What Ketosis Is: Ketone Bodies, BHB and Your Liver
Ketosis is the state in which your liver turns fat into small fuel molecules called ketone bodies, and your brain and muscles burn them in place of some of the glucose they would normally use.
On this page
- Ketosis in one paragraph
- Where ketones are made
- The three ketone bodies
- Why your brain can run on ketones
- What turns ketosis on and off
- Ketones as more than fuel
- How you can tell you are in ketosis
- Common questions
Ketosis in one paragraph
When glucose is scarce, your body breaks down stored fat into fatty acids. Many tissues can burn fatty acids directly. Your brain mostly cannot. So the liver converts some of those fatty acids into ketone bodies, which travel in the blood and can be used by the brain, heart and muscles. Ketosis is the name for having more of these ketones in your blood than usual.
This is a normal backup system. It switches on during fasting, during long stretches without food, and on a very-low-carb diet.
Where ketones are made
Ketones are made almost entirely in the liver, inside the mitochondria (the parts of the cell that handle energy). The NIH StatPearls chapter on ketogenesis describes the key step: an enzyme called HMG-CoA synthase drives the pathway that produces acetoacetate, BHB and acetone (Dhillon & Gupta).
The process in plain steps:
- Fat cells release fatty acids into the blood.
- The liver takes them up and breaks them into two-carbon pieces.
- With little glucose around, those pieces build up faster than the liver can burn them.
- The liver joins them into acetoacetate, the first ketone body.
- Much of the acetoacetate is converted to BHB; a small amount breaks down into acetone.
- Ketones leave the liver and are used by other organs.
The liver itself does not burn the ketones it makes. It exports them for the rest of the body.
The three ketone bodies
| Ketone body | What it is | Where you find it | How it is measured |
|---|---|---|---|
| Acetoacetate | The first ketone the liver makes | Blood; extra spills into urine | Urine strips |
| Beta-hydroxybutyrate (BHB) | Made from acetoacetate; the main ketone in the blood | Blood | Blood ketone meters |
| Acetone | A small breakdown product of acetoacetate | Breath | Breath meters |
BHB is technically not a "ketone" in the chemistry sense, but it is always grouped with the ketone bodies because it is made in the same pathway and used the same way.
KetoDick on this: he names the same three, BHB, acetoacetate and acetone, as what the body releases once glycogen is depleted (video s_sNd1v8Low). He explains that urine strips read acetoacetate "spilling" into urine, so the strip's shade says little about blood levels (video 8wOheKcBEd8; video GojQvo_2GS0).
Why your brain can run on ketones
Your brain uses a lot of energy and normally relies on glucose. Fatty acids do not cross into the brain well, so in a long fast the brain would be short of fuel without another source. Ketones fill that gap.
Two papers are the standard references:
- Owen et al. 1967. In people fasting for several weeks, BHB and acetoacetate replaced glucose as the brain's main fuel (Owen et al. 1967).
- Cahill 2006. A review of how the body shifts during starvation from glucose to fatty acids and liver-made ketones, which spares body protein that would otherwise be broken down to make glucose (Cahill 2006).
The second point matters for dieters. Ketones let the body cover much of the brain's fuel need without breaking down as much muscle protein to make glucose.
Those studies looked at fasting, not at a ketogenic diet with full meals. The same pathway is used in both, but the size of the effect in a fed person on keto is not what those papers measured.
What turns ketosis on and off
Ketone production is controlled mainly by two hormones: insulin and glucagon (Dhillon & Gupta).
| Signal | Effect on ketone production |
|---|---|
| Insulin high (after a carb-rich meal) | Production slows; fat stays stored |
| Insulin low (few carbs, fasting) | Fat is released; liver makes ketones |
| Glucagon high (fasting, low carb) | Encourages the liver to make ketones |
This is why carbohydrate is the main lever. Carbs raise blood glucose, which raises insulin, which slows ketone production.
KetoDick on this: he says insulin "impedes" lipase, the enzyme he says breaks fat down, and that lowering insulin is what lets stored fat be used (video s_sNd1v8Low). He also describes ketosis as building on itself "like a chain reaction" once you are adapted (video xyEZL0HuejI).
Ketones as more than fuel
Researchers have found that BHB also acts as a signal in the body, separately from being burned for energy. Two highly cited lab studies:
- Youm et al. 2015. BHB blocked activation of an inflammation switch called the NLRP3 inflammasome in mice and in human immune cells (Youm et al. 2015).
- Shimazu et al. 2013. BHB inhibited a class of enzymes (class I histone deacetylases) in mice and raised genes linked to resisting oxidative stress (Shimazu et al. 2013).
These are mechanism studies in animals and cells. They do not show that a ketogenic diet reduces inflammation or disease in people. A 2017 review in Cell Metabolism covers this signaling research and the proposed uses (Puchalska & Crawford 2017).
How you can tell you are in ketosis
There are three ways to measure ketones, one for each ketone body:
- Urine strips read acetoacetate. They are cheap but become less useful over time, because your body wastes fewer ketones once it is used to them. See Urine Strips.
- Blood meters read BHB. They are the most direct measure. See Blood Meters.
- Breath meters read acetone. See Breath Meters.
Whether you need to test at all is a separate question, covered in Do You Need to Test? and the Testing Ketosis chapter.
KetoDick on this: his view has changed. In 2017 he called ketone meters and strips useful biofeedback for many people, though not needed to reach ketosis (video 8wOheKcBEd8). In 2019 he said you do not need to check your levels with a device and that ketone monitors are optional (video 1JbK1_ClDmY).
Common questions
Does being in ketosis mean you are burning body fat?
Not on its own. Ketosis means your body is using fat for fuel, but that fat can come from your plate as well as your body. You lose body fat when you use more energy than you eat. See Does Ketosis Mean Burning Fat? and the Fat Loss chapter.
Is ketosis the same as ketoacidosis?
No. Nutritional ketosis and diabetic ketoacidosis are different states. See Ketosis vs Ketoacidosis.
How long does it take to get into ketosis?
Ketones usually start rising within days of cutting carbs, once glycogen runs low. Feeling adapted takes longer. See Fat Adaptation and How Long to Get Into Ketosis.
Do ketone drinks or pills put me in ketosis?
They raise blood ketones for a while, but they do not make your liver produce ketones from your own fat. See Exogenous Ketones and Weight Loss.
Related
- Guide: Chapter 1: What a Ketogenic Diet Is · Ketosis vs Ketoacidosis · Fat Adaptation · Testing Ketosis
- Blog: Signs You Are in Ketosis · Optimal Ketone Level for Weight Loss · Does Ketosis Mean Burning Fat?
- Videos: What is the ketogenic diet (s_sNd1v8Low) · Three ways to test for ketosis (8wOheKcBEd8)
Last verified: 2026-10-31