Mother, it is OK to eat less than 500 calories on a fast day if that is what works better for you. There are a few people who eat nothing at all on a fast day because that works out better for them; if they eat anything at all, they feel very hungry and have difficulty stopping eating. There are medically supervised fasts in which people go with no food at all for several days. Personally I prefer to focus on getting some of the protein, vitamins and minerals that my body needs for repair with the 500 calories I’m allowed. With 5:2, you can work out what will work best for you, yourself, in your own life, within the parameters of 500 (or 600, for men) calories two days a week. It’s OK to try it one way and then another way until you find what suits you best. If you try it one way and it doesn’t work, and you end up going way over 500 calories, just try it another way another day. You didn’t fail, you learned something.