Ketones are molecules your body produces as an alternative fuel when glucose is in short supply. Your liver makes them by breaking down fat, and they travel through your bloodstream to power organs that would otherwise rely on sugar, including your brain. Under normal conditions, ketones account for 5% to 20% of your body’s total energy use, but that share rises dramatically during fasting, prolonged exercise, or very low carbohydrate intake.
How Your Body Makes Ketones
Ketone production starts when two conditions line up: insulin drops and fatty acid levels in the blood rise. This happens naturally when you haven’t eaten for a while, when you’re exercising intensely, or when you eat very few carbohydrates. Low insulin triggers fat cells to release stored fatty acids into the bloodstream. Those fatty acids travel to the liver, enter the energy-producing compartments of liver cells (mitochondria), and get chopped into smaller molecules called acetyl-CoA.
Normally, the liver would burn acetyl-CoA through its main energy cycle. But during fasting or carb restriction, a key ingredient in that cycle (oxaloacetate) gets pulled away to help make new glucose for tissues that absolutely require it, like red blood cells. With the main cycle running at reduced capacity, acetyl-CoA backs up. The liver’s solution is to convert that excess into ketone bodies and release them into the blood for other organs to use.
Glucagon, cortisol, and stress hormones all accelerate this process. Insulin does the opposite: it slows fat breakdown and suppresses ketone production. This is why people with normal insulin function rarely accumulate dangerous levels of ketones, while people with type 1 diabetes, who produce little or no insulin, are vulnerable to runaway ketone buildup.
The Three Types of Ketone Bodies
Your liver produces two primary ketones. The first is acetoacetate, which serves as the starting point. The second is beta-hydroxybutyrate, made when an enzyme converts acetoacetate into a more stable form. Beta-hydroxybutyrate is the most abundant ketone in your blood and the one most commonly measured in clinical testing.
The third ketone, acetone, forms when acetoacetate breaks down spontaneously. Acetone is the least useful as fuel, and your body mostly gets rid of it through your breath. It’s what gives some people on very low-carb diets a distinctive fruity or metallic smell when they exhale.
Why Ketones Matter for Your Brain
Your brain is unusually picky about fuel. It can’t burn fat directly because fatty acids don’t cross the blood-brain barrier efficiently. For most of human history, this posed a survival problem: during food scarcity, the brain still needed energy even as glucose ran low. Ketones solve this. They cross into the brain readily and can supply up to 60% of the brain’s energy needs during prolonged fasting, with glucose covering the rest.
This backup fuel system is one reason humans can survive extended periods without food. It also explains the interest in ketones for neurological conditions, since some researchers suspect the brain may function differently, sometimes favorably, when running partly on ketones rather than glucose alone.
Ketosis vs. Ketoacidosis
These two terms sound similar but describe very different situations. Ketosis simply means your body is producing ketones at a higher-than-baseline rate. This happens during fasting, on a ketogenic diet, or after prolonged exercise. Blood ketone levels in this range typically stay between about 0.5 and 3.0 mmol/L. For most people, this is a normal metabolic state and not dangerous.
Ketoacidosis is a medical emergency, most commonly seen in people with type 1 diabetes. Without insulin to put the brakes on fat breakdown and ketone production, ketones accumulate rapidly and make the blood dangerously acidic. Diagnostic criteria from major diabetes organizations define ketoacidosis as blood ketone levels at or above 3.0 to 3.8 mmol/L combined with blood glucose above 200 to 250 mg/dL and acidic blood pH below 7.3. The key difference isn’t just higher ketone levels; it’s the loss of the body’s ability to regulate them. A healthy person’s insulin response prevents ketones from spiraling out of control.
How Ketones Are Measured
There are three common ways to check your ketone levels, and each one measures a different ketone body.
- Blood meters use a finger prick to measure beta-hydroxybutyrate in capillary blood. This is the most accurate method and the one used in clinical settings. Results appear in mmol/L.
- Urine strips detect acetoacetate through a color-changing chemical reaction. They’re cheap and easy to use, but research has shown they are not accurate at detecting mild ketosis. As your body becomes more efficient at using ketones, less acetoacetate spills into your urine, so the strips can underread even when blood ketone levels are elevated.
- Breath analyzers measure acetone in exhaled air. They’re noninvasive and reusable, but less precise than blood testing.
The mismatch between urine and blood results comes down to biology. Urine strips detect acetoacetate, while blood meters detect beta-hydroxybutyrate. These two ketones are produced and cleared at different rates, so they don’t always rise and fall in sync.
What Triggers Ketone Production
Several common situations raise ketone levels:
- Fasting is the most straightforward trigger. After roughly 12 to 24 hours without food, liver glycogen stores deplete and the body shifts toward fat burning and ketone production.
- Ketogenic diets keep daily carbohydrate intake low enough (typically under 20 to 50 grams) to mimic the metabolic state of fasting, even while you’re eating plenty of calories from fat and protein.
- Prolonged exercise can deplete glycogen and push the body into mild ketosis, especially during endurance activities.
- Illness or uncontrolled diabetes can cause pathological ketone elevation. In type 1 diabetes, absent insulin removes the brake on fat breakdown entirely, leading to the dangerous ketoacidosis described above.
Exogenous Ketones
You can also raise blood ketone levels by consuming ketones directly, without changing your diet or fasting. These supplements come in two forms.
Ketone salts combine beta-hydroxybutyrate with a mineral like sodium, calcium, or magnesium. They produce a moderate rise in blood ketones, reaching roughly 0.5 to 1.0 mmol/L within about 30 minutes. The effect is relatively mild and fades within a few hours.
Ketone esters are more potent. A moderate dose can raise blood levels to around 1.5 mmol/L within 20 minutes, while higher doses can push levels to 3.5 mmol/L or above within 10 minutes and peak around 6.0 mmol/L after 40 to 70 minutes. Esters deliver a stronger and faster ketone spike, but they’re more expensive and have a notoriously bitter taste. Athletes have been the primary users, experimenting with them as a supplemental fuel source during endurance events.
Neither type of supplement forces your body into the same metabolic state as fasting or a ketogenic diet. They raise ketone levels in the blood, but your liver isn’t necessarily ramping up its own production, and the downstream hormonal shifts that accompany natural ketosis may not occur.