Ketones as brain fuel

Ketones as Brain Fuel: The Entrance Insulin Resistance Cannot Close

Ketones fuel the brain without insulin: they enter nerve cells through carriers that depend on neither insulin nor the GLUT4 glucose transporter. When insulin resistance closes part of the glucose supply to the memory circuits, ketones still get in.

The memory circuits partly depend on an insulin-sensitive glucose door. Ketones use a different entrance, and that single difference explains why researchers keep testing them in the insulin-resistant brain.

An insulin-resistant brain is not out of fuel; it is locked out of one fuel while the other sits in the bloodstream unused. Using ketones for brain fuel matters because they walk in through an entrance insulin does not guard, while in the memory circuits the glucose doors partly answer to insulin. That is plumbing, not diet-book language, and the insulin side of it shows up on fasting laboratory panels. When insulin signaling fails, that difference decides whether a nerve cell is starving or simply switching fuels. Measura [Cardiometabolic and Autonomic Health Analysis] does not prescribe diets or supply ketones; it measures the insulin and fuel picture that makes the question relevant to you.

Two fuels, two ways in

Most of the time the brain burns glucose. Transporter proteins carry it across the blood-brain barrier and into cells, and most of that traffic does not need insulin. The exception matters. Nerve cells in the hippocampus, the structure that turns experience into memory, also use an insulin-responsive glucose transporter called GLUT4 when demand spikes, the way a grocery store opens extra checkout lanes at rush hour. If insulin stops working, those extra lanes stay closed exactly when the line is longest.

Ketones, mainly beta-hydroxybutyrate and acetoacetate, are made by the liver from fat when insulin runs low: overnight, during longer fasts or with carbohydrate restriction. They enter nerve cells through a separate family of carriers that depends on neither GLUT4 nor insulin. That independence is the reason researchers began asking whether ketones could keep an insulin-resistant brain running. How normal ketosis differs from the medical emergency of ketoacidosis is explained in ketosis vs ketoacidosis.

What happened when the insulin door was shut

This is mouse work. In a 2024 study, researchers at the University of Rochester and Stony Brook University took thin slices of mouse hippocampus and chemically blocked GLUT4, creating a short-term version of insulin resistance in living tissue. The circuit linking two hippocampal regions, called CA3 and CA1, degraded in several ways at once. Synaptic activity fell. Long-term potentiation, the strengthening of connections that underlies learning, weakened. Signals traveled along nerve fibers less reliably, and groups of cells fell out of step with one another.

Then the researchers added D-beta-hydroxybutyrate, the main ketone the body makes. It did not restore everything, but it rescued conduction along the fibers, the synchronization between cells and long-term potentiation. The non-obvious point is where the rescue landed. Memory depends on timing, on many cells firing together within thousandths of a second, and timing is expensive. When fuel runs short, precision fails long before cells die. A brain with enough energy on paper can still lose the synchrony that learning requires.

A slice of mouse tissue is not a person. Human trials of ketones usually isolate one diagnosis and exclude the metabolically complicated patient, so the mechanism has to carry more of the explanation here than it would in a simpler field. Why the insulin gate deserves a blood test of its own is argued in fasting insulin as an early screen.

Human brains: glucose uptake falls, ketone uptake holds

A research group in Sherbrooke, Quebec, measured both fuels in living brains using PET scans with two tracers, one for glucose and one for the ketone acetoacetate. In 10 people with mild Alzheimer’s disease, average age 76, compared with 29 cognitively normal adults, average age 75, gray matter took up 13% less glucose. Ketone uptake did not differ. In a second comparison of 24 healthy older adults, 20 with mild cognitive impairment and 19 with early Alzheimer’s disease, a glucose shortfall of about 7% had already appeared in one region, the cingulate gyrus, at the mild impairment stage, while ketone metabolism still matched the healthy group.

Supply mattered as much as access. When people with mild to moderate Alzheimer’s disease took 30 g a day of a medium-chain triglyceride supplement for one month, brain ketone consumption doubled without any change in brain glucose use. In adults with mild cognitive impairment randomized to a ketogenic drink or placebo, brain ketone metabolism rose by 230% after 6 months, and several memory, language and processing-speed scores improved from baseline. The deficit in these brains is specific to glucose, and the second entrance stays open. Whether a whole diet does the same in daily life is covered in what to measure before believing keto helps Alzheimer’s.

