Your brain makes up about 2% of your body weight but uses roughly 20% of its energy. It never really rests. It keeps working while you sleep, while you skip breakfast and while you push through the last mile of a long hike. So what happens when its usual fuel, glucose, runs low? It has a backup: ketones.
This guide explains, in plain words, how your brain switches between these two fuels, what the human studies show about that switch, and why scientists who study brain and nerve injuries are so interested in it.
Why we write about this
Audacious Nutrition was founded on the science of ketones as brain fuel. Our founder, Dr. Csilla Ari, is a neuroscientist who studies ketones and the nervous system. Understanding how the switch works makes the research we share easier to follow, and it helps you spot headlines that promise more than the science shows.
Glucose: the everyday fuel
Most of the time, your brain burns glucose from the carbohydrates you eat or from the glucose your liver stores and releases. That works well as long as supply is steady.
Think about a normal day. You eat breakfast, carbohydrates are broken down into glucose, and glucose travels in your blood to your brain. Between meals and overnight, your liver releases stored glucose to keep the supply going. Your brain does not store much fuel of its own, so it depends on this steady delivery.
That is a strength and a weakness. Glucose is quick and reliable, but the liver's stores are limited. If you go many hours without carbohydrates, your body needs another plan.
Ketones: the backup fuel
When glucose is scarce, during fasting, a ketogenic diet or long exercise, your liver turns fat into ketone bodies, mainly beta-hydroxybutyrate (BHB). A ketogenic diet is a very low-carbohydrate, high-fat way of eating that keeps the body making ketones. Learn what BHB is.
Here is the basic path:
- The liver releases BHB into the blood.
- Special carriers called monocarboxylate transporters (MCTs) move BHB across the blood-brain barrier, the tightly sealed lining of the brain's blood vessels that controls what gets in.
- Brain cells convert BHB into acetyl-CoA, a small molecule that enters the mitochondria, the cell's power plants.
- The mitochondria turn it into ATP, the energy currency cells run on.
A simple way to picture it: glucose and BHB are two different trucks delivering to the same factory. Once the cargo is inside the mitochondria, both feed the same assembly line that makes ATP.
What human studies show about the switch
During longer fasting, ketones can cover a large share of the brain's energy needs. The more ketones in the blood, the more the brain takes up. Two classic human studies show this.
- The 1967 fasting study. Researchers, including the metabolism pioneer Dr. George Cahill, measured what went into and came out of the brains of three adults with obesity who fasted under medical supervision for five to six weeks. By then, BHB and acetoacetate (another ketone body) had replaced glucose as the brain's main fuel. This small study changed how scientists thought about the brain, which had long been viewed as a glucose-only organ.
- A brain scan study from 2000. A research team used magnetic resonance spectroscopy, a scan that measures chemicals inside living tissue, in healthy adults who fasted for up to three days. Four people completed the study. Brain BHB rose from barely detectable before fasting to about 1 mmol/L by day three, and brain levels tracked closely with blood levels.
Both studies were small, and long fasting is not something to try on your own. But together they show the core idea: the brain does not just tolerate ketones, it uses them, and supply drives uptake.
Why researchers study this switch
After an injury, brain and nerve cells can struggle to use glucose normally, at the very moment they need energy to repair. Scientists have asked whether ketones could help fill that gap.
- In one rat study of spinal cord injury, a ketogenic diet started four hours after the injury improved forelimb use over 14 weeks. The treated rats also had more of the MCT1 transporter, and blocking these transporters with a drug prevented the diet's protective effect on the injured tissue. It is one of the studies covered in our summary of a systematic review on nerve injury.
- A meta-analysis of 49 animal studies found that higher blood ketone levels were linked to larger protective effects after acute brain and spinal cord injury.
- In a dish, Dr. Csilla Ari's 2024 study found that injured neurons given BHB moved into the damaged area more than untreated cells.
All of this is animal and lab research. It shows why the question matters, not that ketones help a human brain recover. Not every animal study is positive either: in a 2020 mouse study of a lower-back spinal cord injury, a ketogenic diet raised blood ketones but did not improve recovery over 28 days. To learn why results in animals often differ from results in people, read our guide to animal vs. human studies.
