How can drinking diet soda lead to weight gain?

Drinking diet soda does not have any calories associated with it, so drinking diet soda in and of itself will not directly cause weight gain. With that said, if you think you are gaining weight, look closely at what else you are eating. If you are thinking “because I drank that diet soda, I can have……” and then choose to eat or drink foods/beverages with high calories, you may have to teach yourself to rethink. In a nutshell, for the most part, when someone consumes more calories than they use, they will gain weight. Calories are your body’s energy. When you have too many, your body will store them for later. Any time you have more than you need at that moment, you will store them as body fat, aka weight. If you are unsure where the extra calories are coming in, make the decision to write down EVERYTHING you consume for 3 days. Include sauces, dressings, beverages, etc. This will give you a good idea of what you are eating.  We often forget the cookie here or the chips there and wonder why our pants are a little tighter. Once you get a good idea of what is going on, make 1 change at a time. One week, you could decide to only drink calorie-free beverages. The next week, you could choose to replace one snack per day with a fresh fruit or vegetable snack.

Zero calories, it turns out, don't have zero impact when it comes to weight. A recent study shows that people who drank diet sodas had a 70% greater increase in waist circumference over a few years than people who didn't drink them. Any excess fat is bad, but belly fat is the worst. It's linked to diabetes, heart disease and some cancers.

How can something that's basically chemicals and coloring be fattening? First, sweet diet soda fuels your desire for other sweets. Second, it has a "health halo" effect. You view choosing a "diet" drink as virtuous (all those calories saved!). So it feels like you can afford to reward that virtue with a hot fudge sundae or a fast-food burger that's bigger than Texas.

It's not the diet soda that's fattening. It's the sense that drinking it somehow erases the calories of what you eat with it. A diet drink now and then won't make you fat -- as long as you're not using it to wash down a fried calorie bomb from the drive-through.