A ketone is a type of organic molecule built around a specific arrangement: a carbon atom double-bonded to an oxygen atom, with two other carbon-containing groups attached on either side. In chemistry, this carbon-oxygen pair is called a carbonyl group, and it shows up in everything from nail polish remover to industrial solvents. But if you searched “what is a ketone,” you’re probably more interested in the ketones your body makes, the ones tied to fasting, low-carb diets, and blood sugar management. Those are a specific subset called ketone bodies, and they serve as an alternative fuel source when glucose runs low.
Ketones in Chemistry vs. Ketones in Your Body
In a chemistry textbook, a ketone is any molecule where a carbonyl group (C=O) sits between two carbon-based groups. If one of those groups is a hydrogen atom instead, the molecule is an aldehyde, not a ketone. Acetone, the solvent in nail polish remover, is the simplest ketone.
When doctors, dietitians, or fitness articles mention “ketones,” they’re almost always referring to three specific molecules your liver produces: acetoacetate, beta-hydroxybutyrate (BHB), and acetone. These are collectively called ketone bodies. Beta-hydroxybutyrate is the most abundant in circulation and the one most commonly measured in blood tests. Acetone is the least metabolically useful of the three and is mostly exhaled through your lungs, which is why people in deep ketosis sometimes notice a fruity or metallic taste on their breath.
How Your Body Makes Ketones
Ketone production ramps up when your body doesn’t have enough glucose to meet its energy demands. This happens during extended fasting, prolonged exercise, very low-carb diets, or starvation. The process starts in your fat cells, which release stored fat into the bloodstream as free fatty acids. Those fatty acids travel to the liver, where they’re broken down into a building-block molecule called acetyl-CoA.
Normally, acetyl-CoA feeds into the liver’s main energy cycle. But when carbohydrate intake is very low, that cycle can’t process all the acetyl-CoA being generated. The surplus gets shunted into a different pathway, producing acetoacetate first. From there, acetoacetate is either converted into BHB or breaks down spontaneously into acetone. Several hormones accelerate this process. Glucagon (which rises when blood sugar drops), cortisol, adrenaline, and thyroid hormones all promote the release of fatty acids from fat stores, feeding the ketone production line. Insulin does the opposite: when insulin levels are high (after eating carbs, for instance), ketone production slows dramatically.
What Ketones Do as Fuel
Ketones are not a backup generator that kicks on reluctantly. They’re a genuinely efficient fuel. Per unit of oxygen consumed, ketone oxidation produces more energy than burning fat does, and it’s nearly as efficient as burning glucose. This matters most for organs with high energy demands, particularly the brain and heart.
Your brain normally runs almost entirely on glucose because most fats can’t cross the blood-brain barrier. Ketones are the exception. They cross into the brain through specialized transporters and are taken up by neurons directly. During prolonged fasting or strict carbohydrate restriction, ketones can supply a significant share of the brain’s energy needs, sparing the body from having to break down muscle protein to manufacture glucose.
The heart is actually quite fond of ketones too. Cardiac muscle readily oxidizes BHB, and research into heart failure has explored whether shifting the heart’s fuel mix toward ketones could improve function.
Nutritional Ketosis vs. Ketoacidosis
This distinction is critical. Nutritional ketosis is the mild, controlled elevation of blood ketones that happens on a ketogenic diet or during fasting. Blood BHB levels in this state typically range from about 0.5 to 5 millimoles per liter (mmol/L). For reference, someone eating a standard mixed diet usually has BHB levels around 0.1 mmol/L. In nutritional ketosis, insulin is still present in the bloodstream at low levels, which acts as a brake preventing ketone production from spiraling out of control.
Diabetic ketoacidosis (DKA) is a medical emergency. It occurs primarily in people with type 1 diabetes (and occasionally type 2) when insulin is severely deficient or absent. Without that brake, ketone production runs unchecked. BHB levels can soar to 20 to 25 mmol/L, four to five times higher than even aggressive therapeutic ketosis. At those concentrations, ketone bodies (which are acidic) overwhelm the blood’s buffering systems, dropping arterial pH below 7.35. Blood glucose simultaneously spikes to dangerously high levels. DKA requires emergency medical treatment. It is physiologically distinct from the ketosis someone achieves by skipping bread.
Alcoholic ketoacidosis is a less common variant that can occur after heavy, prolonged drinking combined with poor food intake. Ketone levels can reach around 15 mmol/L, though the blood pH changes are less predictable than in DKA.
How Ketone Levels Are Measured
There are three main ways to check your ketone levels, and they don’t all measure the same molecule.
- Blood meters measure beta-hydroxybutyrate directly from a finger prick. This is the most accurate method and the one used in clinical settings. It reflects your current ketone levels in real time.
- Urine strips detect acetoacetate. They’re cheap and easy to use, but less reliable. As your body becomes more efficient at using ketones (a process called keto-adaptation), less acetoacetate spills into urine, so the strips can read low even when blood ketone levels are adequate. Studies comparing the two methods found that blood BHB testing led to fewer emergency visits and faster recovery from DKA episodes in people with type 1 diabetes.
- Breath meters measure acetone in exhaled air. A reading of 9 parts per million or higher generally corresponds to nutritional ketosis (BHB at or above 0.5 mmol/L). These devices are reusable but less precise than blood meters.
Therapeutic Uses of Ketones
The ketogenic diet was originally developed in the 1920s as a treatment for epilepsy, and it remains a legitimate medical therapy today. Ketones appear to reduce seizure activity through several mechanisms: they boost levels of GABA (the brain’s primary calming neurotransmitter), reduce levels of excitatory signals, and activate adenosine receptors that have natural seizure-suppressing effects. They also influence potassium and sodium channels in neurons, making those cells less likely to fire uncontrollably. The blood BHB levels associated with seizure reduction are relatively high, around 4 mmol/L, which typically requires a strict medical ketogenic diet.
Research has also explored ketosis for neurodegenerative conditions. Preliminary studies suggest potential benefits for Alzheimer’s and Parkinson’s disease at lower thresholds, with some improvement in Parkinson’s symptoms observed at BHB levels around 1.0 mmol/L. These findings are early-stage, and the required ketone levels vary significantly depending on the condition. Migraine management, like epilepsy, appears to require higher levels near 4 mmol/L.
Exogenous Ketones: Supplements That Raise BHB
You don’t have to fast or restrict carbs to raise blood ketone levels. Exogenous ketone supplements deliver BHB directly, and they come in two main forms.
Ketone salts combine BHB with a mineral like sodium, potassium, or calcium. They’re widely available and relatively affordable. A typical dose raises blood BHB to about 0.4 to 1.0 mmol/L, which is modest, roughly the low end of nutritional ketosis. The active form of BHB stays elevated for about 2 to 4 hours.
Ketone monoesters are more potent. A single dose can push blood BHB to 3 to 6 mmol/L in a fasted state, reaching peak levels within about 40 to 70 minutes. During exercise, levels typically settle in the 1.5 to 4.0 mmol/L range. These supplements are more expensive and often have a strong, bitter taste that limits their appeal. Athletes have been the primary market, though evidence on whether exogenous ketones consistently improve performance is still mixed.
Both forms raise BHB without requiring you to change your diet, which makes them useful for research settings and for people who want some of the metabolic effects of ketosis without full dietary restriction. They don’t, however, replicate every effect of a ketogenic diet, since they don’t lower insulin or increase fat breakdown the way carbohydrate restriction does.