🧬 biology

β-Hydroxybutyrate maintains energetically demanding neural functions during glucose deprivation

This study demonstrates that β-hydroxybutyrate can independently sustain essential metabolic and functional properties of brain tissue, including the ability to recover from severe energetic challenges, in the complete absence of exogenous glucose.

Original authors: Rachel L. Ricks, Dallin S. Nevers, Tyler Poulos, T. Luke Shafer, Clayton Dunford, Paul R. Reynolds, Benjamin T. Bikman, R. Ryley Parrish

Published 2026-06-26

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Rachel L. Ricks, Dallin S. Nevers, Tyler Poulos, T. Luke Shafer, Clayton Dunford, Paul R. Reynolds, Benjamin T. Bikman, R. Ryley Parrish

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The Big Question: Can the Brain Run on "Backup Fuel" Alone?

Imagine your brain is a high-performance sports car. For decades, scientists have believed this car runs exclusively on glucose (sugar). If the gas tank runs dry, the car stops.

However, we know that when people fast or eat a "keto" diet (very low carb), their bodies switch to burning ketones (specifically a type called β-hydroxybutyrate or BHB) instead of sugar. The big mystery this paper tried to solve was: If you completely remove the sugar (glucose) and give the brain only ketones, can the brain still run? Or does it need a little bit of sugar just to keep the engine from stalling?

In real life, it's hard to test this because even when people are starving, their bodies make a tiny bit of sugar internally. So, the researchers had to build a "test track" outside the body to see what happens when sugar is truly gone.

The Experiment: A Brain on a Test Track

The researchers took brain tissue from mice and placed it in a special bath (like a tiny swimming pool) that mimics the fluid surrounding the brain. They set up three different "fuel tanks" for these brain slices:

The Sugar Tank: Filled with glucose (the standard fuel).
The Ketone Tank: Filled only with BHB (no sugar allowed).
The Empty Tank: Filled with sucrose (table sugar that brain cells can't eat, acting as a control to show what happens with no fuel).

They let the brain slices sit in these tanks for several hours to see if they could survive and function.

Test 1: The Engine Check (Mitochondria)

First, they checked the brain cells' "engines" (called mitochondria). These are the tiny power plants inside cells that burn fuel to create energy.

The Finding: When the brain ran on ketones, the engines actually burned less oxygen than when they ran on sugar.
The Analogy: Think of it like driving a car in "Eco Mode." When the car runs on ketones, it doesn't rev as loudly or burn as much air; it runs more efficiently. It wasn't struggling; it was just being smarter with its energy. The engines were still running strong, just quieter.

Test 2: The Stress Test (Spreading Depolarization)

Next, they put the brain slices through a massive stress test. They wanted to see if the brain could handle a "blackout" event called Spreading Depolarization (SD).

What is SD? Imagine a massive power surge that knocks out the lights in a city, causing all the traffic lights to go haywire. In the brain, this is a wave of electrical chaos that wipes out the cell's energy reserves. To recover, the brain has to work overtime to reset the traffic lights (ions) and get the city running again. This requires a huge burst of energy.
The Challenge: They induced this "blackout" twice in a row to see if the brain had enough battery power to recover from the first one and handle a second one.

The Results:

The Sugar Group: Recovered from the first blackout and easily handled a second one.
The Empty Tank Group: Failed immediately. Without fuel, the brain couldn't even start the recovery process.
The Ketone Group: This is the big surprise. The brain slices running only on ketones recovered from the first blackout and successfully handled a second one, just like the sugar group!

The Nuance: While the ketone-brains could do the job, they were a little slower. It took them slightly longer to start the recovery wave, and the wave moved a bit slower across the tissue. But, they did recover.

The Conclusion: The Brain is Flexible

The paper concludes that β-hydroxybutyrate (BHB) is a complete, standalone fuel for the brain.

No Sugar Needed: The brain does not need a "drop" of sugar to function. It can run entirely on ketones for hours.
Efficiency: Ketones might actually make the brain's energy engines more efficient (burning less oxygen for the same job).
Resilience: Even under extreme stress (like the "blackout" test), a brain fueled only by ketones can recover and keep working.

What This Means (Based Strictly on the Paper)

The researchers found that the brain is much more flexible than we thought. It's not a car that only takes premium unleaded (glucose); it's a hybrid that can run perfectly well on a different fuel source (ketones) if the premium unleaded is unavailable.

The paper suggests that because ketones can sustain the brain's most demanding tasks without sugar, this supports the idea that ketogenic strategies (using ketones as fuel) could be useful for conditions where the brain struggles to get or use sugar, or where the brain's energy needs are unusually high. However, the paper stops at proving the brain can do it; it does not claim to have cured any specific disease in this study.

