Ketogenesis: How the Liver Makes Ketones

Ketogenesis is the pathway the liver uses to build ketone bodies from acetyl-CoA produced through fatty acid beta-oxidation. It runs mainly in the mitochondria of liver cells, and the acetoacetate and BHB it produces are exported for use elsewhere in the body rather than burned on site.¹ Here is how the pathway works, what changes ketone production, and how ketogenesis differs from ketosis and ketoacidosis.

Reading evidence in context

1. 1Observed outcome
2. 2Certainty
3. 3Duration
4. 4What remains unknown

What ketogenesis is

Ketogenesis occurs mainly in liver-cell mitochondria. It uses acetyl-CoA produced through fatty acid beta-oxidation to form acetoacetate. Acetoacetate can be converted to beta-hydroxybutyrate (BHB), while some breaks down spontaneously to acetone.¹

One naming quirk is worth knowing: BHB is chemically a hydroxy acid rather than a ketone, but it is conventionally grouped with acetoacetate and acetone as a ketone body. The label is a convention, not a strict chemistry classification.

The liver also sits in an unusual position. Liver cells lack the enzyme SCOT, so they do not oxidize ketone bodies for their own energy. Instead, the liver exports acetoacetate and BHB for use by extrahepatic tissues, the tissues outside the liver.¹

How the pathway works

Three processes are related but distinct, and keeping them separate makes the rest of this easier to follow:

• Lipolysis releases fatty acids from stored triglycerides.
• Beta-oxidation breaks fatty acids into acetyl-CoA.
• Ketogenesis converts acetyl-CoA into ketone bodies.

The fatty acids entering this chain may come from dietary fat or from stored fat. Once acetyl-CoA is formed inside liver mitochondria, the pathway builds acetoacetate, which can become BHB or break down to acetone, and the liver exports acetoacetate and BHB to tissues elsewhere in the body.

Ketogenesis is often discussed alongside gluconeogenesis. They are distinct liver pathways that can occur in overlapping physiological settings; neither supplies fixed fuels to fixed tissues, and they do not always rise together.

What changes ketone production

Ketogenesis is regulated as a continuum, not an on/off switch. Insulin and glucagon signaling, fatty-acid supply, substrate availability, energy balance, activity, fasting, illness, and medicines can all affect ketone production. No single trigger runs the pathway, and the body does not have to empty its glycogen first.

Carbohydrate restriction, fasting, and some exercise contexts can be associated with higher ketone production, but there is no universal number of hours or days for glycogen depletion, ketone rise, brain use, nutritional ketosis, or adaptation. Those three routes do not lead to the same outcome for every person.

Ketogenesis, ketosis, and ketoacidosis are different

These three terms get used loosely, so the distinctions matter. Ketogenesis is the biochemical production pathway. Ketosis describes elevated circulating ketones in a given context. Neither term alone establishes dietary adherence, adaptation, body-fat loss, performance, benefit, or safety.

Diabetic ketoacidosis (DKA) is an acute medical emergency involving elevated ketones and metabolic acidosis in a person with diabetes or qualifying hyperglycemia. High glucose is not required in every case: euglycemic DKA can occur with glucose below 200 mg/dL, including in contexts such as SGLT2-inhibitor use, pregnancy, or reduced food intake.²³

Certain signs require emergency care: persistent vomiting, severe abdominal pain, deep or difficult breathing, confusion, fainting, or marked drowsiness, especially with known or possible diabetes, pregnancy, or SGLT2-inhibitor use. Do not wait for very high glucose, and do not use a ketone reading alone to rule out DKA.²³ The contrast between ketosis and ketoacidosis is worth a closer look on its own.

What a ketone measurement cannot prove

Blood BHB, urine acetoacetate, and breath acetone are different measurements and are not interchangeable. A ketone value cannot by itself establish blood pH, bicarbonate, the cause of symptoms, personal safety, or a clinical outcome. A reading is one measurement of one compound, not a verdict on what is happening in the body. For the compounds themselves, start with what are ketones.

FAQ

Frequently Asked Questions

Is ketogenesis the same as ketosis?

No. Ketogenesis is the biochemical pathway that produces ketone bodies; ketosis describes elevated circulating ketones in a given context. Neither term alone establishes dietary adherence, adaptation, body-fat loss, performance, benefit, or safety.

Does the liver run on the ketones it makes?

No. Liver cells lack the enzyme SCOT and do not oxidize ketone bodies for their own energy. The liver exports acetoacetate and BHB for use by extrahepatic tissues.^{[1](#footnote-1)}

Is BHB actually a ketone?

Chemically, it is a hydroxy acid rather than a ketone, but it is conventionally grouped with acetoacetate and acetone as a ketone body.

How quickly does ketogenesis start?

There is no universal number of hours or days for glycogen depletion, ketone rise, brain use, nutritional ketosis, or adaptation. Production is regulated as a continuum and responds to many inputs, not to one trigger.

Can someone have ketoacidosis without very high glucose?

Yes. Euglycemic DKA can occur with glucose below 200 mg/dL, including with SGLT2-inhibitor use, pregnancy, or reduced food intake. Persistent vomiting, severe abdominal pain, deep or difficult breathing, confusion, fainting, or marked drowsiness requires emergency care, and a ketone reading alone cannot rule DKA out.^{[2](#footnote-2)}^{[3](#footnote-3)}

Do blood, urine, and breath ketone tests measure the same thing?

No. Blood BHB, urine acetoacetate, and breath acetone are different measurements and are not interchangeable, and no single value can establish blood pH, bicarbonate, the cause of symptoms, safety, or a clinical outcome.

Works cited

1. Biochemistry, Ketogenesis — StatPearls, NCBI Bookshelf https://www.ncbi.nlm.nih.gov/books/NBK493179/
2. Hyperglycemic Crises in Adults With Diabetes: A Consensus Report — Diabetes Care, 2024 https://pmc.ncbi.nlm.nih.gov/articles/PMC11272983/
3. FDA revises SGLT2-inhibitor labels to include ketoacidosis warnings — U.S. Food and Drug Administration https://www.fda.gov/files/drugs/published/FDA-revises-labels-of-SGLT2-inhibitors-for-diabetes-to-include-warnings-about-too-much-acid-in-the-blood-and-serious-urinary-tract-infections.pdf