Diet soda does not directly contribute to weight gain, but it may alter the body's ability to judge calorie content. This can lead to overeating of high-calorie foods, which causes weight gain. Most experts, however, do not believe that there is conclusive evidence to support this theory.
Studies have also linked diet soda with metabolic syndrome, a group of symptoms that increase the risk for heart disease and diabetes. These symptoms include high triglycerides, high blood pressure and high blood sugar. Dangerous side effects such as diarrhea, glaucoma and bleeding disorders have been associated with diet soda and other caffeinated products.