You can still cleanse if you are skinny as long as you are not currently suffering with an eating disorder or are suffering with malnutrition or certain deficiencies. The main goal of cleansing is detoxification and removal of impacted waste within the colon. We have many skinny, non-overweight people visiting us for fasting and generally they fair just as well as other people. My experience to date has been that the biggest contributing factor to how 'hard' the fast will be, is the mental attitude of the faster; those who approach the fast with doubts and apprehensions, often find it a little tougher; those who start the fast with a positive and relaxed mind have few problems along the way. If you are drastically underweight, then you may choose to participate in our Raw cleanse program instead which doesn’t involve abstaining from food.