You may have heard the brain requires carbs in order to function. This can sound like a convincing argument not to go full keto. But is it true? Read on to learn why your brain keeps working when you stop eating carbs, or click on the links above to go right to a certain section.
Your brain needs a constant energy supply
Your brain is arguably the busiest organ in your body. It enables you to make decisions, read, speak, and perform hundreds of other actions instantly. In addition, it's responsible for several involuntary processes that are crucial for survival, including breathing, regulating body temperature, and secreting hormones. It serves as the headquarters of the central nervous system, receiving and sending messages throughout your body that allow you to do things like spot a car that suddenly pulls in front of you and then brake or swerve out of the way to avoid it.
It weighs only 2% of our body weight, yet our brains consume 20% of our daily energy. Studies show that two thirds of the brain's energy budget is used to help nerve cells "fire" or send signals. The remaining third is for "housekeeping," or cell maintenance.
In order to carry out these important functions, the brain requires a steady fuel supply. The brain can use two main fuels, glucose or ketones, both of which cross the blood-brain barrier. In people who eat a diet moderate to high in carbohydrates, the brain's main energy source is glucose. In people who eat a low carb-ketogenic diet, the brain's main energy source is ketones.
What happens when you don't eat any carbs?
It's estimated that when fueled by carbohydrates, the brain needs roughly 110-145 grams of glucose (from the breakdown of carbs you eat) per day in order to function optimally. Most people who follow a typical modern-day high carb diet eat roughly twice as many carbs as their brains use, providing them with an ample glucose supply.
What happens if you eat far fewer than 110 grams of carbs per day, or even no carbs at all? Does the brain starve? Absolutely not!
Your liver and muscles store glucose in the form of glycogen. Although the amount varies from person to person, an average-sized man weighing 154 lbs (70 kg) stores about 100 grams of glycogen in his liver.
When you stop eating carbs for several hours, liver glycogen is broken down into glucose and released into the bloodstream to prevent blood glucose from dropping too low. Although far more glycogen is stored in your muscles than in your liver, it remains in the muscles to meet their energy needs and cannot be released into the bloodstream to raise blood glucose.
After going 24-48 hours without any carbs, glycogen levels become depleted, and blood glucose and insulin levels decrease significantly.
At this point, the liver steps up its production of water- soluble compounds known as ketones, created by the breakdown of fatty acids. The source of ketones comes from either the fat you eat or the mobilization of body fat out of your fat stores. The resulting ketones can cross the blood-brain barrier to provide the brain with an alternative source of energy.
This means that there's another fuel source available for the brain, when the body runs low on stored carbohydrates.
Can your brain rely on ketones alone?
The brain always require some glucose. However, researchers have shown that when following a strict ketogenic diet or under conditions of fasting or starvation, ketones can be used to meet up to 70% of the brain's energy needs.
For the remainder of the brain's energy requirement, your liver can make all the glucose needed through a process known as gluconeogenesis (literally "making new glucose").
Compounds that the liver uses to synthesize glucose include:
- Amino acids from eating protein (or, under conditions of inadequate protein intake or periods of starvation, from muscle breakdown.)
- Glycerol (part of a triglyceride molecule) from the breakdown of body fat or dietary fat.
- Pyruvate and lactate, which are molecules created by the breakdown of glucose during energy metabolism that can be joined back together to re-create glucose.
It's clear that your brain can easily have its energy demands met by the liver, whether or not you eat any carbs.
Indeed, the US Food and Nutrition Board's 2005 textbook " Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids," stated that:
"The lower limit of dietary carbohydrate compatible with life apparently is zero, provided that adequate amounts of protein and fat are consumed."
Video interviews
You may already know the answer, but in this video a number of doctors answer the question of whether the brain needs carbohydrates:
With a free membership trial you can watch 12 more similar videos on different low-carb topics
Using glucose alone vs. glucose and ketones for brain fuel
If you eat a moderate-carb to high-carb diet, your brain isn't adapted to using ketones. Therefore, glucose will be the major fuel source for the brain at all times. When glucose levels fall, your brain will send out strong signals that it needs more glucose: hunger, irritability, dizziness, lightheadedness, confusion.
Once your body has adapted to eating a very-low-carb or carb-free diet, the brain easily uses ketones to meet a large portion of its energy needs, and the liver makes as much glucose as is needed to meet the remainder. Consequently, blood sugar levels remain stable even though carbs aren't being consumed, and symptoms of low blood sugar are avoided.
This makes a lot of sense from an evolutionary point of view. It's known that hunter-gatherers often went for several hours or even days without eating while searching for food. The ability to use a combination of ketones and glucose to fuel their brains was key to their survival.
Are ketones good for brain health and function?
Some experts believe that using a combination of ketones and glucose may be great for the brain, especially in people with neurologic and mental health disorders. Research suggests that in certain situations this combo could be quite beneficial. Let's take a look at some of these circumstances:
- Epilepsy: Although usually not completely devoid of carbs, the classical ketogenic and modified Atkins diet restrict carbs to less than 20 grams per day - providing well below the 100+ grams of glucose needed by the brain. Well-designed trials have shown that strict carb restriction can be very effective in reducing and in some cases eliminating seizures in children and adults.
- Mental health conditions: While research is very preliminary, anecdotal evidence as well as growing basic neurochemistry studies and a few promising clinical trials have suggested a ketogenic diet may provide some benefit in symptom control for some mental health conditions, particularly bipolar disorder, which research is showing shares a number of features with epilepsy.
- Traumatic brain injury (TBI): Trauma to the brain can impair its ability to use glucose efficiently and may lead to elevated blood sugar levels. According to some studies, a carb-free or ketogenic diet may provide an alternative fuel to the brain as it heals, thus giving benefits to people who have sustained TBI's, although the most promising results to date have been shown in animal research.
- Alzheimers: In Alzheimer's disease there is documented insulin resistance in the brain that hampers the uptake of glucose for fuel, so much so that some researchers have called Alzheimer's "Type 3 diabetes." It has been known since the early 1980s, through the use of PET scanning, that brain glucose metabolism is impaired by up to 40% in individuals with Alzheimer's and the problem shows up on imaging studies of the brain many years before cognitive problems begin to show.Studies have found, however, that while glucose uptake is impaired in early Alzheimer's, the brain's use of ketones for energy is not. Two recent notable clinical studies, one in 2012 and one in 2017, showed preliminary but promising results of using a ketogenic diet for people with Alzheimers. Two more clinical trials are now underway.
- Hunger control: A carbohydrate-free diet has been shown to suppress the "hunger hormone" ghrelin that is secreted mainly by the stomach. Ghrelin has multiple impacts in the body but one impact is to go to the brain's hypothalmus to regulate appetite control. It also goes to the amygdala, the brain's reward center. This means that in a body that is burning ketones, the brain is receiving and responding to reduced hunger signals, which is a benefit for weight loss18 and for diabetes control. Importantly, although considered high-quality research, these studies are very small. Nevertheless, they provide clinical evidence that helps confirm what many people report after adopting a carb-free diet -they feel much less hungry.