The brain requires about 120 g. of glucose per day ... the body has a "backup plan" which involves molecules known as ketone bodies. Ketone bodies are short-chain derivatives of fatty acids. These shorter molecules can cross the blood-brain barrier and can be used by the brain as an emergency metabolic fuel.
After 2 or 3 days of fasting, the liver begins to synthesize ketone bodies from precursors obtained from fatty acid breakdown. The brain uses these ketone bodies as fuel, thus cutting its requirement for glucose. After fasting for 3 days, the brain gets 30% of its energy from ketone bodies. After 4 days, this goes up to 70%.
Thus, the production of ketone bodies cuts the brain's glucose requirement from 120 g per day to about 30 g per day. Of the remaining 30 g requirement, 20 g per day can be produced by the liver from glycerol (itself a product of fat breakdown). But this still leaves a deficit of about 10 g of glucose per day that must be supplied from some other source. This other source will be the body's own proteins.
This is about fasting, so on a ZC diet, the last 10g comes from dietary protein.
The "Seven Deadly Sins"
• Grains (wheat/rice/oats etc) . . . . . • Dairy (milk/yogurt/butter/cheese etc) . . . . .• Nightshades (peppers/tomato/eggplant etc)
• Tubers (potato/arrowroot etc) . . . • Modernly palatable (cashews/olives etc) . . . • Refined foods (salt/sugars etc )
• Legumes (soy/beans/peas etc)