from dr. eades pplp: up to 50% or protein can be converted for use as glucose and up to 10% of fats can be converted. so the body makes all it needes. however when the body is encouraged to enter a ketotic state (lipolysis), the brain will begin after about 3 days to use ketone bodies as a fuel state, quite easily. here is an interesting article about tests of a ketogenic diet. (ketogenic diet is high fat, low controlled protein, low carbohydrate) which is the kind of diet originally espoused by atkins in atkins72.

http://dietandbody.com/article1077_2.html