What fuels brain function is something most people don’t really think about until later in life, when concerns around memory loss, cognitive decline, dementia, and overall brain health begin to surface.
However, long before that point, the brain can begin to show subtle signs that something is not working as it should. It may start with small changes that are easy to overlook. You might notice dips in energy, moments of brain fog, or difficulty concentrating. There can also be shifts in mood, all of which reflect how well the brain is being fuelled.
These changes are not always random. They often relate to what the brain is actually using for energy and how effectively it can access that fuel. Many people ask what fuels the brain, whether it needs fat, and whether it prefers glucose or ketones. The reality is more nuanced than a simple either-or answer.
Every cell in the body can use either glucose, derived from carbohydrates, or fatty acids as a source of energy. The brain operates under slightly different constraints. Neurons rely primarily on glucose and cannot efficiently use fatty acids directly. They can, however, use ketones, which are produced from fat in the liver and act as an alternative fuel.
The brain can run on either glucose or ketones as fuel. While glucose is the default fuel, neurons can also use ketones, which are produced from fat in the liver.
Under normal conditions, glucose supplies most of the brain’s energy; yet this system is not as robust as we might assume. In many people, particularly with age or ongoing metabolic stress, the brain’s ability to use glucose efficiently can begin to decline.
This is closely linked to how the body handles blood sugar over time, as explored in how sugar affects brain health.
Importantly, this can occur years before any diagnosis or clearly recognisable symptoms. What matters is not just how much fuel is available, but whether the brain can access and use it effectively.
Ketones are a natural alternative fuel that the liver produces from fat. The body typically generates them when it shifts away from relying solely on carbohydrates, such as during periods of fasting or lower carbohydrate intake.
While the brain usually depends on glucose, it is not limited to it. When ketones are available, the brain uses them efficiently alongside glucose, providing an additional and often valuable source of energy.
This metabolic flexibility is not unusual or extreme. In fact, it is most clearly seen at the very beginning of life, when a newborn’s brain has exceptionally high energy demands and uses a far greater proportion of total energy intake than at any other stage. During this period, ketones provide a significant share of that fuel, supporting the rapid growth and formation of neural connections [9].
During early development, the brain relies heavily on ketones as a fuel source, which are supplied through breast milk and support rapid brain growth.
Seen in this light, the brain is not designed to depend on a single fuel source but to adapt to what is available and how well each system functions. This becomes particularly relevant later in life, where in many individuals, especially in the context of cognitive decline, the brain’s ability to use glucose begins to reduce, while its ability to use ketones often remains relatively preserved.
In this context, ketones are not replacing glucose entirely but helping to fill a gap by providing energy where it is still needed and accessible.
If ketones are simply an alternative fuel, the natural question is whether they make a meaningful difference in the brain.
A series of studies from Professor Stephen Cunnane and colleagues at the University of Sherbrooke have explored this directly. In people with Alzheimer’s disease, they observed that while the brain’s use of glucose was reduced, its ability to take up and use ketones remained intact. When ketogenic medium-chain triglycerides were provided, overall brain energy metabolism increased, driven by a significant rise in ketone use [2].
Research has shown that increasing ketone availability can improve brain energy metabolism and cognitive function, particularly in those with mild cognitive impairment.
This distinction is important, as it suggests that the issue is not a lack of available fuel, but rather a reduced ability of the brain to use glucose effectively.
In a further randomised controlled trial involving individuals with mild cognitive impairment, a ketogenic drink was provided daily over a six-month period. Those receiving the intervention showed improvements across several areas of cognitive function, including memory, executive function, and processing speed, with these changes closely linked to increased uptake of ketones in the brain [3].
In Parkinson’s disease, a ketogenic approach has improved symptoms, with one study reporting a 41% reduction in symptom severity.
In practical terms, when the brain has access to an additional fuel source, aspects of cognitive performance improve. These findings support a broader shift in how we understand early cognitive decline.
The brain’s ability to function well depends not only on the availability of fuel, but also on how efficiently that fuel can be converted into usable energy.
Within each neuron are mitochondria, often described as the cell’s energy factories, which convert fuel into ATP, the form of energy that powers everything from neurotransmitter production to cellular repair. Ketones appear to offer certain advantages at this level, as they can be converted into energy efficiently and may place less metabolic strain on the system than glucose, particularly when glucose metabolism is impaired [7].
They may also influence key signalling pathways in the brain. Research suggests that ketogenic states can help shift the balance between excitatory and inhibitory neurotransmitters, supporting the production of GABA while reducing excessive glutamate activity. This balance is important for maintaining stable brain function and may help explain the long-standing use of ketogenic diets in epilepsy [7]. There is also evidence that ketones may support mitochondrial function more directly, enhancing energy production and resilience within brain cells [8].
Taken together, these effects suggest that ketones do more than act as an alternative fuel. They may also influence how the brain produces and manages energy at a cellular level.
While the body can produce ketones naturally, there are ways to increase their availability without adopting extreme dietary changes.
Researchers have studied medium-chain triglycerides (MCTs) extensively. The liver converts these fats into ketones more readily than longer-chain fats. Because the body absorbs and metabolises MCTs more quickly, they provide an efficient way to raise circulating ketone levels.
The liver rapidly converts medium chain triglycerides (MCTs), especially caprylic acid (C8), into ketones, creating a quick source of brain fuel.
