What Is Ketosis? How Your Body Switches to Burning Fat
Ketosis is a normal metabolic state, not a trick. Here is what happens, in what order, and what the numbers mean.
Short answer: Nutritional ketosis is a normal state for healthy adults. People with type 1 diabetes, those taking SGLT2 inhibitors, or anyone with liver, kidney or pancreatic disease should get medical advice first.
The short definition
Ketosis is a metabolic state in which the liver produces ketone bodies — mainly beta-hydroxybutyrate (BHB), acetoacetate and acetone — from fatty acids, and the body uses them as a major fuel. It happens whenever carbohydrate availability is low: during fasting, prolonged exercise, or a ketogenic diet.
What triggers it
Your body stores carbohydrate as glycogen in the liver and muscles. When carbohydrate intake falls, insulin levels drop and those stores run down over a day or two. Lower insulin lets fat cells release more fatty acids, and the liver turns some of them into ketones. The brain, which cannot burn fatty acids directly, adapts to take a growing share of its energy from ketones.
The three ketone bodies, and why it matters which you measure
| Ketone | Where it shows up | Measured by |
|---|---|---|
| Beta-hydroxybutyrate | Blood | Blood ketone meter |
| Acetoacetate | Urine | Urine strips |
| Acetone | Breath | Breath meters |
Blood BHB is the reference measurement, which is why the ranges below are expressed in mmol/L of blood.
Ketone ranges
- Below 0.5 mmol/L: not in ketosis.
- 0.5–1.5 mmol/L: light nutritional ketosis.
- 1.5–3.0 mmol/L: the range many people on keto aim for.
- Above 3.0 mmol/L: typical of extended fasting; higher still is unusual from diet alone.
See the full ketone levels chart for context on fasting and post-exercise readings.
Ketosis is not ketoacidosis
In people with working insulin production, ketone levels are self-limiting: insulin rises slightly and brakes ketone production. Diabetic ketoacidosis is a medical emergency in which ketones climb far higher — often above 10 mmol/L — alongside high blood glucose and acidic blood. The difference is explained in ketosis vs ketoacidosis.
How long it takes and how long it lasts
Most people reach 0.5 mmol/L within two to four days of strict carb restriction — the full breakdown by situation is in how long it takes to get into ketosis. A single high-carb meal can push readings back down within hours; returning usually takes a day or two. "Fat adaptation" — the longer process in which muscles rely more on fat — is generally described over several weeks, though the timeline varies a lot between people.
Frequently asked questions
Is ketosis safe?
Nutritional ketosis is a normal state for healthy adults. People with type 1 diabetes, those taking SGLT2 inhibitors, or anyone with liver, kidney or pancreatic disease should get medical advice first.
What ketone level is ketosis?
A blood beta-hydroxybutyrate reading of 0.5 mmol/L or higher is the usual threshold for nutritional ketosis.
Does ketosis mean I am losing fat?
Not automatically. Ketosis shows you are using fat-derived fuel; fat loss still depends on eating less energy than you use over time.
This guide is general information for healthy adults, not medical advice. See our medical disclaimer.