Understanding Ketone Science

For most of human history, the brain ran on one primary fuel: glucose. But buried in decades of metabolic research is a powerful alternative — ketones. Once associated only with strict fasting or extreme low-carbohydrate diets, ketones are now gaining serious scientific attention as a direct, efficient energy source for both the brain and the body.

Here is what peer-reviewed research says — and why it matters.

What Are Ketones?

Ketones are small molecules produced by the liver when the body breaks down fatty acids for fuel. They are not a supplement invented in a laboratory — they are a natural metabolic product your body has always been capable of generating. As published in Experimental & Molecular Medicine, when the body is low on glucose, it breaks down fatty acids for energy, generating byproduct metabolites called ketones, and the ketone beta-hydroxybutyrate (BHB) regulates cellular signalling and gene and protein expression.

The three primary ketone bodies are beta-hydroxybutyrate (BHB), acetoacetate (AcAc), and acetone. They circulate in the blood at an approximate ratio of 2:1 (BHB to AcAc), making BHB the primary ketone metabolite in the blood. AcAc can spontaneously break down into acetone, which is responsible for the characteristic breath odour some people notice when they enter ketosis. Tissues such as the brain, heart, kidneys, and skeletal muscle are particularly active ketone users.

Ketone metabolism is especially important during early human development — in the neonate, the brain consumes up to 70% of the body's total energy needs, with ketones playing a critical role as a substrate for brain development.

Endogenous vs Exogenous: What Is the Difference?

Endogenous ketones are those your body makes itself — typically during prolonged fasting, high-intensity exercise, or sustained carbohydrate restriction. This process, called ketogenesis, is the body's natural response to low glucose availability. Caloric restriction and intermittent fasting also produce a transient mild-to-moderate ketosis.

Exogenous ketones, by contrast, are taken as a supplement — delivering ketones directly into the bloodstream without requiring any dietary change or fasting period. Research published in Frontiers in Nutrition demonstrated that an oral D-beta-hydroxybutyrate supplement is rapidly absorbed and metabolised in humans, increasing blood ketones to millimolar levels — achieving a peak blood BHB of approximately 1.2 mmol/L. Critically, this creates what researchers have described as a novel physiological state: high circulating ketone concentrations and replete carbohydrate stores simultaneously, something that cannot occur with dietary ketosis alone.

BHB: The Power Molecule

Beta-hydroxybutyrate is the ketone body that does most of the work. It is the most abundant ketone found in the blood during ketosis, and it is the form primarily used in exogenous ketone supplements because of its stability and bioavailability.

BHB serves two distinct functions. First, it acts as a direct energy substrate — neurons, heart, and muscle cells take it up and oxidise it to produce ATP, the cell's primary energy currency. A review in Experimental & Molecular Medicine notes that BHB is a potent metabolite small enough to filter through cell membranes and circulate throughout the body, including the brain, influencing signalling pathways.

Second, BHB functions as a signalling molecule independent of its role as a fuel. Research published in Cell Chemical Biology identified BHB as a regulator of protein solubility, finding it plays a role in maintaining protein homeostasis — a process disrupted in ageing and neurodegenerative diseases including Alzheimer's. Separately, a PubMed review titled β-Hydroxybutyrate in the Brain: One Molecule, Multiple Mechanisms described BHB as possessing an intrinsically high heat of combustion, making it an efficient mitochondrial fuel that can also reduce the production of reactive oxygen species — a key driver of cellular ageing and inflammation.

Understanding Blood Ketone Levels

Blood ketone levels are measured in millimoles per litre (mmol/L). A peer-reviewed review published in PMC (PubMed Central) — Measuring Ketone Bodies for the Monitoring of Pathologic and Therapeutic Ketosis — confirms the following reference ranges:

Normal baseline: ~0.1 mmol/L — the resting BHB level in healthy adults eating a standard balanced diet.

Nutritional ketosis begins: ≥0.5 mmol/L — the established threshold at which the body is producing and utilising ketones as a meaningful energy source.

Light ketosis: 0.5–1.0 mmol/L — beneficial but moderate ketone availability.

Optimal nutritional ketosis: 1.0–3.0 mmol/L — the range commonly referenced in the research literature and associated with therapeutic and cognitive benefits.

The practical implication: reaching and sustaining meaningful ketosis through diet alone requires significant commitment — typically restricting carbohydrates to under 50g per day. Exogenous ketone supplements offer a way to raise blood BHB rapidly, with research demonstrating measurable increases within 30 minutes of consumption and sustained elevation for several hours.

The Brain and Ketone Metabolism

The brain is an extraordinary energy consumer. Despite representing approximately 2% of adult body weight, it accounts for roughly 20% of the body's total energy use — a figure confirmed by multiple peer-reviewed studies published in PNAS, the Frontiers in Systems Neuroscience, and by researchers at Washington University School of Medicine.

