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PET image of the human brain showing energy consumption. The brain consumes up to 20% of the energy used by the human body, more than any other organ. [131] In humans, blood glucose is the primary source of energy for most cells and is critical for normal function in a number of tissues, including the brain. [132]
The brain typically gets most of its energy from oxygen-dependent metabolism of glucose (i.e., blood sugar), [85] but ketones provide a major alternative source, together with contributions from medium chain fatty acids (caprylic and heptanoic acids), [90] [91] lactate, [92] acetate, [93] and possibly amino acids.
The human brain requires nutrients obtained from the diet to develop and sustain its physical structure and cognitive functions. [1] [3] [4] Additionally, the brain requires caloric energy predominately derived from the primary macronutrients to operate. [1] [4] The three primary macronutrients include carbohydrates, proteins, and fats.
β-Hydroxybutyrate and acetoacetate can pass through membranes easily, and are therefore a source of energy for the brain, which cannot directly metabolize fatty acids. The brain receives 60-70% of its required energy from ketone bodies when blood glucose levels are low.
Energy intake is measured by the amount of calories consumed from food and fluids. [1] Energy intake is modulated by hunger, which is primarily regulated by the hypothalamus, [1] and choice, which is determined by the sets of brain structures that are responsible for stimulus control (i.e., operant conditioning and classical conditioning) and cognitive control of eating behavior.
In the event of low glucose concentration in the blood, most other tissues have alternative fuel sources besides ketone bodies and glucose (such as fatty acids), but studies have indicated that the brain has an obligatory requirement for some glucose. [13] After strict fasting for 3 days, the brain gets 25% of its energy from ketone bodies. [14]