Now that we know the brain needs sugar and carbs to function, it’s increasingly important to consider the impact of sugar and carbs on the brain and mental health.

The idea that the brain needs sugar and carbohydrates for normal function is, in my opinion, a common myth, perpetuated by the food industry. Then why do we continue to consume a diet that is high in sugar and carbohydrates? The idea that the brain needs sugar and carbohydrates to function is one of the most commonly cited reasons to maintain a food product or diet.

From , medical opinion from – Updated 17. June 2023

You may have heard that carbohydrates are necessary for brain function. This may seem like a compelling argument not to embark on a full keto diet. But is it true? Read on to find out why your brain keeps working when you stop eating carbs, or click on the links above to go to a specific section.

Your brain needs a constant source of energy

Your brain is probably the busiest organ in your body. It allows you to make instant decisions, read, talk and do hundreds of other things.

It is also responsible for several involuntary processes that are important for survival, such as breathing, regulation of body temperature and secretion of hormones. It acts as the hub of the central nervous system, receiving and sending messages throughout the body that enable you to do things such as… For example, you see a car suddenly pass in front of you and you brake or move off the road to avoid it.

Although the brain accounts for only 2% of body weight, it consumes 20% of daily energy.

To perform their important functions, the brain needs a constant supply of fuel. The brain can use two fuels: glucose or ketones, both of which cross the blood-brain barrier.

In people following a moderate to high carbohydrate diet, glucose is the main source of energy for the brain. In people who follow a low-carb ketogenic diet, the brain can use ketone bodies to meet most of their energy needs.

What happens when you don’t eat carbs?

It is estimated that on a high-carbohydrate diet, the brain needs about 110 to 145 grams of glucose (formed by the breakdown of the carbohydrates you eat) per day to function optimally. Most people on a typical modern high-carbohydrate diet consume about twice as much carbohydrate as their brains use to supply enough glucose.

What happens if you eat much less than 110 grams of carbs a day, or no carbs at all? Is the brain starving? Certainly not!

Your liver and muscles store glucose as glycogen. Although this amount varies from person to person, the average 70 kg (154 lb) person stores about 100 grams of glycogen in the liver.

When you don’t eat carbs for a few hours, glycogen is broken down into glucose in the liver and released into the bloodstream to prevent blood sugar from getting too low. Although much more glycogen is stored in the muscles than in the liver, it remains in the muscles to meet energy needs and cannot be released into the bloodstream to raise blood sugar levels.

After 24-48 hours without carbohydrates, glycogen stores are depleted and insulin levels drop (this happens much faster during strenuous exercise).

At this stage, the liver increases production of water-soluble compounds called ketones, which are formed when fatty acids are broken down. Ketones can come from the fat you eat or from your body’s fat reserves. The ketones produced can cross the blood-brain barrier and provide the brain with an additional source of energy.

This means that the brain has another source of fuel when the body runs out of stored carbohydrates.

Can your brain rely on ketones alone?

The brain always needs a certain amount of glucose. However, researchers have shown that in some people who follow a strict ketogenic diet, ketones can provide up to 70% of the brain’s energy needs.

To meet your brain’s remaining energy needs, your liver can make all the glucose it needs in a process called gluconeogenesis (literally, creating new glucose).

The compounds that the liver uses for the synthesis of glucose are the following:

  • Amino acids obtained from protein foods (or from muscle breakdown in the case of insufficient protein intake or during periods of starvation).
  • Glycerol (part of a triglyceride molecule), derived from the breakdown of body fat or fat from food.
  • Pyruvate and lactate are molecules created when glucose is metabolized in the body. These molecules can be linked together to form glucose again.

So all your brain’s energy needs can be met by your liver via stored glucose, gluconeogenesis or ketone production, whether you consume carbohydrates or not.

In fact, the 2005 American Council on Food and Nutrition handbook on dietary standards for energy, carbohydrates, fiber, fatty acids, cholesterol, protein, and amino acids states the following

The lower limit of life-compatible carbohydrates in the diet is, of course, zero, provided that one eats enough protein and fat.

Video maintenance

You may already know the answer, but in this video, several doctors answer the question of whether the brain needs carbohydrates:


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Use of glucose alone or glucose and ketones as brain fuel

If you follow a moderate or high-carb diet, your brain is not adapted to use ketones. Therefore, glucose will be the main source of fuel for the brain.

