The major problem with drinking that much diet soda is that the phosphoric acid leaches important vitamins and minerals such as vitamin a, calcium and magnesium from the body. This can lead to weaker bones, and muscle and nerve problems. I first read an article about this in a powerlifting magazine a few months ago and being someone that drank a 20oz diet mountain dew every day, I decided to see if there really were major side effects to drinking too much of the stuff. Knowing from previous research that the experiment I was going to conduct on myself wouldn't cause fatal damage, I started drinking 2 liters or so a day. Now, I was still drinking my usual 3-4 liters of water a day so the diet soda was not my only fluids. After about 2 months of this "diet soda addiction" I definitly experienced side effects. I felt like my toes on both feet were numb all the time as well as my left shoulder. I also wasn't "feeling" the muscular contractions when I was training with weights at the gym.  After feeling this way for most of a week, I stopped drinking all diet soda completely and within 3 days all the "wierd" side effects I was feeling went away. I would speculate that the diet soda was causing some type of nerve damage or interference leading to my feeings of numbness. All in all, I tend to lean to the everything in moderation side of things. Am I going to stop drinking diet sday forever? No, but I am definitly going to cut back to maybe a few a week after a couple of months of being off of it completely. I would treat diet soda as a treat for when you really want something sweet, but not as a way of getting your fluids. Also, I agree that 1000 calories is pretty low. Are you trying to diet for a physique competition of some kind. As someone that has helped more than a few girls get into shape for shows, I never had someone so low in calories.