The brain has a very active metabolism, using 20% of the body’s energy consumption at rest even though the brain accounts for less than 2% of our total body weight.
To sustain life, our body needs food as fuel, and oxygen to metabolize (burn) the fuel. Energy demands change with rest and activity and differs when we are fed or starved. The most important fuels for the body are glucose and fatty acids. The brain is unique because it only utilizes glucose as its energy source.
Unlike most other organs and muscles, the brain has very limited fuel storage – only enough to support brain function for 2 minutes. The brain is completely dependent on blood flow to ensure an adequate and steady supply of glucose and is very sensitive to low blood sugar (hypoglycemia).
Our blood glucose level is tightly regulated by hormones. When this regulation is upset, as in diabetes, blood sugar levels may swing up and down. Antidiabetic medications can also cause hypoglycemia. Binge drinkers are also at risk, especially when meals are missed.
Symptoms of hypoglycemia include fast heart beat, sweating, shaking, anxiety, and confusion. In severe cases, loss of consciousness and even death may occur. In prolonged, severe starvation, brain cells gradually adapt to the use of ketone bodies as fuel to satisfy up to 50% of their energy needs.
Ketone bodies are produced from fat that is mobilized and metabolized during starvation. Before mother’s milk production and feeding are established, newborn infants readily burn ketone bodies.
Simple purpose, complex systems
Neuroscience is beginning to appreciate the complexity of the systems required to accomplish the simple purpose of providing fuel (glucose) to brain nerve cells (neurons).
To ensure ideal cell surroundings, blood vessels in the brain have a special, tight defense system called the blood brain barrier. Even the necessary nutrient glucose needs special transporters to go through.
How the glucose molecules reach the neurons to be burned is not straight forward. Research reveals that after glucose molecules enter the star-shaped glia cells called astrocytes, which surround the blood vessels, they are not simply handed over to the neurons.
The transfer and “consumption” is a complex, interrelated metabolic process. Ongoing research is trying to figure out exactly how.