On my fast days I consume about 600 good cals, 200 for breakfast and the rest at dinner. Nothing except water in between. Apparently, there is no hard rule about the number of calories to take, but I can't imagine it would work if the cals went any higher. Incidentally, I haven't lost much weight on the 5:2. I mostly maintain my weight. The amount I do lose is miniscule; however, it is steady. My goal is not to lose weight so much as it is to find a way not to revert to my pre-diet weight. Thanks for asking. Hope this helps.