The specifics of the Fat Fast can be found in Dr. Atkins New Diet Revolution book. The text describing this short term ketone-forcing plateau busting diet describes a VLC (very low calorie) low protein diet. It's an interesting approach that I tried many times over the years. Frankly, other than water loss I never experienced any positive effects, such as fat-loss. As a result, I don't recommend this plan for anything other than VERY short term (<3 days) and only as a method of getting rid of excess water.