It is well established that the brain is capable of consuming ketone bodies, especially during low glucose availability, e.g. fasting. Cerebral metabolism of ketone bodies depends on passage of the blood brain barrier and especially the global blood concentration of ketone bodies.

Ketone bodies can be administered exogenously, and the most commonly used in clinical trials is 3-hydroxybutyrate (3-OHB). 3-OHB is carried by simple diffusion and facilitated diffusion through several monocarboxylic acid transporters (MCTs) across the blood-brain barrier.

To our knowledge, no studies in human adults exist that concurrently measure 3-OHB concentrations in blood and cerebrospinal fluid (CSF) after ingestion or infusion of exogenous ketone supplementation, necessitating further study.

Aims:

• The 3-OHB CSF/blood ratio after oral ingestion of 30 g ketone ester - primary endpoint
• The window of effect: Ketone supplementation 1 h or 2 h before CSF sampling
• If concentration measurements by point-of-care testing are non-inferior to mass spectrometry
• If acute 3-OHB ingestion increases plasma brain-derived neurotrophic factor (BDNF) levels

Inclusion Criteria:

• All sexes
• Referred to undergo an elective lumbar puncture procedure in the outpatient clinic at Department of Neurology, Aarhus University Hospital.
• Age 18-80 years
• Written and oral consent

Exclusion Criteria:

• Referred to the clinic suspecting severe neuroinflammation
• Special diet habits, including ketogenic diet, fasting, intermittent fasting etc.
• Daily use of insulin or other medication affecting blood glucose and/or glucose metabolism
• Not able to speak or understand Danish and/or give written and oral consent