Once the body gets used to a diet with very little or no carbohydrates, the brain uses ketones to provide much of its energy needs, and the liver produces as much glucose as it needs to provide the rest. This keeps blood sugar levels stable and gives the brain the fuel it needs, even without carbohydrates.

From an evolutionary point of view, this makes perfect sense. It is known that hunter-gatherers, in search of food, often did not eat for hours, even days. The ability to use a combination of ketones and glucose to fuel the brain was probably the key to their survival.

Are ketones good for brain health and function?

Some experts believe that the combination of ketones and glucose may be particularly beneficial to the brain, especially in people with neurological and mental health conditions. Research shows that this combination can be very useful in certain situations.

Let’s look at some of these circumstances:

  • Epilepsy: Although the classic ketogenic diet and the modified Atkins diet are generally not completely carb-free, they limit carbs to less than 20 grams per day. Well-designed studies have shown that this form of carbohydrate restriction can be very effective in reducing – and in some cases eliminating – seizures in children and adults.
  • Mental illness: Despite preliminary research, anecdotal evidence, basic neurochemical research and some promising clinical trials, it is believed that a ketogenic diet may better control the symptoms of some mental illnesses.
    For example, bipolar disorder, which is known to share certain characteristics with epilepsy, may improve with a ketogenic diet, as may schizophrenia. For more information, check out our guide on low-carb diets and mental health.
  • Traumatic Brain Injury (TBI): Trauma to the brain can impair the brain’s ability to use glucose efficiently and lead to hyperglycemia.
    According to some studies, a carbohydrate-free or ketogenic diet can serve as an alternative fuel for the brain during the healing process, which has benefits for people who have suffered brain injuries. However, the most promising results to date have been obtained in animal experiments.
  • Alzheimer’s: Alzheimer’s disease involves insulin resistance in the brain, which prevents it from absorbing glucose for fuel, so much so that some researchers refer to Alzheimer’s disease as type 3 diabetes. Since the early 1980s, the use of PET scans has shown that glucose metabolism in the brain is altered by 40% in people with Alzheimer’s disease, a problem that is revealed on brain imaging several years before the onset of cognitive impairment.

However, studies have shown that in the early stages of Alzheimer’s disease, glucose uptake is impaired, but not the brain’s use of ketones for energy. Clinical studies, including a meta-analysis of intervention studies, have shown preliminary but promising results with a ketogenic diet for people with Alzheimer’s disease.

  • Help against hunger: A diet without carbohydrates suppresses the hunger hormone ghrelin, which is released primarily by the stomach. Ghrelin has many effects on the body, but one of them is its action on the brain’s hypothalamus to regulate appetite. It also goes to the amygdala, the reward center of the brain.
    This means that the brain receives fewer hunger signals in a ketone body, which can improve weight loss and diabetes control.It is important to note that while these studies are considered high quality, they are very small. However, they do provide clinical evidence that confirms what many people find after following a carbohydrate-free diet: they are much less hungry.


In a nutshell: Eating carbs to fuel your brain is an option, not a requirement.

It is true that the brain cannot function on ketones alone; it also needs some glucose. However, your brain is not threatened by a very low-carb diet, or even a diet without carbs. Through gluconeogenesis, your body reliably produces and delivers the glucose your brain needs.

/ Franziska Spritzler, MD

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Frequently Asked Questions

Does low-carb diet affect brain function?

A low-carb diet can have a negative impact on brain function. This is because the brain relies on glucose for fuel, and when there is not enough glucose in the blood, it can lead to a lack of energy and mental fatigue. What are the benefits of a low-carb diet? A low-carb diet can help with weight loss, improve mood, and reduce inflammation.

What happens to your brain without carbs?

Without carbs, your brain will use ketones as its primary fuel source. Ketones are produced when the body breaks down fat for energy. This is a good thing because ketones are a more efficient fuel source than glucose (sugar). Ketogenic diets have been shown to improve cognitive function in people with epilepsy and Alzheimer’s disease. What happens to your body without carbs? Without carbs, your body will use ketones as its primary fuel source. Without carbs, your brain will use ketones as its primary fuel source.

How many carbs a day does your brain need to function?

The brain needs about 20 grams of carbs per day.

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