What is Ketones?

What is Ketones?

Ketones, also called ketone bodies, are small fat-derived fuel molecules your liver produces when carbohydrate is scarce. There are three of them: beta-hydroxybutyrate (BHB), acetoacetate, and acetone. Chemically BHB is actually a hydroxy acid rather than a true ketone, but physiologists group all three together because they travel and burn as a set. They exist for one reason: to keep the body, and especially the brain, fueled when blood glucose runs low.

The brain cannot burn fatty acids directly because long-chain fats do not cross the blood-brain barrier well, so during fasting or carbohydrate restriction the liver converts fat into ketones, which cross that barrier easily and plug straight into energy production. You make a trickle of ketones every night during sleep and a larger amount whenever you fast, eat very few carbohydrates, or train long enough to drain liver glycogen.

In a fed person eating normal carbohydrate, blood ketones sit below about 0.1 mmol/L. Push carbohydrate low enough or fast long enough and the liver ramps ketone output up several hundredfold. This shift from running mostly on glucose to running substantially on fat-derived ketones is what people mean by ketosis, and it is the metabolic state a ketogenic diet is built to create and sustain.

How it works

Ketones are produced by ketogenesis, a pathway that runs in the mitochondria of liver cells. When insulin falls and glucagon rises — the hormonal signal of fasting or low carbohydrate intake — stored and dietary fat is broken down into free fatty acids, which travel to the liver and undergo beta-oxidation into acetyl-CoA. Normally acetyl-CoA enters the citric acid (Krebs) cycle, but when carbohydrate is scarce the cycle's capacity is limited, so surplus acetyl-CoA is redirected.

Two acetyl-CoA molecules condense and, through the rate-limiting enzyme HMG-CoA synthase, form acetoacetate. Acetoacetate is then either reduced to beta-hydroxybutyrate (the dominant circulating form, usually 70–80% of blood ketones) or spontaneously loses carbon dioxide to become acetone, the volatile molecule you smell on the breath as fruity keto breath. The liver exports these ketones into the blood but cannot use them itself, because it lacks the enzyme SCOT (succinyl-CoA:3-oxoacid CoA transferase).

Peripheral tissues — brain, heart, skeletal muscle, kidney — take them up and reverse the process: BHB is oxidized back to acetoacetate, converted to acetoacetyl-CoA by SCOT, split into two acetyl-CoA, and burned in the Krebs cycle for ATP. Ketone oxidation actually yields slightly more energy per unit of oxygen than glucose, which is part of why the heart and brain use them readily. The whole system is a fuel-switching mechanism that let humans survive long stretches without food.

How to apply it

• Follow a ketogenic diet: Cut carbohydrate to roughly 20–50 grams a day so liver glycogen stays low and insulin stays down. Fill the gap mostly with fat and adequate protein. Blood ketones typically rise into nutritional ketosis (0.5–3.0 mmol/L) within two to four days.
• Fast or time-restrict eating: Once liver glycogen is drained — usually 12 to 16 hours without food — the liver shifts toward ketone production. Extended fasts push ketones higher still, reaching several mmol/L after a few days, which is the ancient survival version of ketosis.
• Do prolonged endurance exercise: Long, glycogen-depleting sessions such as multi-hour cycling or running raise post-exercise ketones as the liver taps fat. This is a transient, exercise-driven bump rather than sustained dietary ketosis, and it clears once you refuel with carbohydrate.
• Add MCT oil or coconut-derived fats: Medium-chain triglycerides are absorbed straight to the liver and converted to ketones faster than long-chain fats. A tablespoon can nudge blood ketones up within an hour, which is why MCT is common on ketogenic and therapeutic protocols.
• Take exogenous ketones (esters or salts): Ketone monoesters and BHB salts raise blood ketones directly, often to 1–3 mmol/L within 30 minutes, without any carbohydrate restriction. They mimic the blood level of ketosis but do not by themselves put you into fat-burning metabolism (see misconceptions).
• Measure to confirm, do not guess: A blood BHB meter is the gold standard for nutritional ketosis. Breath acetone meters track a related signal, and urine strips detect acetoacetate but become unreliable once you adapt, because the kidney reclaims ketones instead of dumping them.

Types

Worked example

Blood BHB is the number that tells you where you sit on the ketone spectrum. The table below maps common metabolic states to their typical blood ketone range so you can read a meter honestly. Note the enormous gap between nutritional ketosis, which is safe and self-limiting, and diabetic ketoacidosis, which is a medical emergency.

Nutritional ketosis is capped by your own insulin, which rises just enough to keep ketones in a safe band. Ketoacidosis only happens when that brake fails, almost always in type 1 diabetes or severe illness, not from eating low-carb.

Ketones vs glucose as fuel

Ketones and glucose are complementary backup and primary fuels, not rivals. Most people burn mainly glucose; ketones become important when carbohydrate is scarce, letting the brain and heart keep working through fasts and famine.

