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.
Your brain may make up only around 2% of your body weight, but it uses roughly 20% of your energy at rest.[1] That energy keeps billions of brain cells working, communicating and supporting everything from memory and concentration to mood and movement. Unlike some other parts of the body, the brain has very limited energy reserves of its own, so it relies on a steady supply of fuel.[2] Keeping that energy supply working well is therefore an important part of keeping your brain healthy.
Most of the time, your brain runs mainly on glucose. Glucose travels in your blood and crosses into the brain, where it can be used to make the energy your brain cells need.[2] But more glucose isn’t necessarily better. Regularly eating lots of sugar and refined carbohydrates can contribute to poor blood sugar control and insulin resistance.[3] The aim is not to deprive your brain of glucose, but to support a healthy, steady energy supply.
Most cells 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.[4] They can, however, use ketones, which are produced from fat in the liver and act as an alternative fuel.[5]
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. Learn more about how ketones affect the brain and what the research says about ketones and brain health.
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.[6]
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.[6] What matters is not just how much fuel is available, but whether the brain can access and use it effectively.
Your brain cannot simply burn fat for fuel in the same way that many other parts of your body can.[4] Instead, when fat is broken down, the liver can convert some of it into ketones. These ketones travel through the blood and cross into the brain, where they can be used as an alternative source of energy.[5,7] This is why saying that the brain “runs on fat” is only partly true. Much of the brain-fuel story is really about how fat can be turned into ketones.
Glucose: primary/default fuel under normal dietary conditions.[2]
Ketones: alternative fuel produced by the liver from fat, particularly during fasting, carbohydrate restriction or ketogenic diets.[7]
Fatty acids: unlike many tissues, neurons cannot simply rely on fatty acids directly for their energy needs.[4]
Lactate: another substrate the brain can use under certain conditions.[8]
For a deeper look at how the brain uses ketones, their role in cognitive ageing and the evidence around ketogenic diets, see our guide to ketones and brain health.
Glucose and ketones aren’t the whole story. Your brain can also use lactate as an energy source. Lactate is produced naturally in the body, particularly during exercise, and can be transported into the brain and used by brain cells for energy.[8] This is another reminder that the brain has a more flexible fuel system than the simple idea that it runs only on glucose might suggest.
Getting fuel into the brain is only the beginning. Brain cells still have to turn that fuel into usable energy, and much of this happens inside tiny structures called mitochondria. Often described as the powerhouses of our cells, mitochondria help convert fuels into ATP, the form of energy cells can actually use.[2] Healthy brain energy therefore depends not only on having enough fuel available, but also on how efficiently brain cells can turn that fuel into energy.
The brain has exceptionally high energy demands and needs a continuous supply of fuel to support memory, concentration, communication between neurons and other essential functions.[1,2] Under normal conditions, glucose provides most of this energy.
As we age, however, the brain may become less efficient at using glucose. Research has found reduced brain glucose metabolism in people with mild cognitive impairment (MCI) and Alzheimer’s disease, sometimes before significant cognitive symptoms become apparent.[6,9] This does not mean the brain is starved of glucose altogether, but that its ability to take up and use glucose for energy may decline.
This matters because brain cells need a reliable supply of energy to function properly. When glucose metabolism is impaired, a mismatch can develop between the brain’s energy requirements and the energy available from glucose.[6]
Importantly, glucose is not the brain’s only possible fuel. The brain can also use ketones, which are produced mainly by the liver from fat. Research suggests that the ageing brain may retain its ability to use ketones even when glucose metabolism is reduced.[9,10] This has led researchers to investigate whether ketones could provide an additional source of energy under these circumstances.[10,11]
Ketones are therefore an interesting part of the brain-energy picture, but they are not a simple replacement for glucose. Brain energy metabolism also depends on factors including mitochondrial function, blood flow, metabolic health and the brain’s ability to switch between available fuels.[6,12]
Supporting brain energy isn’t simply about choosing glucose or ketones. Your brain needs a reliable supply of fuel, good blood flow and healthy mitochondria to turn that fuel into energy. Regular exercise, good blood sugar control and a nutrient-rich diet can all help support healthy energy metabolism. Your metabolism is also designed to adapt to changes in fuel availability, using mainly glucose after carbohydrate-containing meals while making greater use of alternative fuels, including ketones, during fasting or carbohydrate restriction. This ability to switch between available fuels is sometimes called metabolic flexibility.[13]
Glucose is the brain’s main everyday fuel, but chronically high blood sugar and poor insulin sensitivity are associated with poorer metabolic health and changes in brain glucose metabolism.[3,14] Reducing added sugars and refined carbohydrates, while choosing fibre-rich vegetables, whole foods, healthy fats and good sources of protein, can help support more stable blood glucose levels.
Exercise supports brain energy metabolism in several ways. It improves insulin sensitivity and cardiovascular health, increases blood flow and can support mitochondrial function. During longer periods of activity, changes in fuel availability can also increase the brain’s access to alternative energy substrates, such as lactate.[8]
Mitochondria are the structures within cells that convert available fuels into ATP, the usable energy that powers cellular activity. Their function depends on adequate nutrition as well as wider metabolic health. A varied, nutrient-dense diet provides vitamins, minerals and other compounds involved in normal energy metabolism.
