GLUT1 transport across the blood–brain barrier must be capable of producing a net flux that is equal to the cerebral metabolic rate for glucose, CMRglc, at all times both at rest and during nervous activity. Furthermore the system must be capable of increasing net flux quickly to match demand. If the net inward flux were not increased, then for a glucose content in brain of 1.3 mM × 0.77 mL g−1 and an increase in glucose consumption rate of 0.65 µmol g−1 min−1 (i.e. to twice resting level, figures for rats), the entire glucose reserve would be consumed in < 2 min.