Here is the story of ketosis in four steps. Scroll down and watch it light up.
Carbs drop
You eat far fewer carbohydrates, so less glucose enters your blood.
Stores run down
Your body uses up its stored glucose (glycogen), and insulin levels fall.
Fat is released
Fat is broken down into fatty acids and sent to the liver.
Ketones form
The liver makes ketones, which fuel your brain and muscles.
Step by step
1. Carbs drop. Carbohydrates break down into glucose, the body’s default fuel. When you eat very few, your blood sugar and insulin stay low.
2. Stores run down. Your liver and muscles hold a reserve of glucose called glycogen. This lasts roughly a day or two on a low-carb diet. Glycogen holds water, which is why many people see a quick drop on the scale in the first week. It is mostly water, not fat.
3. Fat is released. With insulin low, your body releases fat from storage and from your meals.
4. Ketones form. The liver converts fatty acids into ketones. Your brain, which cannot burn fat directly, can run on ketones and a small amount of glucose.
How long does it take?
Typically a few days of eating very low-carb, but this varies with your starting point, activity level and how strictly you keep carbs down. Eating even a modest amount of extra carbs can pause the process.
“Nutritional ketosis” is not “ketoacidosis”
These sound alike but are very different. Nutritional ketosis is the controlled, mild elevation of ketones that comes from a low-carb diet. Diabetic ketoacidosis is a dangerous medical emergency that happens when someone lacks enough insulin, most often in type 1 diabetes. If you have diabetes, please talk to your doctor before changing your diet. See Is keto safe for you?
Do you have to measure ketones?
No. Some people use urine strips, breath meters or blood meters, but plenty of people simply follow the plan and notice how they feel. Testing is optional and not necessary for success.