Constant eating is not necessary for maintaining the brain’s energy supply. Extending the natural overnight period without food can lower insulin levels and encourage the body to draw more heavily on stored energy. With longer periods without food, the liver also begins producing more ketones from fat.[5,7]
Fasting is not appropriate for everyone, however, particularly for people taking certain medications or with medical conditions that affect blood glucose regulation.
A healthy metabolism is able to adapt to changes in fuel availability. After eating carbohydrate, glucose is readily available. During exercise, overnight fasting or periods of lower carbohydrate intake, the contribution of other fuels can increase.[13]
This ability to shift between available energy sources is sometimes described as metabolic flexibility. Rather than viewing glucose and ketones as competing fuels, it may be more useful to think about the brain as part of a flexible energy system that responds to diet, activity and metabolic state.
Ketones provide the brain with an alternative energy source when their concentration in the blood rises. This can occur during fasting, carbohydrate restriction or a ketogenic diet, and ketone availability can also be increased by consuming certain medium-chain triglycerides (MCTs).[7,15]
There is growing interest in whether this alternative fuel could be particularly useful when brain glucose metabolism becomes less efficient.[10,11,12] However, increasing ketones is only one aspect of supporting brain energy and does not mean that the brain no longer needs glucose.[5]
For a deeper look at how the brain uses ketones and what research suggests about cognition and the ageing brain, read our guide to ketones and brain health.
Supporting healthy brain energy metabolism is about more than choosing between glucose and ketones. The brain needs a reliable supply of fuel, good blood flow and healthy mitochondria to produce the energy neurons need. Regular physical activity, good metabolic health and a diet based on nutrient-dense whole foods can all support this process. The brain can also adapt to changes in fuel availability, using mainly glucose under normal conditions while making greater use of alternative fuels such as ketones during fasting, carbohydrate restriction or a ketogenic diet
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.[16] 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.[16] 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.[17] The body’s management of glucose and insulin directly influences whether it can deliver and use fuel effectively in the first place.[14]
There probably isn’t one “best” brain fuel. Most of the time, glucose provides the majority of the brain’s energy, while ketones become a more important alternative fuel when their levels rise.[5] Lactate can also contribute to the brain’s energy supply.[8] Rather than thinking of one fuel as good and another as bad, a healthier way to think about brain energy is flexibility: maintaining the metabolic health that allows your brain to access and use the fuels available to it.
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.[6] 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] Raichle ME, Gusnard DA. Appraising the brain’s energy budget. Proc Natl Acad Sci USA. 2002;99(16):10237–10239.
[2] Magistretti PJ, Allaman I. A cellular perspective on brain energy metabolism and functional imaging. Neuron. 2015;86(4):883–901.
[3] Stanhope KL. Sugar consumption, metabolic disease and obesity: the state of the controversy. Crit Rev Clin Lab Sci. 2016;53(1):52–67.
[4] Schönfeld P, Reiser G. Why does brain metabolism not favor burning of fatty acids to provide energy? Reflections on disadvantages of the use of free fatty acids as fuel for brain. J Cereb Blood Flow Metab. 2013;33(10):1493–1499.
[5] Owen OE, Morgan AP, Kemp HG, Sullivan JM, Herrera MG, Cahill GF Jr. Brain metabolism during fasting. J Clin Invest. 1967;46(10):1589–1595.
[6] Cunnane SC, Trushina E, Morland C, et al. Brain energy rescue: an emerging therapeutic concept for neurodegenerative disorders of ageing. Nat Rev Drug Discov. 2020;19(9):609–633.
[7] Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1–22.
[8] van Hall G, Strømstad M, Rasmussen P, et al. Blood lactate is an important energy source for the human brain. J Cereb Blood Flow Metab. 2009;29(6):1121–1129.
[9] Castellano CA, Nugent S, Paquet N, et al. Lower brain 18F-fluorodeoxyglucose uptake but normal 11C-acetoacetate metabolism in mild Alzheimer’s disease dementia. J Alzheimers Dis. 2015;43(4):1343–1353.
[10] 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.
[11] 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.
[12] 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.
[13] Goodpaster BH, Sparks LM. Metabolic flexibility in health and disease. Cell Metab. 2017;25(5):1027–1036.
[14] Willette AA, Bendlin BB, Starks EJ, et al. Association of insulin resistance with cerebral glucose uptake in late middle-aged adults at risk for Alzheimer disease. JAMA Neurol. 2015;72(9):1013–1020.
[15] 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.
[16] Dyall SC. Long-chain omega-3 fatty acids and the brain: a review of the independent and shared effects of EPA, DPA and DHA. Front Aging Neurosci. 2015;7:52.
[17] Smith AD, Refsum H, Bottiglieri T, et al. Homocysteine and dementia: an international consensus statement. J Alzheimers Dis. 2018;62(2):561–570.
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