Ketones are molecules your liver produces from stored fat when your body doesn’t have enough glucose for energy. They serve as an alternative fuel source, powering your brain, heart, and muscles during fasting, prolonged exercise, or very low carbohydrate intake. In healthy people at rest, blood ketone levels sit around 0.1 mmol/L, but they can rise significantly depending on diet, activity, and metabolic health.
How Your Body Makes Ketones
Ketone production starts with fat. When glucose runs low, your fat cells release fatty acids into the bloodstream. Those fatty acids travel to the liver, where they’re shuttled into the energy-producing compartments of cells called mitochondria. Once inside, the fatty acids are broken down into a raw building block called acetyl-CoA, which is normally burned for energy. But when there’s more acetyl-CoA than the liver can use, the excess gets converted into ketone bodies instead.
Three key steps control how many ketones your body produces: how quickly fat cells release fatty acids, how efficiently those fatty acids enter liver mitochondria, and how much of the resulting material gets diverted toward ketone production rather than other uses. This is why ketone levels respond so directly to what and when you eat. Restrict carbohydrates or skip meals, and each of these steps accelerates.
The Three Types of Ketones
Your liver produces three distinct ketone bodies, each with a slightly different role:
- Beta-hydroxybutyrate (BHB) is the most abundant ketone in your blood and the one most commonly measured. In people without diabetes, blood levels average around 0.44 mmol/L. It’s the primary ketone your cells actually burn for fuel.
- Acetoacetate is the first ketone produced in the liver. It either gets converted into BHB or breaks down into acetone. Blood levels are typically much lower than BHB, averaging around 0.05 mmol/L in healthy individuals.
- Acetone is a byproduct that your body can’t efficiently use for energy. It’s expelled through your breath, which is why people in ketosis sometimes notice a fruity or metallic smell. Breath acetone in non-diabetic individuals is generally below 0.8 parts per million.
Why Your Brain Relies on Ketones
Your brain is extremely energy-hungry, consuming roughly 20% of your body’s total fuel. It can’t burn fat directly because fatty acid molecules are too large to cross the blood-brain barrier. Ketones solve this problem. They cross into brain tissue through specialized transport channels called monocarboxylate transporters, where nerve cells pull them into their mitochondria and convert them back into usable energy.
The brain’s ability to use ketones actually increases when demand is high. After a traumatic brain injury, for example, the number of these transporter channels in the brain increases by roughly 85%, allowing neurons to take in more ketone fuel. Diet, fasting, and exercise also influence how permeable the blood-brain barrier is to ketones, meaning your brain gets better at using them the more consistently they’re available.
Ketones Do More Than Provide Fuel
For decades, scientists viewed ketones purely as backup energy. That picture has changed. BHB also acts as a signaling molecule, binding to receptors on cell surfaces and influencing which genes get turned on or off. One of its most studied roles is blocking a group of enzymes that normally keep genes tightly wound and silent. By inhibiting these enzymes, BHB essentially loosens the packaging around DNA, allowing cells to ramp up production of proteins involved in stress resistance and cellular repair.
This creates a direct link between what you eat and how your genes behave. When you fast or restrict carbohydrates, rising BHB levels trigger changes in gene expression that go well beyond simple energy metabolism. Researchers now consider ketone bodies crucial regulators of metabolic health, in part because of their ability to influence this layer of gene control.
How Ketones Protect Muscle During Fasting
Without ketones, extended fasting would be far more destructive to your body. Here’s why: your brain needs a constant supply of glucose, and when you’re not eating, the main way your body manufactures glucose is by breaking down muscle protein. Amino acids from muscle tissue get sent to the liver and converted into glucose through a process called gluconeogenesis.
Ketones short-circuit this cycle. By replacing glucose as the brain’s primary fuel during fasting, they dramatically reduce the brain’s glucose demand. This suppresses the need to cannibalize muscle for raw materials. Ketones also help preserve carbohydrate stores and prevent the breakdown of muscle contractile proteins, the structural components that let muscles generate force. This protein-sparing effect is one reason the body evolved to produce ketones in the first place: it allows survival during food scarcity without rapidly wasting away.
How Quickly Ketone Production Begins
Your body doesn’t flip into ketone production instantly. After your last carbohydrate-containing meal, your liver first burns through its stored glycogen (the storage form of glucose). This process typically takes 12 to 24 hours, depending on how active you are and how full those stores were to begin with.
If you follow a very low carbohydrate diet, eating between 20 and 50 grams of carbs per day, it usually takes two to four days to enter a state of sustained ketosis. For some people, it can take a week or longer. Intermittent fasting can speed up the transition. The variability comes down to individual differences in glycogen storage, activity level, and metabolic flexibility.
Ketosis vs. Ketoacidosis
This distinction matters enormously. Nutritional ketosis and diabetic ketoacidosis (DKA) both involve elevated ketones, but they occupy completely different ends of the spectrum.
In nutritional ketosis, blood ketone levels range from 0.5 to 3 mmol/L. This is a normal, regulated metabolic state. Insulin remains present in the blood, keeping ketone production in check. Your blood pH stays stable.
Starvation ketosis pushes levels higher, typically between 5 and 10 mmol/L, but the body still maintains some degree of control. DKA is a medical emergency that occurs primarily in people with type 1 diabetes (and occasionally type 2). Without sufficient insulin, the liver produces ketones in an uncontrolled cascade, driving blood levels to 15 to 25 mmol/L. At these concentrations, ketones make the blood dangerously acidic, which can damage organs and become life-threatening.
In a healthy person with functioning insulin production, reaching DKA-level ketones through diet alone is essentially impossible. The body’s insulin response acts as a built-in brake on ketone production.
How Ketone Levels Are Measured
Three practical methods exist for tracking ketones, each measuring a different type:
- Blood meters measure BHB directly from a finger prick. This is the most accurate method and the one used in clinical settings. Results appear in mmol/L.
- Urine strips detect acetoacetate. They’re inexpensive but become less reliable over time because as your body gets more efficient at using ketones, fewer spill into urine.
- Breath analyzers measure acetone. Breath acetone correlates positively with blood BHB levels (a correlation of about 0.64 in research), making it a reasonable, non-invasive proxy, though less precise than a blood reading.
For most people interested in whether they’re in ketosis, a blood BHB reading between 0.5 and 3 mmol/L confirms it. Values below 0.5 suggest you haven’t fully transitioned to fat-based fuel production.