The supply of fuel to various parts of the brain is determined not only by its concentration in the plasma, but also by blood flow. Simultaneous reduction of both can lead to impairment of cerebral function, even when there is no discernible abnormality in either one alone. This probably accounts for the inability of many elderly subjects to withstand lowering of their blood glucose concentrations to levels that are well tolerated by young people. Similarly, it explains why elderly diabetic patients with unilateral cerebrovascular disease, who have been rendered hypoglycaemic by insulin, occasionally present with hemiplegia that is rapidly reversed by intravenous glucose.
