What kind of sugar does the brain use?

December 17, 2025 •

4 min read

Despite making up only 2% of the body's mass, the brain accounts for a remarkable 20% of the body's total energy expenditure at rest. This enormous and constant demand for energy is met almost exclusively by one specific type of sugar under normal physiological conditions: glucose.

Quick Summary

The brain relies primarily on glucose for its energy needs, but demonstrates metabolic flexibility by using ketone bodies as a highly efficient alternative fuel, especially during fasting or carbohydrate restriction.

Key Points

Primary Fuel Source: Under normal conditions, the brain relies almost exclusively on glucose for its energy needs.
Alternative Energy: When glucose is scarce, such as during prolonged fasting or ketogenic diets, the liver produces ketone bodies from fat, which the brain can use for energy.
Ketone Transport: The brain takes up ketones efficiently via specialized monocarboxylate transporters (MCTs) across the blood-brain barrier.
Minimal Storage: The brain has very limited glycogen reserves and is therefore highly dependent on a constant supply of fuel from the blood.
Fructose is Not Preferred: Fructose is not a primary brain fuel. Its metabolism is largely hepatic, and excessive intake can have negative impacts on brain health.
Metabolic Flexibility: The brain's ability to switch between glucose and ketones is a key adaptation for maintaining cognitive function across different metabolic states.

The Brain's Primary Fuel: Glucose

Under normal circumstances, the brain's main metabolic fuel is glucose. This is crucial for sustaining the high energy requirements of neuronal function, particularly for powering the ion pumps that maintain membrane potentials for communication. The brain's reliance on a steady supply of glucose is so profound that even a temporary reduction (hypoglycemia) can lead to impaired cognitive function, seizures, and unconsciousness.

How Glucose is Delivered to the Brain

Glucose cannot simply diffuse into the brain from the bloodstream. It must be actively transported across the blood-brain barrier (BBB), a protective layer of specialized cells that regulates what enters the brain.

The transport mechanism involves several key players:

GLUT1 Transporters: Abundant on the endothelial cells forming the BBB, these transporters are responsible for ferrying glucose from the blood into the brain tissue.
Astrocytes: These glial cells are wrapped around blood vessels and take up glucose via GLUT1. They can then break down this glucose into lactate and shuttle it to neurons, which can use it as an energy source.
GLUT3 Transporters: These high-affinity transporters are primarily found on neurons, ensuring they can efficiently take up glucose even when local concentrations are low.

The brain's ability to store glycogen is minimal, emphasizing its dependency on a continuous glucose supply from peripheral sources.

Alternative Brain Fuels: The Role of Ketones

While glucose is the favored fuel, the brain is not limited to it. During periods of low glucose availability, the body shifts its metabolism to produce alternative fuel sources, mainly ketone bodies.

What are Ketones?

Ketones are molecules produced in the liver from the breakdown of fatty acids. The three main ketone bodies are beta-hydroxybutyrate (BHB), acetoacetate, and acetone. This metabolic state, known as ketosis, occurs when carbohydrate intake is low or absent, forcing the body to burn fat for energy.

When and Why the Brain Uses Ketones

The brain readily uses ketones during specific metabolic conditions:

Prolonged Fasting: In the absence of food, circulating ketone levels rise, and the brain's energy metabolism can shift significantly, with ketones supplying up to 60% of its energy needs.
Ketogenic Diets: Deliberate carbohydrate restriction can induce ketosis. Studies have shown potential therapeutic applications for conditions like drug-resistant epilepsy and neurodegenerative diseases like Alzheimer's and Parkinson's.
Exertion: Following strenuous, prolonged exercise, the brain may also rely more heavily on ketones as a fuel source.

Ketones cross the blood-brain barrier via monocarboxylate transporters (MCTs), which are expressed in endothelial cells, astrocytes, and neurons. Their utilization offers a glucose-sparing effect, which can be advantageous during periods of low glucose availability.

The Misunderstood Role of Fructose

Unlike glucose, the brain's use of fructose is not significant. While fructose does enter the bloodstream after digestion, it is primarily metabolized by the liver. Excess fructose can eventually spill over and affect the brain, but it is not a primary fuel and can have negative effects, including neuroinflammation and mitochondrial dysfunction. Some transporters for fructose, like GLUT5, do exist in the brain, but its role remains limited compared to glucose.

