Ketogenesis is the metabolic process by which your liver converts fatty acids into small, water-soluble fuel molecules called ketone bodies. It kicks in when glucose is scarce, whether from fasting, prolonged exercise, or a very low-carbohydrate diet, and it provides an alternative energy source for organs that would otherwise struggle without sugar. The process is more than a simple backup generator, though: research over the past two decades has revealed that ketone bodies also act as signaling molecules, influencing gene expression, inflammation, and even how the heart adapts to disease.
The Three Ketone Bodies
Your liver produces three distinct ketone bodies. The two main ones are acetoacetate and beta-hydroxybutyrate (often shortened to BHB). Acetone is the third and least abundant of the group.1PubMed. Ketone bodies: a review of physiology, pathophysiology and application of monitoring to diabetes Acetoacetate is produced first in the pathway. Some of it is then converted to BHB by an enzyme in the liver’s mitochondria, and some spontaneously breaks down into acetone. BHB ends up being the most plentiful ketone body circulating in your blood during fasting, which is why blood ketone meters typically measure BHB. Acetone, being volatile, gets exhaled through the lungs, and that fruity or metallic breath some people notice during fasting or on a ketogenic diet is literally acetone leaving the body.
How the Liver Builds Ketones
Ketogenesis happens inside the mitochondria of liver cells. The raw material is fatty acids, released from your fat stores when insulin drops and signals like glucagon rise. Those fatty acids travel through the blood to the liver, enter liver cells, and get shuttled into the mitochondria through a transport system that depends on an enzyme called carnitine palmitoyltransferase (CPT-1). Once inside, the fatty acids are chopped into two-carbon units called acetyl-CoA through a process called beta-oxidation.
When your body has plenty of glucose, those acetyl-CoA molecules mostly feed into the citric acid cycle to produce energy. But when glucose is low and the liver’s own energy needs are already met, the excess acetyl-CoA gets diverted. Two acetyl-CoA molecules combine to form acetoacetyl-CoA, which then gains another acetyl-CoA to become HMG-CoA. A key enzyme, mitochondrial HMG-CoA synthase, catalyzes that step and acts as the main bottleneck controlling how fast ketogenesis runs.2Biochimie. Transcriptional regulation of mitochondrial HMG-CoA synthase in the control of ketogenesis HMG-CoA is then cleaved into acetoacetate and a fresh acetyl-CoA. From acetoacetate, BHB and acetone are formed.
An important detail: the liver itself cannot use the ketone bodies it produces. It lacks the enzyme needed to convert them back into usable fuel. So the liver exports them into the bloodstream, essentially manufacturing fuel for the rest of the body.
What Switches Ketogenesis On and Off
The on-off switch for ketogenesis is not a single hormone but a combination of signals, with fatty acid supply playing the starring role. Research in humans found that the availability of free fatty acids in the blood is a major determinant of how fast ketone bodies are produced.3PubMed Central. Effects of free fatty acid availability, glucagon excess, and insulin deficiency on ketone body production in postabsorptive man When insulin levels are high, as they are after a carbohydrate-rich meal, fat stays locked in your adipose tissue and very few fatty acids reach the liver. Ketogenesis stays suppressed.
When insulin drops, fat cells release their stores. Glucagon, the hormone that rises when blood sugar falls, further promotes the process, but it needs insulin to be low to have much effect. Animal studies showed that anti-insulin serum produced sharp rises in blood ketone bodies, while glucagon alone had little impact unless insulin was already suppressed.4PubMed Central. Hormonal control of ketogenesis. Rapid activation of hepatic ketogenic capacity in fed rats by anti-insulin serum and glucagon In practical terms, ketogenesis is governed by the ratio between glucagon and insulin rather than by either hormone alone. That ratio swings dramatically during fasting, exercise, or carbohydrate restriction, and it is what determines whether your liver runs in “glucose mode” or “ketone mode.”
At the gene level, fasting and fatty acids increase the transcription of mitochondrial HMG-CoA synthase, the bottleneck enzyme, through a receptor called PPARα. Refeeding and insulin suppress it.2Biochimie. Transcriptional regulation of mitochondrial HMG-CoA synthase in the control of ketogenesis So the regulation works on two time scales: a fast hormonal switch that responds within minutes, and a slower gene-expression shift that ramps up the liver’s ketogenic capacity over hours to days of fasting.
