A few recent studies suggest that consuming fake sugar actually trains your insulin response to store more fat, not less. Basically when you consume real sugar, your tastebuds send an alert to your pancreas that says, "Hey, calories are on the way! Prepare to produce insulin!" The insulin then helps break down the sugars, which either provide immediate energy or go into fat cells for storage. If your body interprets something as sweet when there's not really sugar on the table, though, it may end up producing that same insulin response. So that diet soda is still prompting your pancreas to store fat, even though you're not getting to enjoy real sugar—your brain can tell the difference. Artificial sweeteners don't trigger our reward circuits the same way, so you don't get the satisfaction of ingesting sugar.