What this means for your day
You do not need to be fasting for weeks to make some ketones. Here is what that looks like in everyday life:
- Overnight. After a long night without food, many people have a small rise in ketones by morning.
- On a ketogenic diet. Cutting carbohydrates way down for several days pushes the liver to make ketones steadily. Many people notice a rough adjustment period, often called the keto flu. See what the keto flu is and how to get into ketosis safely.
- With exogenous ketones. "Exogenous" means from outside the body. Ketone drinks raise blood BHB for a few hours, even if you just ate. Compare the two routes in keto diet vs. exogenous ketones.
A practical tip: if you are curious about your own ketone levels, a finger-prick blood meter measures BHB directly. Our guide to blood ketone levels explains what the numbers mean.
Common questions
Does my brain stop using glucose when I am in ketosis?
No. Even after weeks of fasting in the 1967 study, the brain still used some glucose. Ketones take over part of the load, and your liver keeps making a small amount of glucose to cover the rest.
How fast does the brain start using ketones?
The brain takes up ketones in proportion to how much is in the blood, so it starts as soon as levels rise. In the 2000 scan study, brain BHB was clearly higher after two days of fasting and higher still after three.
Are ketones a "better" fuel than glucose?
Not better, just different. Your brain is built to use both. Researchers study ketones because they can help in situations where glucose use is limited, such as long fasting, in aging brain and possibly after injury.
Can a ketone drink help my brain after a concussion or injury?
That has not been shown in people. The injury findings above come from animal and lab studies. If you or someone you love has had a head or spinal injury, follow your medical team's plan.
The bottom line
Your brain is flexible: it runs on glucose but can switch to ketones when needed. Human studies show that it takes up more ketones as blood levels rise, and animal studies suggest this fuel may matter after injury. That flexibility is why ketones are an active area of brain research. For practical context on nerve injuries, read Nerve Damage Recovery: What Helps, and What Ketone Research Is Exploring.
Following a ketogenic lifestyle? KETOSTART is designed to help you reach and maintain ketosis. If you prefer a warm drink, KETOSPIKE Instant Coffee with BHB is another option.
References
- Owen OE, Morgan AP, Kemp HG, Sullivan JM, Herrera MG, Cahill GF Jr. Brain metabolism during fasting. Journal of Clinical Investigation. 1967;46(10):1589-1595. View the abstract
- Pan JW, Rothman DL, Behar KL, Stein DT, Hetherington HP. Human brain beta-hydroxybutyrate and lactate increase in fasting-induced ketosis. Journal of Cerebral Blood Flow & Metabolism. 2000;20(10):1502-1507. View the abstract
- Streijger F, Plunet WT, Lee JHT, et al. Ketogenic diet improves forelimb motor function after spinal cord injury in rodents. PLoS One. 2013;8(11):e78765. View the abstract
- Sayadi JJ, Sayadi L, Satteson E, Chopan M. Nerve injury and repair in a ketogenic milieu: a systematic review of traumatic injuries to the spinal cord and peripheral nervous tissue. PLoS One. 2021;16(1):e0244244. View the abstract
- Gambardella I, Ascione R, D'Agostino DP, Ari C, et al. Neuroprotection of ketosis in acute injury of the mammalian central nervous system: a meta-analysis. Journal of Neurochemistry. 2021;158(2):105-118. View the abstract
- Ari C, D'Agostino DP, Cha BJ. Neuroregeneration improved by sodium-D,L-beta-hydroxybutyrate in primary neuronal cultures. Pharmaceuticals. 2024;17(9):1160. View the abstract
- Mayr KA, Kwok CHT, Eaton SEA, Baker GB, Whelan PJ. The effects of a ketogenic diet on sensorimotor function in a thoracolumbar mouse spinal cord injury model. eNeuro. 2020;7(4):ENEURO.0178-20.2020. View the abstract
This article is for education only and is not medical advice. These statements have not been evaluated by the FDA. Our products are not intended to diagnose, treat, cure or prevent any disease. Talk to your doctor before changing your diet or supplements.
Disclosure: Dr. Csilla Ari, a co-author of two studies cited here, is the founder of Audacious Nutrition.