The reason why the brain consumes such a large portion of the body's energy, despite weighing only about 2% of the total weight, lies in its constant functioning. This organ does not completely shut down, and even at rest, it is responsible for approximately 20% of the body's oxygen consumption.
This energy expenditure occurs even when a person is sitting, relaxing, or not performing any specific activity. Much of this energy comes from glucose, which is one of the main fuels used by the brain. Research published in the journal Neurology indicates that an adult brain requires about 120 grams of glucose daily, with this value being an average subject to variation according to individual conditions.
The brain requires energy to sustain the function of its billions of neurons and enable continuous communication between them. These cells exchange signals incessantly, even when there is no conscious thought taking place.
For this exchange to occur, it is essential that the brain maintains an ion balance, which are electrically charged substances both inside and outside the cells. In simple terms, this is equivalent to preserving an essential charge difference so that neurons can transmit signals, a process that permanently demands energy.
In this process, glucose is used together with oxygen. In an adult, the brain's oxygen consumption reaches approximately 50 milliliters per minute during rest. Oxygen helps the cells convert the energy stored in glucose into a form usable by the body, ensuring brain function even in moments that seem unproductive.
This also clarifies why sleep does not imply a cessation of brain energy consumption. During sleep, various areas continue to operate and perform vital functions. Thus, the state of rest should not be interpreted as an inactive brain.
A notable example is the default mode network. This term refers to a group of brain regions that show significant activity when the individual is not focused on a specific task. This concept was detailed in neuroimaging studies and gained prominence with the article 'A default mode of brain function', published in 2001 in the Proceedings of the National Academy of Sciences (PNAS), by Marcus E. Raichle and other researchers from the Washington University School of Medicine.
This network is linked to internal mental processes, including aspects related to memory, reflections on personal experiences, and future planning, which helps explain the continuous production of thoughts even without a defined task.
Additionally, the brain maintains other crucial functions during this period, such as blood circulation, which must continue to supply oxygen and glucose, while neurons preserve their signaling capacity. Because of this, cerebral metabolism remains high even in a state of rest, contradicting the idea that little happens biologically.
This functioning helps demystify the belief that dedicating long hours to studying or solving complex problems would cause the brain to burn hundreds of extra calories. Although mental activity can intensify the function of certain brain areas, the brain already has a high basal energy consumption. Research suggests that increases in energy expenditure due to specific tasks are modest compared to resting metabolism, generally estimated at less than 5% of the basal level.
This does not negate the brain changes caused by studying or solving difficult problems; they do occur. However, greater mental activity does not result in a proportionally large increase in energy expenditure. Fatigue after long study sessions is a real feeling, but it does not necessarily correspond to massive caloric consumption. Therefore, in daily life, the brain already operates at a high energy cost before any intellectual effort, continuously consuming glucose and oxygen.