What is the Best Fuel for the Brain, Ketones or Glucose?

What is the Best Fuel for the Brain, Ketones or Glucose?

The majority of the doctors and nutritionists are totally convinced that the brain feeds on glucose exclusively, and this is not true, as it also feeds on ketones and lactate.

In this post about what is the best fuel for the brain: glucose or ketones, we analyse the benefits and disadvantages of each.

Glucose: fuel of the brain

Your brain is one of the most important organs that you have, and it is responsible for many of the important functions that process sensory information, learn new things, move you or allow you to feel emotions.

All of our learning, memories, and what makes us ourselves, capable of good and bad, resides in the brain…

And all of this makes it consume more than 20% of the daily calories of our entire body and that only represents 2% of our weight.

Obtaining glucose

Carbohydrates from the foods we eat are converted to glucose, which is energy that organs can use immediately, allowing intense workouts and serving as brain fuel.


Other names for glucose include dextrose, starch or glycogen.

Glucose is the main form of glucose storage in our body, and is stored in the liver and muscles.

Every time we need energy the liver releases it by transforming it into glucose to be used by all the cells of our body.

In modern life and with today’s diet, full of foods rich in carbohydrates, the brain is perfectly fed with glucose, which is its main energy.

What happens when you can’t eat carbohydrates?

Carbohydrates are not entirely bad, in fact the brain prefers sugar as the main fuel, and consumes about 120 grams of glucose per day (1).

This carbohydrate-filled diet ensures your glucose dose perfectly, but what if we can’t access food for a long time?

«It’s okay, whether we don’t eat carbohydrates because we don’t have access to them and we have to fast or we decide not to eat them, our body goes into ketosis and feeds our brain mainly with ketones instead of glucose.»

Fatty foods

Ketones: the alternative fuel

When we do a prolonged fast, because:

  • Of our own decision; or
  • We don’t have access to food; or
  • We feed on a diet very low in carbohydrates (ketogenic diet).
Glycogen reserves (very limited, about 400-500 grams) of our body (liver and muscles) are depleted.

When this happens, the liver produces ketones from the fatty acids you eat or from the fat stored in your adipose tissue, this is what is known as ketogenesis (entering ketosis).

There are organs (2) that use these ketones as alternative fuel: brain and the heart or the skeletal muscle.

It is an advantage inherited from our own evolution, as in the past there was no unlimited access to food as there is today, there was no supermarket and even hunting was not always fruitful.

Types of ketones

There are three types:

  • Acetoacetate: is the first ketone body produced by the liver, which is then reduced to beta-hydroxybutyrate (another type). Interestingly, it appears (3) that immediately after birth acetoacetate and beta-hydroxybutyrate (rather than glucose) are preferred for brain growth.
  • Beta-hydroxybutyrate: is formed from acetoacetate, constitutes 78% of the ketone bodies generated, and is a very efficient carrier of energy, even more so than glucose (4).
  • Acetone: is the least abundant (2% of ketones in the bloodstream) and is the result of the breakdown of acetoacetate


Energy source of the brain

While we recognize glucose as the main source of energy in the brain, the brain can also use ketones perfectly, mainly beta-hydroxybutyrate and acetoacetate.

This occurs when glycogen reserves are very low, or depleted, then the body metabolizes stored fat by converting it into fatty acids, which can be broken down into ketones that are capable of crossing the blood-brain barrier.


One reason why there is the myth that the brain can only work with glucose is that there are cells in our body that do not have mitochondria (cell energy centres), such as red blood cells and some small brain cells (they are very small and have no room for mitochondria).

These cells require glucose and cannot metabolise fats.

70% of brain requirements can be met using beta-hydroxybutyrate as the primary energy.

Meanwhile the rest of the mitochondrial-free cells would feed on glucose, which the body itself can create, without the need to eat carbohydrates, by gluconeogenesis (either from proteins or glycerol released from fatty acid metabolization).

Ketones vs glucose

Glucose vs Ketones

Let’s evaluate the benefits and downsides of both, in relation to the brain:

Benefits of glucose

Improves the attention span of children

A 1987 study (5) examined 60 children aged six and seven, giving them a drink containing 25 grams of glucose or a placebo.

The results showed that those who took the glucose-filled drink could hold their attention for longer.

Helps perform challenging mental tasks

The brain spends about 11kcal/h to function, but if the task is complex, this increases its energy need, so scientists believe (6) that eating sugary foods should improve the performance of these tasks.

Benefits of ketones

Improved cognitive performance

This was part of the conclusions of a meta-analysis (7), where it was seen during a maze test that the brain easily uses ketone bodies instead of glucose and that a ketogenic diet increases the PCR of the human brain, as observed using MR spectroscopy.

Keep your neurons alive

One study (8) examined offspring of postnatal rats, one group was given a standard diet while the others were fed a ketogenic diet.

After five days, they induced hypoglycaemia through insulin injection and found that the rats receiving the ketogenic diet showed less neural loss from hypoglycaemia.

