You're drinking a zero calorie beverage, which your body has to use energy to process. Over time, this extra work will put your body into starvation mode, causing you to collect extra calories through the air you breathe, things you touch, and even pictures of food that you see on TV. What happens is, your glands will cause your metabolism to slow, and your body will permanently hold onto these calories, to make sure you never starve to death. THIS is how you will get fat. It's no fault of your own, and every body is beautiful.