Ketones are chemicals your liver produces when your body breaks down fat for energy instead of sugar. This happens whenever your carbohydrate intake drops low enough that your body needs an alternative fuel source. Your brain, heart, and muscles can all run on ketones, making them a critical backup energy system that kicks in during fasting, very low-carb diets, or prolonged exercise.
How Your Body Makes Ketones
When you eat fewer carbohydrates, your blood sugar and insulin levels drop. Low insulin signals your body to start breaking down stored fat into fatty acids, which travel to the liver. Inside liver cells, those fatty acids get chopped into smaller molecules that are then assembled into ketone bodies. This whole process is tightly regulated by the balance between insulin (which slows it down) and glucagon (which speeds it up).
Your body produces three types of ketone bodies. The first two, acetoacetate and beta-hydroxybutyrate, are the ones your cells actually use for energy. They dissolve easily in blood and pass freely into tissues throughout the body. The third, acetone, is a byproduct that your body can’t use for fuel. It gets expelled through your breath, which is why people in ketosis sometimes notice a fruity or metallic taste in their mouth.
Why Your Brain Cares About Ketones
Your brain is picky about fuel. It can’t burn fat directly because fatty acids are too large to cross the protective barrier between your bloodstream and brain tissue. Ketones, however, pass through this barrier easily using specialized transport proteins. After an overnight fast, ketone levels are typically low (under 0.5 millimoles per liter) and supply less than 5% of the brain’s energy. But during prolonged fasting of five to six weeks, ketone levels rise dramatically and can cover nearly 60% of the brain’s energy needs, replacing glucose as the primary fuel.
This ability to fuel the brain is likely the reason ketone metabolism evolved in the first place. It allowed early humans to maintain mental function during periods without food.
What Triggers Ketone Production
The most common trigger is restricting carbohydrates. Eating fewer than 50 grams of carbs per day, roughly the amount in a single bagel, is generally enough to shift your body into ketosis. At that intake level, most people enter ketosis within two to four days. Intermittent fasting can also raise ketone levels, with the most popular approach being an eight-hour eating window followed by 16 hours of fasting.
Intense or prolonged exercise, illness, and skipping meals can all trigger mild ketone production too. For most healthy people, this is completely normal and harmless.
Normal vs. Dangerous Ketone Levels
There’s an important difference between nutritional ketosis and a medical emergency called diabetic ketoacidosis (DKA). In nutritional ketosis, blood ketone levels typically stay below 3 millimoles per liter. The body’s insulin system keeps production in check, and blood pH stays stable.
DKA is a different situation entirely. It occurs mostly in people with type 1 diabetes (and occasionally type 2) whose bodies produce little or no insulin. Without that brake on ketone production, levels can skyrocket to 20 to 25 millimoles per liter, turning the blood dangerously acidic. Alcoholic ketoacidosis, a related condition, can push levels to around 15 millimoles per liter. Both require emergency treatment.
For people with diabetes who monitor ketones, here are the general reference ranges for blood testing:
- Below 0.6 mmol/L: Normal, no action needed
- 0.6 to 1.5 mmol/L: Low to moderate risk, worth contacting a healthcare provider
- 1.6 to 2.9 mmol/L: High risk range
- Above 3.0 mmol/L: Very high risk, requires emergency care
How Ketones Are Tested
Three methods exist, and they differ quite a bit in accuracy. Blood meters are the most reliable option. They measure beta-hydroxybutyrate, which is the predominant ketone in your bloodstream, and give you a real-time reading of your current level. The downside is the finger prick and the cost of test strips.
Urine strips are painless and easy to use, but they only detect acetoacetate, not beta-hydroxybutyrate. They also show an average concentration since you last urinated rather than your current level, which can lead to delayed or misleading results. As your body becomes more efficient at using ketones, urine strips often show lower readings even though blood ketone levels remain steady.
Breath meters measure acetone and some small studies show a positive correlation with blood ketone levels. However, no breath device has been cleared by the FDA for this purpose, so accuracy is less certain.
The “Keto Flu” Transition Period
When your body first shifts into ketosis, you may feel noticeably worse before you feel better. This cluster of symptoms, commonly called “keto flu,” affects many people during the first few days. In a study analyzing online reports from people starting a ketogenic diet, the most frequently mentioned symptoms were flu-like feelings (reported by about 45% of people), headache (25%), fatigue (18%), nausea (16%), and dizziness (15%). Smaller numbers reported brain fog, stomach discomfort, low energy, feeling faint, and changes in heartbeat.
The prevailing explanation centers on electrolyte loss. When insulin drops and your body depletes its stored carbohydrates, you also lose water and minerals like sodium, potassium, and magnesium through increased urination. Staying hydrated and replenishing electrolytes are the most commonly recommended remedies, and symptoms typically resolve within a week.
Medical Uses for Ketones
The longest-established medical use of ketosis is treating epilepsy. Ketogenic diets have been used since the 1920s for children whose seizures don’t respond to medication. A meta-analysis found that roughly 53 to 55% of children on a ketogenic diet experienced at least a 50% reduction in seizure frequency over 12 months. That’s a meaningful result for patients who had already failed standard drug treatments.
Researchers are also investigating ketones’ effects on neurodegenerative diseases, given their ability to fuel the brain through an alternative pathway when glucose metabolism is impaired.
Exogenous Ketones
You don’t have to fast or cut carbs to raise your ketone levels. Exogenous ketones, sold as supplements, deliver ketone bodies directly. They come in two main forms. Ketone salts are minerals bonded to ketone molecules and are the most widely available consumer product. Ketone esters are a more concentrated form that raises blood ketone levels higher and tends to cause fewer gastrointestinal side effects like bloating and stomach upset. Both types elevate ketones temporarily regardless of what you’ve eaten, though esters are generally considered more potent.
These supplements are popular among athletes and people experimenting with cognitive performance, though the evidence for clear performance benefits is still mixed. They don’t replicate all the metabolic changes of true dietary ketosis, since your insulin levels and fat-burning patterns remain different when you’re still eating carbohydrates alongside a ketone supplement.