Technical Summary: β-Hydroxybutyrate Maintains Energetically Demanding Neural Functions During Glucose Deprivation

Problem Statement
The mammalian brain is traditionally characterized as a glucose-dependent organ with limited energy reserves, relying on glucose as the primary substrate for ATP production under normal physiological conditions. While it is established that ketone bodies, particularly β-hydroxybutyrate (BHB), serve as alternative fuels during fasting or ketogenic diets, it remains uncertain whether BHB can independently sustain brain tissue metabolism in the near-complete absence of glucose. This question is difficult to resolve in vivo because endogenous gluconeogenesis maintains circulating glucose levels even during starvation, preventing the isolation of the brain's dependence on glucose versus alternative fuels. Consequently, the limits of cerebral metabolic flexibility and the capacity of BHB to function as a standalone fuel source under true glucose deprivation remain undefined.

Methodology
To address this, the authors utilized an ex vivo brain preparation using neocortical slices from C57BL/6 mice, isolating the tissue from endogenous glucose sources. The study employed two primary experimental approaches:

High-Resolution Respirometry: Neocortical slices were incubated for 4 hours in artificial cerebrospinal fluid (aCSF) containing either 10 mM glucose, 10 mM BHB, or 10 mM sucrose (as a fuel-deprived control). Following incubation, tissue homogenates were analyzed using an Oroboros O2k respirometer. A substrate-uncoupler-inhibitor titration (SUIT) protocol was used to assess mitochondrial function across various respiratory states, including leak respiration (LEAK), oxidative phosphorylation (OXPHOS) supported by Complex I and/or II, and maximal electron transfer (ET) capacity.
Electrophysiology and Optical Imaging: To test tissue resilience under extreme energetic demand, the authors induced spreading depolarization (SD), a massive wave of neuronal depolarization requiring rapid restoration of ionic homeostasis. SD was triggered by perfusing slices with high-potassium (26 mM K+) aCSF. The ability of tissue to generate and recover from repeated SD events was monitored using intrinsic optical signal (IOS) imaging. Metrics included SD latency, propagation speed, peak amplitude of the optical signal, and recovery half-time (T50).

Key Results

Mitochondrial Respiration: Tissue incubated in BHB exhibited significantly lower oxygen flux ( $J_{O2}$ ) across all respiratory states (LEAK, OXPHOS, and ET capacity) compared to glucose-incubated tissue. Despite this reduction in oxygen consumption, mitochondrial integrity was preserved, and the tissue remained viable.
Spreading Depolarization (SD) Induction and Recovery:
- Single Event: Both glucose and BHB-treated slices successfully generated and fully recovered from a single SD event within 10 minutes. There were no significant differences in SD latency, propagation speed, peak IOS amplitude, or recovery kinetics between the two groups during the first event.
- Repeated Events: Upon a second SD induction, glucose-treated slices maintained their performance. BHB-treated slices also successfully generated a second SD, whereas sucrose-treated (fuel-deprived) slices failed to elicit a second event. However, BHB-treated slices showed a significantly longer latency to onset and slower propagation speed for the second SD compared to glucose controls.
- Recovery Metrics: Following the first SD, the time to return to 50% baseline reflectance did not differ significantly between glucose and BHB groups.

Key Contributions
The study provides direct experimental evidence that BHB can serve as a standalone cerebral fuel, independent of exogenous glucose. The authors demonstrate that:

Brain tissue can acutely rely on BHB to maintain essential metabolic functions, including the ability to undergo and recover from the energetically demanding process of spreading depolarization.
Mitochondria utilizing BHB operate with lower oxygen flux compared to those using glucose, suggesting a shift in metabolic efficiency rather than a deficit in energy generation capacity.
The capacity to recover from repeated SDs in the absence of glucose challenges the dogma that glucose is strictly necessary for brain metabolic function during acute stress.

Significance and Claims
The authors claim these findings broaden the understanding of cerebral fuel flexibility, demonstrating that BHB is sufficient to sustain essential metabolic and functional properties of brain tissue without glucose. The observed reduction in oxygen flux coupled with maintained functional resilience suggests that BHB metabolism may represent a more efficient metabolic strategy, potentially involving increased coupling efficiency or altered thermodynamic states of ATP hydrolysis.

The paper posits that these results have implications for understanding ketogenic neuroprotection and metabolic resilience. Specifically, the data supports the therapeutic use of ketogenic strategies in neurological disorders characterized by altered tissue excitability, energy metabolism, or glucose availability, such as drug-resistant epilepsy and GLUT1 deficiency syndrome. The authors modestly suggest that the observed slowing of SD propagation in BHB-treated tissue during repeated events may indicate a reduced susceptibility to SD, potentially offering a protective mechanism during extreme brain perturbations.