It is technical true that it has no sugar, but this misleading because that doesn't mean diet coke doesn't make you fat. The taste of sweetness causes your body to release insulin and immediately store blood sugar as fat to prepare for the incoming sugar. Because excess blood sugar is dangerous to the body and it must prevent that. Once the expected sugar doesn't hit your blood, your low blood sugar makes you lethargic and hungry, making it difficult to function until you satisfy your craving. I saw one study that said diet soda was about half as effective as regular soda at making you fat. Which is still a lot because a can of soda has as much sugar as a candy bar, and because the false sense of security causes many people to drink many more cans of diet soda than they would regular soda. The estrogen-like BPA in the soda can cause some weight gain too, unless you drink bottled. But bottles tend to be bigger than cans. A variety of studies at pubmed.gov likewise link diet soda and artificial sweeteners in general to insulin spikes and weight gain. Diabetics know to be careful with diet soda in fact, when monitoring their insulin levels.