How Ketones Reach the Brain and Other Tissues
Muscles, the heart, and the kidneys can all take up ketone bodies directly from the blood and oxidize them for energy. The brain, however, is a special case. It sits behind the blood-brain barrier, which blocks most fatty acids from entering. This is precisely why ketone bodies matter so much: they are small and water-soluble enough to cross that barrier using dedicated transporters called monocarboxylate transporters (MCTs).5PubMed. Expression of monocarboxylate transporter mRNAs in mouse brain: support for a distinct role of lactate as an energy substrate for the neonatal vs. adult brain
Under normal fed conditions, the brain runs almost entirely on glucose. But during prolonged fasting, ketone bodies can supply a substantial fraction of the brain’s energy needs.6PubMed Central. Effects of Ketone Bodies on Brain Metabolism and Function in Neurodegenerative Diseases The brain adapts to this shift in part by upregulating MCT1, the transporter responsible for moving ketones across the blood-brain barrier. Research in rats showed that fasting activates this upregulation through PPARδ, meaning the longer you fast, the more efficiently your brain can pull ketones out of the blood.7PubMed Central. Fasting upregulates the monocarboxylate transporter MCT1 at the rat blood-brain barrier through PPAR δ activation That adaptation is part of why the first day or two of fasting can feel mentally foggy, while people who fast regularly often report clearer thinking once the system has had time to ramp up.
Newborns Run on Ketones
Adults think of ketosis as an unusual state triggered by fasting or dietary intervention. For newborns, it is the default. Studies measuring ketone body turnover in human neonates found rates comparable to what adults reach only after several days of total fasting, even when the infants had fasted for fewer than eight hours.8PubMed Central. Ketone body transport in the human neonate and infant If most of those transported ketones are oxidized, as they are in adults, ketone bodies could account for roughly a quarter of a newborn’s resting energy needs in the first days of life.
This makes biological sense. Breast milk is high in fat, and the neonatal brain is growing rapidly and consuming a disproportionate share of total energy. Ketone metabolism during early development is not just a stopgap; it is part of normal brain development.9PubMed Central. Cerebral ketone metabolism during development and injury Neonatal brains express high levels of the enzymes needed to metabolize ketones, and those levels decline gradually as the child matures and the brain shifts to relying more heavily on glucose. It is a reminder that ketogenesis is not a metabolic curiosity or a fad-diet trick. It is an ancient, fundamental pathway woven into human development from birth.
Why Ketogenesis Likely Shaped Human Brain Evolution
Humans have unusually large brains relative to body size, and large brains are metabolically expensive. One hypothesis is that the ability to produce and use ketone bodies was a key enabler of human brain expansion. During periods of food scarcity, which were routine in ancestral environments, the liver’s HMG-CoA pathway could keep the brain supplied with fuel by converting stored fat into ketone bodies.10PubMed. Metabolism as a tool for understanding human brain evolution: lipid energy metabolism as an example Unlike most tissues, the brain cannot directly burn circulating fatty acids for energy. Without ketogenesis, a fasting human’s large brain would depend entirely on glucose made from breaking down muscle protein, which is slow and destructive. Ketone bodies bridge that gap, allowing humans to tolerate extended fasts without catastrophic muscle wasting or brain energy failure.11PubMed Central. Brain Ketone Bodies in Health, Evolution and Disease
Post-Exercise Ketosis
You do not need to fast for days to trigger ketogenesis. Intense or prolonged exercise can do it too, particularly once liver glycogen stores are depleted. After a hard workout, your liver’s glycogen is low, and the glucagon-to-insulin ratio is elevated. That combination reduces production of malonyl-CoA, a molecule that normally blocks fatty acid transport into liver mitochondria. With that block removed, fatty acids pour in, acetyl-CoA accumulates, and ketogenesis picks up.12PubMed Central. Metabolism of ketone bodies during exercise and training: physiological basis for exogenous supplementation
There is also a less obvious contributor. Oxaloacetate, which is needed to feed acetyl-CoA into the citric acid cycle, gets diverted toward gluconeogenesis (making new glucose) when liver glycogen is low. With less oxaloacetate available in the mitochondria, acetyl-CoA has nowhere to go except into the ketogenic pathway. This post-exercise ketosis is usually mild and temporary, resolving once you eat and glycogen is restored, but it explains why blood ketone levels can be detectably elevated for hours after a long endurance session, even without dietary carbohydrate restriction.
