As ElizEllen quoted above, the FatFast, based on the Kekwick diet is 90% fat. Subject to the other caveats about who should use it and when, you are not eating too little but too much protein and carbs. If you just started Atkins on 2/22, unlikely your body is even fat-adapted yet! Start with Atkins induction at moderate protein and around 75% fat. If you like tracking macros, (not that you have to) aim for 75-20-5 at around 1200 cal first before trying a FatFast. Also consider No nuts, no cheese, no Splenda, or no dairy; various tweaks that work for some people.