Not all MCTs have the same effect. Research shows that caprylic acid (often referred to as C8) produces a greater increase in circulating ketones compared to other MCTs or coconut oil [1].
At this point, many people assume that the logical next step is to follow a strict ketogenic diet.
While this approach can increase ketone production, it is not the only way to support the brain’s energy supply, nor is it necessary or appropriate for everyone. Patrick Holford, our founder, explores this in more detail in The Hybrid Diet.
It is also important to recognise that ketogenic diets are not simply a trend. Clinicians have used them therapeutically for over a century to treat epilepsy, where they can be highly effective at reducing seizures, particularly in individuals who do not respond to medication [5][6].
More recently, researchers have been actively studying ketogenic approaches across a range of neurological and psychiatric conditions. These include Alzheimer’s disease, bipolar disorder, and schizophrenia. Early findings suggest there may be benefits for supporting brain energy metabolism.
In individuals who already have diagnosed cognitive impairment or specific clinical conditions, a well-formulated ketogenic diet can be a powerful therapeutic tool. It is most effective when used appropriately and, ideally, under professional guidance.
However, the key insight is not that the brain needs to run exclusively on ketones. Rather, it benefits from metabolic flexibility. This means having the ability to use both glucose and ketones efficiently, depending on the situation.
This reflects more than carbohydrate intake alone; it depends on how well the body regulates blood sugar, the quality of fats in the diet, and the availability of nutrients involved in energy production.
For some individuals, a ketogenic approach can be helpful, particularly in a clinical setting. For many others, a wholefood diet rich in fibre, protein, healthy fats, and unprocessed carbohydrates is sufficient to support stable energy levels and metabolic flexibility.
The goal is not to force the body into a single metabolic state, but to restore the conditions under which the brain can access and use fuel effectively.
Understanding how the brain is fuelled gives us a more useful way to think about long-term cognitive health. It also helps move us away from rigid dietary thinking.
Ketones are one part of that picture, but they are not the only factor that matters. The brain depends on a steady and reliable supply of energy. This is supported by stable blood sugar regulation and adequate nutrient levels. It also relies on the right balance of fats to maintain both structure and function. Restorative processes such as sleep are equally important.
It also raises a common question around how much fat the brain actually needs each day. There is no single fixed requirement. However, the brain depends on a consistent supply of essential fats, particularly omega-3 fatty acids, to maintain its structure and function. The focus is therefore less on quantity alone, and more on the quality and balance of fats within the diet.
For example, omega-3 fatty acids play a critical role in maintaining the integrity of brain cell membranes and supporting communication between neurons. B vitamins are essential for processes such as methylation, which help regulate homocysteine. Homocysteine is a marker that is consistently linked to cognitive decline when elevated. The body’s management of glucose and insulin directly influences whether it can deliver and use fuel effectively in the first place.
Rather than focusing on a single nutrient or dietary approach, the emphasis shifts towards understanding how these systems work together. One of the challenges is that changes in brain energy metabolism can begin long before symptoms become obvious. You may notice subtle shifts in energy, focus, or mood, but these are often easy to overlook or attribute to other factors.
Recent advances now allow us to measure many of these underlying processes
The DRIfT 5-in-1 test looks at key markers linked to brain health, including omega-3 status and indicators of metabolic function. It helps provide a clearer picture of how well your brain is supported. It also shows where to focus first, based on your own biology rather than general advice.
Protecting brain health is not about reacting once problems appear, but about understanding what is happening early enough to do something about it.
[1] Vandenberghe C, St-Pierre V, Pierotti T, et al. Tricaprylin alone increases plasma ketone response more than coconut oil or other medium-chain triglycerides: an acute crossover study in healthy adults. Curr Dev Nutr. 2017;1(4):e000257.
[2] Croteau E, Castellano CA, Richard MA, et al. Ketogenic medium chain triglycerides increase brain energy metabolism in Alzheimer’s disease. J Alzheimers Dis. 2018;64(2):551–561.
[3] Fortier M, Castellano CA, St-Pierre V, et al. A ketogenic drink improves cognition in mild cognitive impairment: results of a 6-month RCT. Alzheimers Dement. 2021;17(3):543–552.
[4] Phillips M, Murtagh DKJ, Gilbertson LJ, et al. Low-fat versus ketogenic diet in Parkinson’s disease: a pilot randomised controlled trial. Movement Disorders. 2018.
[5] Neal EG, Chaffe H, Schwartz RH, et al. The ketogenic diet for the treatment of childhood epilepsy: a randomised controlled trial. Lancet Neurol. 2008;7(6):500–506.
[6] Lambrechts DAJE, de Kinderen RJA, Vles JSH, et al. A randomized controlled trial of the ketogenic diet in refractory childhood epilepsy. Acta Neurol Scand. 2017;135(2):231–239.
[7] Augustin K, Khabbush A, Williams S, et al. Mechanisms of action for the medium-chain triglyceride ketogenic diet in neurological and metabolic disorders. Lancet Neurol. 2018;17(1):84–93.
[8] Jensen NJ, Wodschow HZ, Nilsson M, Rungby J. Effects of ketone bodies on brain metabolism and function in neurodegenerative diseases. Int J Mol Sci. 2020;21(22):8767.
[9] Bougneres PF, Lemmel C, Ferré P, Bier DM. Ketone body transport in the human neonate and infant. J Clin Invest. 1986;77(1):42–48.
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