Ketones (sometimes misspelled “keytones”) are molecules your body produces when it burns fat for fuel instead of carbohydrates. Your liver creates them when glucose is in short supply, whether from fasting, a very low-carb diet, or prolonged exercise. They serve as an alternative energy source for your brain, heart, and muscles, and understanding how they work helps explain everything from popular diets to serious medical conditions.
The Three Types of Ketones
Your body produces three distinct ketone molecules. The first two do the heavy lifting as energy carriers, transporting fuel from the liver to other tissues throughout the body. The third is mostly a byproduct.
- Acetoacetate is the initial ketone produced during fat breakdown. It serves as the starting point for the other two.
- Beta-hydroxybutyrate is formed from acetoacetate inside your cells’ energy centers (mitochondria). It’s the most abundant ketone in your blood and the one doctors measure to assess ketosis.
- Acetone is created when acetoacetate breaks down spontaneously. It’s the least useful as fuel and is responsible for the fruity or sweet smell on the breath of people in deep ketosis.
How Your Body Makes Ketones
Ketone production happens in your liver and follows a straightforward logic. When you eat fewer carbohydrates, your insulin levels drop. That drop signals your body to start pulling fatty acids out of fat stores. Those fatty acids travel to the liver, where they’re broken down into a molecule called acetyl-CoA.
Normally, acetyl-CoA enters your cells’ main energy cycle. But when carbs are scarce and fat breakdown is running high, there’s more acetyl-CoA than that cycle can handle. The excess gets funneled into ketone production. The liver packages it into acetoacetate and beta-hydroxybutyrate, then releases them into the bloodstream for other organs to use as fuel. Your liver itself can’t use ketones; it only manufactures them for the rest of your body.
Why Ketones Matter for Your Brain
Your brain is extremely energy-hungry but also extremely picky. It can’t burn fat directly because fatty acids are too large to cross the blood-brain barrier, the tightly controlled gateway that protects brain tissue. Glucose is normally the brain’s primary fuel. But ketones are small enough to pass through using specialized transport proteins on blood vessel walls and brain cells.
Once inside nerve cells, ketones enter the mitochondria and generate ATP, the molecule your cells use as energy currency. The number of transport proteins on the blood-brain barrier actually increases during fasting or sustained low-carb eating, meaning your brain gets better at absorbing ketones the longer you’re in ketosis. This adaptability is one reason people on ketogenic diets often report mental clarity improving after the first week or two.
What Triggers Ketone Production
The most reliable trigger is restricting carbohydrates to roughly 20 to 50 grams per day. For context, a single banana contains about 27 grams of carbs, so this is a significant restriction. At that level, insulin drops enough to unlock fat stores and ramp up ketone production in the liver. A standard ketogenic diet derives 60% to 75% of its calories from fat, with moderate protein filling the rest.
Fasting also triggers ketogenesis. After about 12 to 24 hours without food, your glycogen (stored glucose) runs low and your liver begins converting fat to ketones. Extended exercise can have a similar effect, especially when glycogen stores are already depleted.
Nutritional Ketosis vs. Ketoacidosis
This distinction is critical because the two conditions sound similar but are vastly different in severity. Nutritional ketosis is a normal, controlled metabolic state. Blood ketone levels typically sit between 0.5 and 3 mmol/L, and the body’s pH stays in a safe range. Millions of people enter mild ketosis through dieting, intermittent fasting, or even skipping a meal.
Diabetic ketoacidosis (DKA) is a medical emergency. It occurs primarily in people with type 1 diabetes whose bodies produce little or no insulin. Without insulin to regulate the process, ketone production spirals out of control, pushing blood levels to 20 to 25 mmol/L, roughly five to ten times higher than nutritional ketosis. Blood sugar simultaneously soars to dangerously high levels, and the blood becomes acidic. DKA requires emergency treatment.
Alcoholic ketoacidosis is a related condition that can occur after heavy, prolonged drinking combined with poor nutrition. Ketone levels in this case can reach around 15 mmol/L. The key takeaway: a healthy person following a low-carb diet is in no danger of ketoacidosis. The body has multiple feedback mechanisms that keep ketone levels within a safe range as long as insulin is functioning normally.
Medical Uses Beyond Weight Loss
The ketogenic diet was originally developed in the 1920s not for weight loss but to treat epilepsy, after doctors noticed that fasting reduced seizures. It remains a well-established therapy for drug-resistant epilepsy, particularly in children. For certain rare metabolic conditions where the body can’t properly use glucose for brain fuel, a ketogenic diet is considered a first-line treatment rather than a last resort.
Research also points to potential benefits in neurodegenerative diseases like Alzheimer’s and Parkinson’s. The working theory is that ketones support mitochondrial function in brain cells, essentially providing a cleaner or more accessible energy source when glucose metabolism is impaired. Some cancer researchers are exploring whether tumors, which rely heavily on glucose, may struggle to use ketones efficiently, potentially making dietary therapy a useful complement to standard treatment. These applications are still being studied, but the underlying biology is promising.
How to Test Your Ketone Levels
Three testing methods exist, and they differ significantly in accuracy.
Blood meters are the gold standard. They measure beta-hydroxybutyrate with a finger prick, similar to a glucose monitor. Clinical studies treat blood meters as the reference method because they closely match laboratory results. If you want reliable numbers, this is the way to go. The target range for nutritional ketosis is 0.5 to 3 mmol/L.
Urine strips are cheap and widely available, but they’re unreliable for detecting mild ketosis. They measure acetoacetate rather than beta-hydroxybutyrate, and research shows their sensitivity at lower ketone levels is poor. At a blood threshold of 0.5 mmol/L, urine strips only detected ketosis about 52% of the time. They’re better at catching higher levels but still miss a lot. As your body becomes more efficient at using ketones, fewer spill into the urine, making the strips even less useful over time.
Breath analyzers measure acetone and offer a middle ground in terms of convenience and accuracy, though they’re less studied than blood meters. They can confirm you’re in ketosis but won’t give you a precise blood level.
Exogenous Ketone Supplements
You can also raise your blood ketone levels without changing your diet by taking exogenous (externally sourced) ketones. These come in two main forms. Ketone esters are more potent and tend to cause less stomach upset, but they’re expensive and often unpleasant-tasting. Ketone salts combine beta-hydroxybutyrate with minerals like sodium and are more affordable and widely available.
Research confirms that ketone salts can meaningfully raise both beta-hydroxybutyrate and acetoacetate levels within about 30 minutes of consumption. One study in healthy adults found that a ketone salt supplement increased acetoacetate by an average of 0.57 mmol/L compared to a negligible change with a placebo. Both forms may occasionally cause mild gastrointestinal discomfort, though many people tolerate them well.
Exogenous ketones won’t replicate all the metabolic effects of a true ketogenic diet. They raise ketone levels temporarily without requiring the sustained low insulin state that drives your body’s own fat-burning process. Athletes and people experimenting with ketosis sometimes use them as a quick energy source or a way to ease into carb restriction, but they’re not a shortcut to the full benefits of dietary ketosis.