If starvation exceeds 7 days, the major nutritional supply to the brain comes from

Glucose Is Always Required by the Central Nervous System and ErythrocytesErythrocytes lack mitochondria and hence are wholly reliant on (anaerobic) glycolysis and the pentose phosphate pathway at all times. The brain can metabolize ketone bodies to meet about 20% of its energy requirements; the remainder must be supplied by glucose. The metabolic changes that occur in the fasting state and starvation serve to preserve glucose and the body's limited glycogen reserves for use by the brain and red blood cells and to provide alternative metabolic fuels for other tissues.Ref: Harper&;s Biochemistry; 30th edition; Chapter 14; Overview of Metabolism & the Provision of Metabolic Fuels