The average adult human brain weighs only three pounds. But as Brain & Behavior Research Foundation Scientific Council member Nora D. Volkow, M.D., notes in a new paper, pound for pound, the adult brain requires about 10 times more energy to function properly than other tissue in the body. 

Dr. Volkow, Director of the National Institute on Drug Abuse and colleagues, used positron emission tomography (PET) and functional magnetic resonance imaging (MRI) scans of 54 healthy volunteers to study where, exactly, the brain burns up its main energy source, the sugar molecule glucose. The results of their work were published online July 29th in a paper in Proceedings of the National Academy of Sciences.

Not surprisingly, the amount of glucose consumed within the brain was found to correspond with the degree of functional connectivity in various brain regions. The researchers estimate that about 30 percent of total energy is devoted to what they call “basal metabolism,” i.e., basic operations that must be carried out at all times, and excluding the myriad operations that involve brain cells “talking” to one another, both within and across brain regions. Spontaneous activity of the brain, such as when we think, make decisions, or respond to our surroundings, eats up the other 70 percent of the total glucose “budget.” Of particular interest to the scientists was the necessity that the most active connectivity hubs needed to use energy efficiently in order to function properly. 

There’s a flip side to this coin. “The higher energy demands of brain communication that hinges upon higher connectivity could render brain hubs more vulnerable to deficits in energy delivery or utilization,” authors Dr. Volkow and Dardo Tomasi, Ph.D. of Brookhaven National Laboratory observed. “This helps explain their sensitivity to neurodegenerative conditions such as Alzheimer’s disease.”  The researchers describe this inherent vulnerability of the brain to energy supply as an example of the “robust yet fragile” nature of finely-tuned complex systems.