Brain networks in living adults, and a limit worth knowing

The Stony Brook group had earlier asked whether fuel type changes how brain networks hold together. Using imaging datasets of 292 and 636 adults spanning young adulthood to old age, they found that network stability, the ability of brain regions to stay in sustained communication, declined with age and tracked cognitive sharpness. In younger adults scanned in an ultrahigh-field scanner, networks became less stable after glucose and more stable with ketones, whether ketosis came from a ketogenic diet or from a ketone ester drink.

Newer work tested people closer to the question. In a 2025 randomized crossover trial of 10 adults with metabolic syndrome and 10 matched adults without it, working memory improved after a single ketone ester drink in both groups. The limit is instructive too. In a 12-week pilot in community-dwelling adults aged 65 and older who were healthy and independent, a daily ketone ester did not change exploratory measures of function or quality of life. Extra fuel appears to matter most where fuel is short, which is exactly why knowing whether your insulin system is failing comes first. The tissue evidence for that failing insulin system is traced in what brain tissue shows about insulin resistance in Alzheimer disease.

Why the insulin entrance closes in the first place

Three forces do most of the work, two of them biological. The first is constant insulin: a pancreas that answers every snack keeps insulin elevated, and cells exposed to a signal that never stops learn to ignore it. The second is metaflammation, the low-grade inflammatory output of overfilled visceral fat, which interferes with the same relay proteins insulin needs inside the cell. The third force is social. Food is engineered and sold to be eaten from waking to bedtime, and a day without a real fasting gap is a day the liver never makes ketones at all. The brain was built to alternate fuels; the modern schedule rarely lets it. How that plays out across the whole body is laid out in insulin resistance and metabolic health. Repeated glucose spikes damage more than the brain, starting with the lining inside every blood vessel.

What a standard visit checks, and what can be measured

A routine visit reports fasting glucose and sometimes hemoglobin A1c. Neither says how much insulin it took to hold glucose there, and nothing in a standard workup shows which fuel your body is burning. Measura measures; it does not treat, and results go to your physician.

Insulin behind the glucose. Laboratory panels can include fasting insulin alongside glucose and hemoglobin A1c. A blood ketone level, if wanted, is blood work your physician orders.
Which fuel you burn at rest. Indirect calorimetry measures oxygen used and carbon dioxide produced after a fast and reports a respiratory quotient, which indicates whether you are predominantly burning fat or carbohydrate. The practical side is in metabolic rate and energy.
A memory starting point. Cognitive assessment records attention, memory and executive function so that any later change is measured against you.

If you take medication for diabetes or blood pressure, any change in eating that raises ketones is a conversation with your prescriber before it starts. Unmeasured is unmanaged, and the brain’s second fuel is only an option for someone who knows the first one is failing.

Frequently asked questions

Can the brain run entirely on ketones?

Not entirely. Some brain cells and all red blood cells still need glucose, and the liver keeps making it even during a long fast. What ketones can do is cover a large share of the brain’s energy need, and they reach nerve cells without depending on insulin. That matters most when insulin-dependent glucose uptake is faltering, the problem described in fasting insulin screening before glucose moves.

Is a ketone drink the same as a ketogenic diet?

They raise the same molecules by different routes. A ketone ester or medium-chain triglyceride supplement raises blood ketones within hours while insulin and diet stay much the same, whereas a ketogenic diet lowers insulin and changes fat, muscle and liver metabolism over weeks. The brain scan studies used both approaches. How physicians weigh the options in memory complaints is covered in ketogenic therapy in mild cognitive impairment.

Is it safe to try ketosis if I take diabetes medication?

That decision belongs to your prescriber before anything changes. Carbohydrate restriction can drop blood sugar quickly in people taking insulin or certain tablets, and some diabetes drugs can cause ketoacidosis even when glucose looks normal. A baseline set of measurements and a plan to repeat them make that conversation specific. The physician’s checklist is in a baseline and re-measure plan before low-carb therapy.

What does a respiratory quotient tell me about my fuel use?