Beyond Fuel: Ketones as Signaling Molecules
For decades, ketone bodies were viewed purely as backup fuel. That picture has changed substantially. BHB, the most abundant circulating ketone, turns out to double as a signaling molecule with effects on gene expression, inflammation, and cellular stress responses.13PubMed Central. β-Hydroxybutyrate: A Signaling Metabolite
One well-studied mechanism is that BHB inhibits a class of enzymes called histone deacetylases (HDACs), specifically class I and IIa. These enzymes normally tighten the packaging of DNA around histone proteins, which silences nearby genes. When BHB blocks them, the packaging loosens and certain protective genes get switched on. In mouse studies, BHB treatment increased histone acetylation in the kidneys and activated genes involved in oxidative stress resistance, including FOXO3a, a gene linked to longevity in multiple human populations.14Heliyon. β-Hydroxybutyrate as an epigenetic modifier: Underlying mechanisms and implications
BHB also binds to specific receptors on cell surfaces, including GPR109A on immune cells, and can suppress the NLRP3 inflammasome, a molecular complex that drives production of inflammatory molecules like interleukin-1β.15Trends in Endocrinology & Metabolism. What Is Ketogenesis? How Your Body Makes Ketones These anti-inflammatory effects may partially explain why ketogenic diets and fasting sometimes improve conditions with an inflammatory component, though the clinical research connecting these molecular findings to specific patient outcomes is still developing.
The Failing Heart Turns to Ketones
The heart is one of the most metabolically flexible organs in the body, capable of burning fatty acids, glucose, lactate, and ketone bodies. Research has shown that when the heart is under stress from hypertrophy or failure, it shifts toward relying more heavily on ketone bodies for energy production.16PubMed Central. The Failing Heart Relies on Ketone Bodies as a Fuel Whether that shift is purely compensatory or actually beneficial is still being sorted out. One study aimed to test whether increased ketone oxidation improves cardiac energy production and efficiency in failing hearts, though the answer appears to be that while ketone oxidation does provide additional energy, it does not necessarily improve the efficiency with which that energy translates into mechanical work.17PubMed Central. Increased ketone body oxidation provides additional energy for the failing heart without improving cardiac efficiency Still, the fact that the failing heart actively upregulates its ketone-burning machinery suggests this fuel switch is part of the organ’s survival strategy, and it has sparked interest in whether therapeutic ketosis could support heart failure patients.
Your Body Clock Controls Ketone Production
Ketogenesis does not run at a constant rate throughout the day, even during prolonged fasting. Your circadian clock actively regulates it. Research in mice on calorie restriction found that blood BHB levels followed strong daily rhythms that correlated with rhythmic expression of ketogenesis genes in the liver.18PubMed Central. Circadian clock controls rhythms in ketogenesis by interfering with PPARα transcriptional network The core clock proteins CLOCK, BMAL1, and cryptochromes directly interfere with PPARα, the same transcription factor that drives ketogenic gene expression during fasting. When researchers knocked out the cryptochrome genes in mice, the daily rhythm in blood ketone levels was significantly blunted.
Time-restricted feeding also produced rhythmic BHB patterns, but with smaller swings than calorie restriction. This suggests that when you eat matters for ketone production, not just how much you eat. For people practicing intermittent fasting, it means that the timing of your eating window relative to your natural circadian rhythm may affect how deeply you enter ketosis during fasting hours. Morning fasters and evening fasters could see different ketone profiles even with the same total food intake.
Exogenous Ketones and How They Differ
You can also raise blood ketone levels without fasting or restricting carbohydrates by drinking exogenous ketone supplements. These come in two main forms: ketone esters and ketone salts. In a direct comparison, a ketone ester drink raised blood BHB to a peak of about 2.8 mmol/L, while a ketone salt drink reached only about 1.0 mmol/L. The ester also peaked faster and delivered roughly 30 to 60 percent more total BHB exposure over the measurement period.19PubMed Central. On the Metabolism of Exogenous Ketones in Humans
The clinical interest in exogenous ketones is growing. A systematic review of their use in adults with various diseases found that acute ketone ester supplementation in people with pre-diabetes or type 2 diabetes consistently improved blood sugar control, insulin response, and some lipid markers.20PubMed Central. Clinical Benefits of Exogenous Ketosis in Adults with Disease: A Systematic Review Existing human studies also suggest that exogenous ketones can lower blood glucose and may improve certain aspects of cognitive function.21PubMed Central. Exogenous ketone supplementation: an emerging tool for physiologists with potential as a metabolic therapy These are still relatively small studies, and longer-term data remain limited, but the ability to raise ketone levels independent of diet opens up research possibilities that were not feasible when the only way to study ketosis was through fasting or dietary intervention.