By goal

• Fat loss: Ketosis is a signal that carbohydrate is low, not a magic fat-burner. You still lose fat only in a calorie deficit. A keto diet can help by curbing appetite and simplifying food choices, but ketones on a meter do not guarantee you are losing body fat.
• Endurance performance: Fat and ketone use spares glycogen at low-to-moderate intensity, which appeals to ultra-endurance athletes. But high-intensity efforts still demand glucose, and evidence that raising ketones improves performance is mixed at best, so keep carbohydrate available for hard sessions.
• Therapeutic and metabolic health: Medically supervised ketogenic diets are established therapy for drug-resistant epilepsy and are studied for other conditions. These are clinical protocols with monitoring, not casual weight-loss diets. Anyone with diabetes or on medication should involve a clinician before pursuing ketosis.

Common misconceptions

• "Being in ketosis means you are burning body fat." Ketosis only means your body is running on fat-derived fuel; that fat can come from your plate. On a high-fat keto diet in a calorie surplus you can be in ketosis and still gain fat. Fat loss depends on a calorie deficit, not on a ketone reading.
• "Exogenous ketone supplements make you lose weight." Drinking ketones raises the number on a blood meter but does not switch on fat-burning metabolism, and the ketones themselves carry calories. Trials show no reliable weight-loss benefit, and elevated blood ketones can actually suppress your own fat breakdown for a while.
• "Ketosis and ketoacidosis are the same thing." They are not. Nutritional ketosis holds blood ketones around 0.5–3 mmol/L and is safe. Diabetic ketoacidosis runs above 10 mmol/L, makes the blood acidic, and is a life-threatening emergency seen mainly in type 1 diabetes, not in healthy people eating low-carb.
• "Beta-hydroxybutyrate is chemically a ketone." It is not, strictly speaking. BHB is a hydroxy acid; the true ketone group belongs to acetoacetate and acetone. Physiologists still count BHB as a ketone body because it is made, transported, and burned as part of the same fuel system.
• "Higher ketone levels are always better." Beyond nutritional ketosis, more ketones bring no extra performance or health payoff and can cause nausea and gut distress from supplements. The useful range for a keto diet is modest — roughly 0.5 to 3 mmol/L — not the highest number you can push.

Related terms

Ketones FAQ

What are ketones in simple terms?

Ketones are fuel molecules your liver makes from fat when you are low on carbohydrate, such as during fasting or a keto diet. They travel in the blood to your brain, heart, and muscles and get burned for energy, acting as a backup when glucose runs short.

What are the three ketone bodies?

The three ketone bodies are beta-hydroxybutyrate (BHB), acetoacetate, and acetone. BHB is the most abundant in blood and the one meters measure. Acetoacetate is what urine strips detect. Acetone is a minor byproduct you breathe out, causing the fruity keto breath smell.

What is the difference between ketones and ketosis?

Ketones are the actual molecules the liver makes from fat. Ketosis is the metabolic state of having enough of them in your blood to run on, usually a BHB level of 0.5 to 3.0 mmol/L. Put simply, ketones are the fuel and ketosis is running on that fuel.

What blood ketone level counts as nutritional ketosis?

Nutritional ketosis is generally defined as a blood beta-hydroxybutyrate level between about 0.5 and 3.0 mmol/L. Below 0.5 you are barely producing ketones. Prolonged fasting can push levels to 5 to 7 mmol/L, which is still safe and very different from dangerous ketoacidosis above 10.

Do ketones burn body fat?

Not on their own. Ketosis shows your body is using fat-derived fuel, but that fat can come from your diet. You lose body fat only in a calorie deficit. A keto diet may help by reducing appetite, yet ketones on a meter do not prove you are losing fat.

Are ketones dangerous?

Ketones from fasting or a keto diet are safe for most healthy people, because insulin keeps them in a self-limiting range. The dangerous version is diabetic ketoacidosis, where ketones climb above 10 mmol/L and acidify the blood. That is an emergency, mostly affecting people with type 1 diabetes.

Do exogenous ketone supplements actually work?

Ketone esters and salts do raise blood ketones quickly, but they do not force your body into fat-burning metabolism, and studies show no reliable weight-loss or clear performance benefit. They also cost a lot and often upset the stomach, so treat marketing claims with caution.

Can your brain run on ketones?

Yes. The brain cannot burn fat directly, but it readily uses ketones. During prolonged fasting, ketones can supply roughly 60 to 65% of the brain's energy, replacing much of its usual glucose demand. This adaptation is why humans survive long stretches without eating.

How do I get into ketosis and raise my ketones?

The reliable routes are cutting carbohydrate to about 20 to 50 grams a day, fasting for 12 or more hours, or draining glycogen with long exercise. MCT oil speeds things up. Most people reach nutritional ketosis within two to four days of consistent low-carb eating.

How do you measure ketones?

A finger-prick blood meter reading beta-hydroxybutyrate is the most accurate and the standard for nutritional ketosis. Breath meters estimate acetone, and cheap urine strips detect acetoacetate but grow unreliable once you adapt, because the kidney starts reclaiming ketones instead of excreting them.

References

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