What it means
When glucose is scarce, the liver turns fat into ketones, which the brain and muscles can use for energy. This happens during long fasts, very low-carb or ketogenic diets and prolonged heavy exercise.
In people without diabetes, mild ketosis is generally not dangerous because insulin keeps ketone levels in check. Some people notice bad breath, headache, tiredness or constipation in the first days.
Ketosis is not the same as diabetic ketoacidosis, in which a lack of insulin lets ketones climb to dangerous levels. People with diabetes, especially those on insulin or SGLT2 inhibitor medicines, should talk with their care team before trying a ketogenic diet or long fasts.
Ketosis is not required for fat loss. Weight loss on a ketogenic diet comes mainly from eating fewer calories, and studies find results similar to other diets over the long term.
Why it matters
Very low intake on a GLP-1 can push some people into ketosis, which matters for anyone with diabetes.