Ketogenic Diets and Lipid Changes
Since ketogenesis depends on mobilizing and oxidizing fat, people who sustain the process through a ketogenic diet sometimes worry about their cholesterol. A large meta-analysis of randomized controlled trials found that ketogenic diets significantly lowered triglycerides (by about 20 mg/dL) and raised HDL cholesterol compared to control diets. However, LDL cholesterol also rose by about 8.5 mg/dL, as did total cholesterol.22PubMed. The Impact of the Ketogenic Diet on the Lipid Profile in Adults: A Comprehensive Review and Meta-Regression Analysis of Randomized Controlled Trials The clinical significance of that LDL increase is debated, particularly because the triglyceride-to-HDL ratio, which some researchers consider a better marker of cardiovascular risk than LDL alone, improved.
Duration matters, too. A separate meta-regression found that the initial HDL increase associated with ketogenic diets faded over time, with a significant negative correlation between HDL changes and the length of the intervention.23PubMed Central. Ketogenic diet-induced changes in adult lipid metabolism: a comprehensive systematic review and meta-regression of randomized controlled trials And in strength-trained middle-aged men on calorie-restricted diets, one study found no significant difference in lipid profiles between a ketogenic diet and a non-ketogenic low-carb diet, suggesting that in some contexts the carbohydrate restriction itself, rather than sustained deep ketosis, may be what drives the lipid changes.24PubMed. Effects of calorie restricted low carbohydrate high fat ketogenic vs. non-ketogenic diet on strength, body-composition, hormonal and lipid profile in trained middle-aged men The bottom line for most people is that short-to-medium-term ketogenic diets tend to produce a mixed lipid picture, and individual responses vary widely. Anyone with existing cardiovascular risk factors should monitor their lipid panel during sustained ketosis.
Measuring Ketones
If you want to know whether ketogenesis is actually happening in your body, you have three measurement options: blood, urine, and breath. Blood BHB meters are the gold standard and give a real-time snapshot of your circulating ketone level. Urine strips detect acetoacetate and are cheap and convenient, but they become less reliable over time because as your body adapts to ketosis, the kidneys reabsorb more ketones and excrete fewer. Breath analyzers measure exhaled acetone, which is appealing because it is completely noninvasive. However, accuracy is a concern. One study in adults and children with type 1 diabetes found that while a breath ketone analyzer showed a significant association with blood ketone readings in non-fasting adults, the association was not significant in children. In adults, the breath device had high sensitivity (about 95%) for detecting ketosis at an optimized cutoff, but specificity was only about 54%, meaning it frequently flagged people as ketotic when blood levels said otherwise.25PubMed. Accuracy of a breath ketone analyzer to detect ketosis in adults and children with type 1 diabetes For casual tracking during a ketogenic diet, breath and urine devices give a rough directional signal. For clinical decisions, especially in diabetes management where distinguishing dietary ketosis from dangerous diabetic ketoacidosis matters, blood measurement is the only reliable option.
The Ketogenic Diet and Epilepsy
The oldest therapeutic application of sustained ketogenesis is epilepsy management, dating back to the 1920s. The ketogenic diet remains a standard treatment option for drug-resistant epilepsy, particularly in children. The mechanisms are not fully mapped, but researchers have identified a variety of candidate pathways. These include direct effects on how excitable neurons are at the synapse and broader neuroprotective effects. The molecular foundation for many of these mechanisms traces back to the shift from glucose-based to fat-based energy generation, which changes the relative abundance of ketone bodies, glycolysis intermediates, ATP, and reactive oxygen species in brain tissue.26PubMed. The mechanisms mediating the antiepileptic effects of the ketogenic diet, and potential opportunities for improvement with metabolism-altering drugs The fact that no single mechanism explains the diet’s anticonvulsant effect is actually considered a strength: multiple overlapping pathways may make it effective against seizure types that resist drugs targeting a single receptor or channel. Research interest has also expanded into whether ketosis helps with neurodegenerative conditions, brain injury recovery, and other neurological states where the brain’s glucose metabolism is compromised.27PubMed Central. Cerebral metabolic adaptation and ketone metabolism after brain injury