Hi, emilycat – you can check-out what your recommended non-fast day intake should be, based on your own statistics – size, age, activity levels, etc. – by using the TDEE (Total Daily Energy Expenditure) Calculator. Click on the link ‘how?’ at the top-right of the page and choose ‘tdee calculator’ from the drop-down menu. Consistently eating too few calories can lead to a slowing down of one’s metabolism, which then means weight-loss becomes less likely. Best wishes.