Glucose vs. Ketones: A Comparison of Brain Fuels


Feature	Glucose	Ketones (e.g., BHB)
Primary Use	Yes, the brain's preferred and primary energy source.	No, used primarily as an alternative fuel when glucose is limited.
Availability	Derived from dietary carbohydrates and stored glycogen in the liver.	Produced by the liver from fatty acids during fasting or low-carb diets.
Transport	Crosses the blood-brain barrier (BBB) via GLUT1 and is absorbed by neurons via GLUT3.	Crosses the BBB via monocarboxylate transporters (MCTs).
Metabolic Flexibility	Essential for daily cognitive function and high-energy tasks.	Enables the brain to sustain function during prolonged fasting and food scarcity.
Cognitive Effects	Associated with optimal cognitive function when levels are stable; drops can cause 'brain fog'.	Can enhance mental clarity and provide a more stable energy supply, possibly due to neuroprotective properties.

The Metabolic Flexibility of the Brain

One of the brain's most remarkable features is its metabolic adaptability. While glucose is the default fuel, the ability to switch to ketones is a critical evolutionary backup mechanism to survive periods of food scarcity. This process is actively regulated, involving a complex interplay between the central nervous system, hormones, and peripheral organs. For example, the astrocyte-neuron lactate shuttle provides another pathway for energy exchange, where astrocytes can supply neurons with lactate derived from glucose for immediate energy use, especially during high neural activity.

The brain is not just a passive consumer of fuel. It contains specialized glucose-sensing neurons that monitor blood glucose levels and regulate responses to maintain energy homeostasis. These mechanisms ensure that regardless of the dietary conditions—whether fed on carbohydrates or fat—the brain has a reliable and continuous energy supply.

For a more detailed look at the metabolic actions of ketone bodies in the brain, particularly in the context of neurodegenerative diseases, see this comprehensive review.

Conclusion

In summary, the answer to what kind of sugar the brain uses is primarily glucose, but also, importantly, ketones as a robust alternative. Under normal dietary conditions, glucose is the undisputed king of brain fuel, necessary for maintaining the high-energy demands of cognitive function. However, during periods of caloric restriction or specific metabolic interventions, the brain can effectively utilize ketone bodies. This metabolic flexibility ensures the brain's survival and optimal function even when its preferred fuel is not readily available. While other carbohydrates like fructose may enter the brain, they are not used as primary fuel and can have detrimental effects when consumed in excess. Maintaining a balanced diet and metabolic health is therefore essential for providing the brain with the optimal energy it needs to function at its best.

Frequently Asked Questions

While glucose is the brain's preferred and primary fuel, it is not the only one. During conditions of low glucose availability, such as prolonged fasting or carbohydrate restriction, the brain can efficiently use ketone bodies as an alternative energy source.

The brain can metabolize a small amount of fructose, but it is not a preferred fuel source. Unlike glucose, excessive fructose consumption has been linked to negative effects on brain health, including reduced synaptic plasticity and altered metabolism.

Ketone bodies are produced by the liver from fatty acids. They can cross the blood-brain barrier using specific transport proteins (MCTs) and are then converted into acetyl-CoA, which enters the Krebs cycle to produce energy in brain cells, similar to glucose metabolism.

When blood sugar drops too low, the brain can't get enough fuel. This can cause cognitive impairment, 'brain fog', dizziness, and in severe cases, seizures or loss of consciousness, because neural communication is disrupted.

Some studies suggest that using ketones can offer neuroprotective benefits, improve cognitive function, and enhance overall brain metabolism, especially in conditions characterized by impaired glucose utilization, like certain neurodegenerative diseases.

Glucose is transported from the bloodstream across the blood-brain barrier primarily by GLUT1 transporters. It is then taken up by brain cells, including neurons and astrocytes, to fuel their high energy demands.

Yes, long-term overconsumption of sugar, particularly refined sugar, can lead to negative effects. It can contribute to neuroinflammation, damage blood vessels, impair memory, and increase the risk of certain neurological disorders.