How it works: switching to fat
Normally the body runs mainly on glucose and stores the surplus as glycogen in the liver and muscles. During fasting, or when carbohydrate intake falls to roughly 20–50 g a day, glycogen runs out within one to three days and insulin levels drop. Fatty acids released from fat tissue then travel to the liver, which converts them into ketone bodies: beta-hydroxybutyrate, acetoacetate and acetone. The brain cannot burn fatty acids directly but can use ketones; George Cahill's fasting studies at Harvard showed that during prolonged starvation ketones supply more than half the brain's energy. A blood ketone level above about 0.5 mmol/L is called ketosis, and a fruity, acetone smell on the breath is a classic sign.
The ketogenic diet: evidence and limits
A diet designed to keep you in ketosis is the ketogenic, or keto, diet. It was developed at the Mayo Clinic in the 1920s to treat drug-resistant epilepsy in children, and a 2008 randomised trial in the UK confirmed it significantly reduces seizures. For weight loss, early results look dramatic because glycogen is stored with water, but trials lasting a year or more often show little difference from other calorie-reduced diets. It can improve blood sugar control, yet some people see large rises in LDL cholesterol. Intermittent fasting also produces mild ketosis during fasting windows. The headaches, fatigue and dizziness of "keto flu" in the first days are mostly due to water and sodium loss.
Ketosis is not ketoacidosis
In nutritional ketosis, the insulin that remains keeps ketone production in check, so blood levels usually stay below 3 mmol/L and blood pH stays normal. Diabetic ketoacidosis (DKA) is a medical emergency in which near-absent insulin lets ketones accumulate unchecked, sometimes beyond 10 mmol/L, turning the blood acidic. It mainly affects people with type 1 diabetes who miss insulin or develop an infection, and people taking SGLT2 inhibitors can develop it even with near-normal blood sugar. Anyone on diabetes medication, pregnant people and those with kidney or liver disease should talk to a clinician before starting a low-carb, high-fat diet.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- How can I tell if I'm in ketosis?
- A blood ketone meter is most accurate (about 0.5 mmol/L or higher). Urine strips are convenient but become less reliable as the body adapts. Ketone breath and reduced appetite are common signs.
- How do I ease keto flu?
- Drink enough water and replace sodium with broth or salt; most symptoms fade within a few days. If they are severe or persistent, stop the diet and see a doctor.
Sources reviewed: 2026-10-26
References
- Cahill, G. F. (2006). Fuel metabolism in starvation. Annual Review of Nutrition, 26, 1–22.
- Neal, E. G., et al. (2008). The ketogenic diet for the treatment of childhood epilepsy: a randomised controlled trial. The Lancet Neurology, 7(6), 500–506.
- Kitabchi, A. E., et al. (2009). Hyperglycemic crises in adult patients with diabetes. Diabetes Care, 32(7), 1335–1343.
- Hall, K. D., et al. (2021). Effect of a plant-based, low-fat diet versus an animal-based, ketogenic diet on ad libitum energy intake. Nature Medicine, 27, 344–353.