends to leave you craving sugar more than before.
3. It makes you feel entitled to eat more.
Counting calories is still the most common method people use to try to lose weight, and it's the basic principle behind both Weight Watchers and the popular weight-loss app Lose It! If you're counting calories, there's a simple equation: Drinking a regular soda means you have to eat 140 calories less of something else that day or that meal. Drinking a diet soda means you've consumed zero calories, so you get a free pass to eat more. But since drinking the diet soda has fooled your body into expecting sugar, it's changed the way you metabolize those other calories--you may store more of them as fat and use fewer of them as energy--which could leave you hungry and wanting even more food.
If all of the above isn't enough to make you avoid the stuff, then consider that other studies also show drinking diet soda is associated with increased risk of stroke and Alzheimer's disease. (Drinking regular soda seems to be bad for the brain as well.)
I don't know about you, but I'm sticking to water, unflavored seltzer, coffee, tea, and the occasional beer or red wine. Soda, both regular and even diet, can be sweet, bubbly, and yummy. But it just isn't worth the risk.