Obligatory utilization of glue os e.-The fact that brain normally derives almost all its energy from aerobic oxidation of glucose does not distinguish between preferential and obligatory utilization. Most tissues are facultative in their choice of substrates and use them interchangeably roughly in proportion to their availн ability from the blood. It was once believed that the brain did not enjoy such flexibility and that its energy metabolism was rigidly restricted to the substrate glucose (1, 2). The basis for this conviction rested on two lines of evidence: 1. the impairment of cerebral function and energy metabolism by glucose depriн vation, and 2. the inability of potential substrates other than glucose to reverse the effects of hypoglycemia.

Hypoglycemia is associated with changes in mental state ranging from mild, subjective sensory disturbances to convulsions, coma, and death, depending on both the degree and duration of the condition. The behavioral effects are paralн leled by abnormalities in the electroencephalogram and cerebral metabolic rate, and cerebral oxygen consumption declines in proportion to the depression of the level of consciousness (1, 2). The cerebral effects of hypoglycemia are indeн pendent of the mode of its induction and are similar whether it is caused by insulin or hepatectomy (1, 2). Allthe effects are rapidly reversed by glucose administraн tion, provided they have not been allowed to persist until irreversible changes have occurred. It appears then that there are no other substances normally present in blood which ean replace glucose as the substrate for cerebral energy metaboн lism.