I've read that the (whole) brain consumes 20% of the total body energy expenditure at rest. How much

Jayatri Das, Ph.D.
senior exhibit & program developer
The Franklin Institute Science Museum

Tom Schnugg, AK, CA

I've read that the (whole) brain consumes 20% of the total body energy expenditure at rest. How much energy do the the different areas of the brain consume for various tasks such as higher executive functioning More specifically, how can I find out about the varying metabolic requirements of different executive functions (goal related behavior) such as response inhibition, prioritizing, reasoning and scenario planning (etc) vs. lower level (limbic) tasks? Thanks.

Jayatri Das
senior exhibit & program developer,
The Franklin Institute Science Museum,
former HHMI predoctoral fellow
The brain, as a whole organ, uses on average 20% of the body’s total oxygen, which generally reflects the amount of energy being used. However, looking at this average number hides a more interesting story about the variation in energy use by the brain, including how much energy is used for different brain functions. New functional brain imaging technology, such as PET and MRI, is giving us glimpses into this story, but many questions remain unanswered. Scientists have documented that the brain’s use of glucose varies at different times of day, from 11% of the body’s glucose in the morning to almost 20% in the evening. In addition, different parts of the brain use different amounts of glucose. The medial and lateral parietal and prefrontal cortices use higher amounts of glucose than average—these regions are involved in the brain’s default (non-task-related) activity as well as cognitive control and working memory. The cerebellum, used for motor control and learning, and medial temporal lobes, involved in long-term memory, use less. In addition, increases in glucose metabolism at the cellular level have been correlated with task-induced neural activity. These lines of evidence do suggest that different brain functions have different metabolic requirements.

But the story’s not so simple, with several factors making it difficult to identify those specific metabolic requirements. First, we know that the brain is constantly active, even during rest, but we don’t have a good estimate of how much energy is used for this baseline activity. The metabolic and blood flow changes associated with functional activation are much smaller for this ongoing activity—local changes in blood flow during cognitive tasks are less than 5%. In addition, the variation in glucose use between different regions of the brain, as discussed above, accounts for only a small fraction of the total observed variation. So, the high level of constant activity throughout the brain makes it very hard to detect changes associated with specific functions. Another major challenge in identifying the metabolic requirements of specific functions is determining which regions of the brain are involved in those functions. The information revealed by functional imaging is powerful but has its limitations. Brain functions are complex, occur on a rapid timescale, and are not always localized; in addition, imaging may not capture the full scope of functional activation. The pictures that we see emerging from fMRI and PET studies look very definitive, but in fact they are the result of a lot of data processing to pull out a small signal from a very noisy background. While we understand functional localization at a very broad level of brain anatomy, we do not have an accurate functional map with which to compare any subtle changes in metabolic consumption.

Both the complexity of brain activity and the complexity of the techniques we use to study brain activity have presented a challenge to figuring out how much energy is used for different functions. This area of research is exciting, however, as it ties together the neurophysiology of the brain with our understanding of human behavior. Hopefully, continuing advances in brain imaging and cell monitoring technology will shed light on this question in the near future.

For additional information:

Vaishnavi SN, Vlassenko AG, Rundle MM, Snyder AZ, Mintun MA, and Raichle ME. Regional aerobic glycolysis in the human brain. Proc Natl Acad Sci. 2010;107:17757–62.

Accessible at http://www.pnas.org/content/107/41/17757.full.pdf.

Raichle ME, and Gusnard DA. Appraising the brain’s energy budget. Proc Natl Acad Sci. 2002;99:10237–9.

Accessible at http://www.pnas.org/content/99/16/10237.full.pdf+html

Raichle ME. The brain’s dark energy. Sci Am. March 2010;302:44–9. Accessible at http://www.braininnovations.nl/Dark-Energy.pdf

Brain Imaging Technologies: http://learn.genetics.utah.edu/content/addiction/drugs/brainimage.html

08/26/11 09:21