ROLF GRUETTER*t, EDWARD J. NOVOTNYt, SUSAN D. BOULWAREt,) DOUGLAS L. ROTHMAN?,

Glucose is the main fuel for energy metabo- lism in the normal human brain. It is generally assumed that glucose transport into the brain is not rate-limiting for metab- olism. Since brain glucose concentrations cannot be determined directly by radiotracer techniques, we used 13C NMR spec- troscopy after infusing enriched D-(1-13C)glucose to measure brain glucose concentrations at euglycemia and at hyperglyce- mia (range, 4.5-12.1 mM) in six healthy children (13-16 years old). Brain glucose concentrations averaged 1.0 ? 0.1 Amol/ml at euglycemia (4.7 ? 0.3 mM plasma) and 1.8-2.7 ,umol/ml at hyperglycemia (7.3-12.1 mM plasma). Michaelis- Menten parameters of transport were calculated to be Kt = 6.2 ? 1.7 mM and Tmax = 1.2 + 0.1 ,umol/g-min from the relationship between plasma and brain glucose concentrations. The brain glucose concentrations and transport constants are consistent with transport not being rate-limiting for resting brain metabolism at plasma levels >3 mM.