The energetic requirements of a body are composed of the basal metabolic rate (BMR) and the physical activity level (ERAT, exercise-related activity thermogenesis).  This caloric requirement can be met with protein, fat, carbohydrates, or a mixture of those. Glucose is the general metabolic fuel, and can be metabolized by any cell. Fructose and some other nutrients can be metabolized only in the liver, where their metabolites transform into either glucose stored as glycogen in the liver and in muscles, or into fatty acids stored in adipose tissue.
Because of the blood–brain barrier, getting nutrients to the human brain is especially dependent on molecules that can pass this barrier. The brain itself consumes about 18% of the basal metabolic rate: on a total daily intake of , this equates to , or about 80 g of glucose. About 25% of total body glucose consumption occurs in the brain.
Glucose can be obtained directly from dietary sugars and by the breakdown of other carbohydrates. In the absence of dietary sugars and carbohydrates, glucose is obtained from the breakdown of stored glycogen. Glycogen is a readily-accessible storage form of glucose, stored in notable quantities in the liver and skeletal muscle.
When the glycogen reserve is depleted, glucose can be obtained from the breakdown of fats from adipose tissue. Fats are broken down into glycerol and free fatty acids, with the glycerol being turned into glucose in the liver via the gluconeogenesis pathway.
When even the glucose made from glycerol reserves start declining, the liver starts producing ketone bodies. Ketone bodies are short-chain derivatives of the free fatty acids mentioned in the previous paragraph, and can cross the blood–brain barrier, meaning they can be used by the brain as an alternative metabolic fuel. Fatty acids can be used directly as an energy source by most tissues in the body, but are themselves too ionized to cross the blood–brain barrier.