I don't eat back exercise cals, given that the calorie guidelines include an option to specify how much exercise you do. I want the health benefits of fasting as well as weight loss, so I stay at or under 500 cals on DDs. I have found I can exercise on DDs as I usually have enough energy in my system to keep me going. I have noticed though, that on an UD which follows a DD, I need to wait until later in the day to exercise or I just don't have enough energy, having been very low cal the day before.