Under normal conditions, glucose is the brain's primary fuel, accessed via specific glucose transporters. However, ketones serve as the brain's only other significant alternative fuel. A review published in the British Journal of Nutrition (Cambridge University Press) describes how the brain's ability to metabolise glucose often diminishes with age and in neurodegenerative conditions, while the capacity to metabolise ketones appears to remain largely intact.

Research published on PubMed (β-Hydroxybutyrate in the Brain: One Molecule, Multiple Mechanisms) describes BHB as an efficient mitochondrial fuel with the ability to alter key redox couples and reduce the production of mitochondrial reactive oxygen species — pointing to its potential neuroprotective properties beyond simply being an alternative to glucose.

A review published in the British Journal of Nutrition specifically examined how cognitive decline in Alzheimer's disease appears to reflect a deficit in brain glucose metabolism, while ketone metabolism remains comparatively preserved — opening the possibility of ketones as a targeted brain energy intervention.

Physical Performance: What Does the Research Show?

Exogenous ketones have attracted considerable attention in sports science, with researchers exploring whether they can act as an additional fuel source, spare glycogen, or support recovery. The evidence here is more complex and mixed than for brain metabolism.

A review published in Current Opinion in Physiology (ScienceDirect) noted that exercise metabolism is altered during exogenous ketosis, with increased fat oxidation and some preservation of intramuscular glycogen observed in studies. The review also noted that this metabolic shift may be advantageous during low-to-moderate intensity exercise.

However, the picture is more cautious at higher intensities. A randomised crossover study published in the Journal of Applied Physiology — conducted with 12 highly trained male cyclists — found that exogenous ketosis did not cause meaningful glycogen sparing or improve high-intensity performance when adequate carbohydrate intake was maintained. A subsequent editorial perspective in PMC concluded that evidence to date does not support a consistent performance benefit from acute ketone supplementation during exercise, though research into recovery and longer-duration efforts continues.

Importantly, a review in Nutrition & Metabolism (Springer Nature, 2025) noted that ketone supplementation promotes efficient energy production by elevating BHB levels and stimulating mitochondrial pathways, with individual responses varying depending on exercise intensity, metabolic state, and supplementation strategy.

An Important Note on Safety

Nutritional ketosis — achieving blood BHB levels of 0.5–3.0 mmol/L — is a normal physiological state and is considered safe for most healthy adults. It is distinct from diabetic ketoacidosis, a dangerous condition that occurs in type 1 diabetics when both ketones and blood glucose are dangerously elevated simultaneously.

Research published in PMC notes that the long-term safety profile of ketone ester supplements has not yet been fully established, and that high-dose ketone ester ingestion can cause transient metabolic acidosis and gastrointestinal discomfort in some individuals.

Anyone with diabetes, kidney disease, or other metabolic conditions should consult a qualified healthcare professional before using exogenous ketone supplements.

The Bottom Line

Ketone science is well-established at the metabolic level. BHB is the most important molecule in this picture — it crosses into the brain, fuels the body's tissues, and acts as a signalling molecule influencing gene expression and protein regulation.

The research base supporting ketones as a brain fuel is substantial, with particular interest in populations where glucose metabolism is compromised. The evidence for physical performance benefits is more mixed, with ongoing research focused on recovery and lower-intensity endurance efforts rather than peak output.

What is clear from the literature is that ketones represent a genuinely distinct metabolic pathway — one with real physiological significance. Understanding how that pathway works is the foundation for using it well.

Key References

1.       Han Y-M et al. β-hydroxybutyrate and its metabolic effects on age-associated pathology. Experimental & Molecular Medicine. 2020.

2.      Stubbs BJ et al. On the metabolism of exogenous ketones in humans. Frontiers in Physiology. 2017.

3.      St-Pierre V et al. Metabolism of Exogenous D-Beta-Hydroxybutyrate, an Energy Substrate Avidly Consumed by the Heart and Kidney. Frontiers in Nutrition. 2020.

4.      Fortier M et al. Ketones: potential to achieve brain energy rescue and sustain cognitive health during ageing. British Journal of Nutrition. 2021.

5.      McNally MA, Hartman AL. Ketone Bodies in Epilepsy. Journal of Neurochemistry. 2012.

6.      Poffé C et al. Exogenous ketosis impacts neither performance nor muscle glycogen breakdown in prolonged endurance exercise. Journal of Applied Physiology. 2020.

7.      Dearlove DJ et al. Context is key: exogenous ketosis and athletic performance. Current Opinion in Physiology. 2019.

8.     Morales-Álvarez MC. Perspective: Ketone Supplementation in Sports — Does It Work? PMC. 2021.

9.      Raichle ME & Gusnard DA. Appraising the brain's energy budget. PNAS. 2002.

10.  Stubbs B et al. Measuring ketone bodies for the monitoring of pathologic and therapeutic ketosis. PMC. 2021.

11.   Hyder F et al. 3D Brain Energy Atlas. Cerebral Cortex. Yale University.

12.  Alzheimer's Disease Neuroimaging Initiative — various brain glucose/ketone PET studies cited in Fortier et al. (2021).