Benefit to brain trauma patients

Brain trauma is caused by a severe blow to the head that damages brain tissue.

The injured brain requires energy to recover but cannot effectively metabolize glucose (9). A ketogenic diet in patients with brain trauma offers (10) an alternative fuel for the brain in the form of ketones.

Improvements in those with Alzheimer’s

A pioneering study (11) indicates that, with some recommendations and support at home, people with mild Alzheimer’s can improve brain function by raising ketone levels in the blood.

Downsides of glucose

Sugar can create addiction

Research (12) explored the influence of sugar on obesity, concluding that sugar is an addictive substance as it increases the reward circuit of the brain, which is related to addiction.

The regular intake of sugar increases the risk of depression

One study (13) concluded that sugar is on the list of factors that may contribute to depression, as too much sugar, especially sugar found in soft drinks, juices and cakes, decreases BDNF (brain-derived neurotrophic factor), a protein that maintains and promotes the growth of neurons.

Downsides of glucose

Downsides of ketones

Possible side effects during keto adaptation

Not everyone has these temporary side effect but it is common occurrence, because as the brain adapts to ketones it may suffer from: headaches, light-headedness, difficulty concentrating, dizziness, and nausea.

This is due to the loss of water from lack of carbohydrates in the diet, which means that electrolytes are lost, so it is important to hydrate and replace lost electrolytes.


Glucose is essential, that’s undeniable.

But we don’t need to eat it, because our body can meet the needs of that group of brain cells (less than 30%) and red blood cells that don’t have mitochondria using glucose because they cannot use ketones or fatty acids as a source of energy.

However, sugar is associated, as we have seen, with problems such as depression and obesity, and too much glucose in the blood does more harm than good.

According to previous studies, ketones are a better source of fuel, as beta-hydroxybutyrate acts as a more efficient fuel than glucose.

There is growing (current) research that shows ketones can benefit the brain in both healthy people and people with Alzheimer’s, epilepsy, traumatic brain injury or Parkinson’s

Bibliography References

  1. Each Organ Has a Unique Metabolic Profile – Berg JM, Tymoczko JL, Stryer L. (2002).
  2. Multi-dimensional roles of ketone bodies in fuel metabolism, signaling, and therapeutics – Puchalska P, Crawford PA (2017).
  3. Preferential Utilization of Ketone Bodies in the Brain and Lung of Newborn Rats – Y Y Yeh, P M SheehanY Y Yeh, P M Sheehan (1985).
  4. Clinical review: Ketones and brain injury – Hayden White and Balasubramanian Venkatesh (2011).
  5. Glucose improves attention and reaction to frustration in children – David Benton, Veronica Brett, Paul F.Brain (1987).
  6. Glycemic Instability and Spontaneous Energy Intake: Association With Knowledge-Based Work – Jean-Philippe Chaput , Vicky Drapeau, Paul Poirier, Normand Teasdale, Angelo Tremblay (2008).
  7. Novel ketone diet enhances physical and cognitive performance – Andrew J. Murray,*†,1,2 Nicholas S. Knight,*,1 Mark A. Cole,‡,3 Lowri E. Cochlin,* Emma Carter,* Kirill Tchabanenko,‡,4 Tica Pichulik,* Melanie K. Gulston,§¶ Helen J. Atherton,* Marie A. Schroeder,* Robert M. J. Deacon,‖ Yoshihiro Kashiwaya,# M. Todd King,# Robert Pawlosky,# J. Nicholas P. Rawlins,‖ Damian J. Tyler,* Julian L. Griffin,§¶ Jeremy Robertson,‡ Richard L. Veech,# and Kieran Clarke* (2016).
  8. Ketones Keep Neurons Alive– Carl E Stafstrom (2006).
  9. Glucose metabolism following human traumatic brain injury: methods of assessment and pathophysiological findings – Ibrahim Jalloh,corresponding author Keri L. H. Carpenter, Adel Helmy, T. Adrian Carpenter, David K. Menon, and Peter J. Hutchinson (2015).
  10. The Ketogenic Diet as a Treatment for Traumatic Brain Injury: A Scoping Review – Alexandre McDougall 1, Mark Bayley 2, Sarah Ep Munce (2018).
  11. Feasibility and efficacy data from a ketogenic diet intervention in Alzheimer’s disease – Author links open overlay panelMatthew K.TaylorabDebra K.SullivanabJonathan D.MahnkenacJeffrey M.BurnsadeRussell H.Swerdlowadef (2018).
  12. Sugar Addiction: From Evolution to Revolution – Wiss DA, Avena N, Rada P. (2018).

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Review of Fuel for the brain

Getting glucose - 100%

What happens if we don’t eat carbohydrates? - 100%

Myth of glucose - 100%

Glucose VS Ketones - 100%

Conclusions - 100%


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About David Diaz Gil
David Diaz Gil
David Díaz Gil contributes with excellent articles in which he deposits the essence of his experience as well as scientific rigor.
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