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Objective.
Glucagon-like peptide-1 (GLP-1) has many effects on glucose homeostasis, and GLP-1 receptors are broadly represented in many tissues including the brain. Recent research in rodents suggests a protective effect of GLP-1 on brain tissue. The mechanism is unknown. We therefore tested whether these neuroprotective effects could relate to changes of glucose transport and consumption.

Research Design and Methods.
We studied 10 healthy men in a randomized, double-blinded, placebo-controlled cross-over experiment. We used positron emission tomography to determine the acute insulin-independent effect of GLP-1 on unidirectional glucose transport into the brain during a pituitary-pancreatic normoglycemic (plasma glucose ~4.5 mmol/l) clamp with 18-fluoro-deoxy-glucose as tracer.

Results.
On average, GLP-1 reduced cerebral glucose transport by 27% in total cerebral gray matter (P = 0.05) and by 25–30% in individual gray matter regions (P = 0.02–0.06). The same regions revealed a uniform trend toward similarly reduced cerebral glucose metabolism. Consequently, the intracerebral glucose concentration remained constant in all regions, with and without GLP-1.

Conclusions.
We have demonstrated that a hormone involved in postprandial glucose regulation also limits glucose delivery to brain tissue and hence provides a possible regulatory mechanism for the link between plasma glucose and brain glucose. Because GLP-1 reduces glucose uptake across the intact blood-brain barrier at normal glycemia, GLP-1 may also protect the brain by limiting intracerebral glucose fluctuation when plasma glucose is increased.

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