Relatively speaking, the brain consumes an immense amount of energy in comparison to the rest of the body. The mechanisms involved in the transfer of energy from foods to neurons are likely to be fundamental to the control of brain function. Human bodily processes, including the brain, all require both macronutrients, as well as micronutrients.

Insufficient intake of selected vitamins, or certain metabolic disorders, may affect cognitive processes by disrupting the nutrient-dependent processes within the body that are associated with the management of energy in neurons, which can subsequently affect synaptic plasticity, or the ability to encode new memories.

The human brain requires nutrients obtained from the diet to develop and sustain its physical structure and cognitive functions. Additionally, the brain requires caloric energy predominately derived from the primary macronutrients to operate. The three primary macronutrients include carbohydrates, proteins, and fats. Each macronutrient can impact cognition through multiple mechanisms, including glucose and insulin metabolism, neurotransmitter actions, oxidative stress and inflammation, and the gut-brain axis. Inadequate macronutrient consumption or proportion could impair optimal cognitive functioning and have long-term health implications.

Through digestion, dietary carbohydrates are broken down and converted into glucose, which is the sole energy source for the brain. Optimal brain function relies on adequate carbohydrate consumption, as carbohydrates provide the quickest source of glucose for the brain. Glucose deficiencies such as hypoglycaemia reduce available energy for the brain and impair all cognitive processes and performance. Additionally, situations with high cognitive demand, such as learning a new task, increase brain glucose utilization, depleting blood glucose stores and initiating the need for supplementation.

Complex carbohydrates, especially those with high dietary fibre, are associated with increased cognitive performance and improved memory function. This is because fibre regulates glucose metabolism, slowing the release of insulin and preserving insulin sensitivity. An improperly functioning glucose and insulin metabolism is a primary mechanism for cognitive impairment, and general metabolic dysfunction, as it can cause inflammation and oxidative stress within the brain, potentially leading to neurodegeneration. Therefore, complex carbohydrates with high fibre can improve glucose and insulin metabolism, which decreases inflammation and oxidative stress, and leads to improved brain aging as measured by the absence of disability, depression, chronic disease, and decreased cognitive decline.

Simple carbohydrates are associated with decreased global cognitive performance. Simple carbohydrates negatively impact many essential cognitive processes, including attention, memory, reaction time, visual-spatial processing, mental processing speed, and executive functions. Simple carbohydrates impair cognition through glucose and insulin metabolism dysfunction, as well as causing inflammation and oxidative stress within the brain. Therefore, excessive or chronic consumption of simple carbohydrates is unanimously linked to negative health consequences.

Through digestion dietary proteins are broken down into individual amino acids and absorbed into the blood. The essential amino acids tyrosine and tryptophan are precursors for the neurotransmitters dopamine, serotonin, and norepinephrine, and these chemicals modulate neural activity and influence cognitive functioning.

Dietary protein can improve cognition by increasing reaction time and inhibition control during mentally demanding and physically stressful situations, as tyrosine and tryptophan will replenish exhausted neurotransmitter levels. Additionally, adequate and consistent consumption of tyrosine and tryptophan correlates to improvements in memory function. Tyrosine is also shown to improve convergent thinking processes through increased cognitive control.