A team of researchers from NY universities and medical laboratories wanted to determine the dynamics of brain beta-hydroxybutyrate and lactate in fasting induced ketosis in five healthy adults.
These subjects all began fasting after an evening meal; they were admitted in a clinic at Albert Einstein College of Medicine during the fast. So, they were under strict supervision.
Data was collected after 2 days and after 3 days from the beginning of the fast. During the fast they drank 2 liters of water per day and took a vitamin and mineral supplement. Brain BOHB and lactate have been measured using a 4-T whole-body image spectrometer.
Plasma BOHB increased (beginning at almost 0) in all subjects during the 3 day fast, reaching a concentration of almost 4 mmol/L in the third day of the fast. Likewise brain BOHB increased from almost 0 to around 1 mmol/L in the last day of the fast. A similar trend was seen in brain lactate concentrations.
As per the researchers, there is a strong correlation between plasma and brain BOHB during fasting:
"Based on the regression analysis between brain and plasma BHB, there appears to be a strong correlation between them with r = 0.86 and slope = 0.26, as shown in Fig. 3. We anticipate that two factors affect this correlation: net ketone transport and oxidative flux. " [source]
So, despite this being a very small study, one cannot derive clear-cut conclusions from it, but it might be relatively say to say that a plasma concentration of BOHB of 3-4 mmol/L is correlated with a brain BOHB concentration of ~1 mmol/L in healthy subjects in prolonged fasting.
It would be interesting to see how these levels fluctuate in even longer fasting or in people who follow ketogenic diets over the long term. It would also be interesting to determine the proportion of brain energy metabolism taken over by beta-hydroxybutyrate as a result of this. I'd suspect it's the predominant fuel (from papers by George Cahill) but it would be great to see solid numbers.
You can read the full paper below if you're interested in this subject.
Human Plasma and Brain Beta-Hydroxybutyrate in Ketosis - [Research]
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Cristi Vlad Self-Experimenter and Author