It reflects the mix of fat and carbohydrate your body is burning at rest, measured from the gases you breathe after a fast. A lower value points toward fat, a higher value toward carbohydrate. It is a snapshot under set conditions, not a fixed trait, and it shifts with recent meals. Why a measured value beats a formula is explained in measured versus estimated metabolic rate.

Does insulin resistance in my body mean my brain is insulin resistant?

Not necessarily, and no blood test can show the brain directly. Body-wide insulin resistance and poorer brain glucose uptake do tend to travel together, and brain tissue from people with Alzheimer’s disease has shown impaired insulin signaling even without diabetes. The body-wide measurement is the one available to you now, which is the argument made in type 3 diabetes and a normal blood sugar.

Know which fuel your body is using

Request Measura testing for fasting insulin, a resting respiratory quotient and a cognitive baseline, with the results sent to your physician.

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References

Kula, B., Antal, B., Weistuch, C., Gackière, F., Barre, A., Velado, V., Hubbard, J. M., Kukley, M., Mujica-Parodi, L. R., & Smith, N. A. (2024). D-ꞵ-hydroxybutyrate stabilizes hippocampal CA3-CA1 circuit during acute insulin resistance. PNAS Nexus, 3(5), pgae196. https://doi.org/10.1093/pnasnexus/pgae196
Castellano, C. A., Nugent, S., Paquet, N., Tremblay, S., Bocti, C., Lacombe, G., et al., & Cunnane, S. C. (2015). Lower brain 18F-fluorodeoxyglucose uptake but normal 11C-acetoacetate metabolism in mild Alzheimer’s disease dementia. Journal of Alzheimer’s Disease, 43(4), 1343-1353. https://doi.org/10.3233/JAD-141074
Croteau, E., Castellano, C. A., Fortier, M., Bocti, C., Fulop, T., Paquet, N., & Cunnane, S. C. (2018). A cross-sectional comparison of brain glucose and ketone metabolism in cognitively healthy older adults, mild cognitive impairment and early Alzheimer’s disease. Experimental Gerontology, 107, 18-26. https://doi.org/10.1016/j.exger.2017.07.004
Croteau, E., Castellano, C. A., Richard, M. A., Fortier, M., Nugent, S., Lepage, M., et al., & Cunnane, S. C. (2018). Ketogenic Medium Chain Triglycerides Increase Brain Energy Metabolism in Alzheimer’s Disease. Journal of Alzheimer’s Disease, 64(2), 551-561. https://doi.org/10.3233/JAD-180202
Fortier, M., Castellano, C. A., Croteau, E., Langlois, F., Bocti, C., St-Pierre, V., et al., & Cunnane, S. C. (2019). A ketogenic drink improves brain energy and some measures of cognition in mild cognitive impairment. Alzheimer’s & Dementia, 15(5), 625-634. https://doi.org/10.1016/j.jalz.2018.12.017
Mujica-Parodi, L. R., Amgalan, A., Sultan, S. F., Antal, B., Sun, X., Skiena, S., et al., & Clarke, K. (2020). Diet modulates brain network stability, a biomarker for brain aging, in young adults. Proceedings of the National Academy of Sciences of the United States of America, 117(11), 6170-6177. https://doi.org/10.1073/pnas.1913042117
Graybeal, A. J., Aultman, R. S., Brandner, C. F., Vallecillo-Bustos, A., Compton, A. T., Swafford, S. H., Newsome, T. A., & Stavres, J. (2025). Effects of Ketone Ester Supplementation on Cognition and Appetite in Individuals with and Without Metabolic syndrome: A Randomized Trial. Journal of Dietary Supplements, 22(3), 382-400. https://doi.org/10.1080/19390211.2025.2473371
Stubbs, B. J., Stephens, E. B., Mansfield, T., Senadheera, C., Diaz, S. R., Peralta, S., et al. (2025). Exploratory functional and quality of life outcomes with daily consumption of the ketone ester bis-octanoyl (R)-1,3-butanediol in healthy older adults: a randomized, parallel arm, double-blind, placebo-controlled, pilot study. The Journal of Frailty & Aging. https://pubmed.ncbi.nlm.nih.gov/41313689/