Nutrition glossary
What is ketosis?
Ketosis is a normal metabolic state in which your body, short on carbohydrate, breaks down fat and turns some of it into ketones in the liver. Your brain and muscles then use those ketones as fuel in place of some of the glucose they usually run on.
How you get into ketosis
Normally your cells run mainly on glucose from the carbohydrate you eat, with some stored as glycogen in your liver and muscles. When carbohydrate stays very low for a few days, those glycogen stores run down, insulin levels fall, and your liver starts turning fatty acids into three kinds of ketone: beta-hydroxybutyrate, acetoacetate and acetone.
A few common routes into ketosis:
- A ketogenic diet. Keeping carbohydrate under roughly 20 to 50 g a day, often counted as net carbs, usually gets most people there within two to four days. For a sense of scale, one medium banana and a slice of bread together come to about 40 g.
- Fasting. Even an overnight fast produces a small rise in ketones. After a day or more without food, levels climb further.
- Long, hard exercise can raise ketones a little, especially on a low-carb diet.
Eating a lot of protein can slow things down a bit, because your body can turn some protein into glucose. That's why keto plans are high in fat and moderate in protein, not high in both.
Signs you might be in ketosis
None of these prove anything on their own, but many people notice:
- Breath that smells fruity or a little like nail polish remover, from acetone.
- Needing to pee more and feeling thirstier in the first week, as you lose water and salt.
- The "keto flu": headache, tiredness, brain fog, irritability and leg cramps for a few days. This is often linked to electrolyte losses and usually passes within about a week.
- Less hunger than usual once the first week is over.
- A quick drop on the scale, most of it water rather than fat.
Testing for ketones
| Method | How it works | The catch |
|---|---|---|
| Urine strips | Change colour with acetoacetate in urine | Cheap, but less reliable after a few weeks as your body adapts and how much water you drink changes the result |
| Blood meter | Finger-prick test for beta-hydroxybutyrate | Most accurate at home; strips cost more |
| Breath meter | Measures acetone in your breath | No needles, but accuracy varies between devices |
For nutritional ketosis, blood readings are usually somewhere around 0.5 to 3 mmol/L. You don't have to test at all to follow a keto diet; many people just keep their carbs low and track how they feel.
Ketosis is not ketoacidosis
These words sound alike but they are very different. In nutritional ketosis, your body still makes some insulin, which keeps ketone levels in check, and your blood sugar stays normal.
Diabetic ketoacidosis (DKA) is a medical emergency. It happens mainly in people with type 1 diabetes, and sometimes type 2, when there isn't enough insulin. Ketones build up to very high levels, usually alongside high blood sugar, and the blood turns acidic. Symptoms include vomiting, tummy pain, deep fast breathing, extreme thirst, confusion and drowsiness. If you have diabetes and notice these, get urgent medical help.
Who should be careful
If you have type 1 or type 2 diabetes, especially if you take insulin, sulfonylureas or SGLT2 inhibitors (such as empagliflozin or dapagliflozin), a very low-carb diet can cause low blood sugar or raise the risk of ketoacidosis, and your doses may need changing. The same goes if you have kidney or liver disease, are pregnant or breastfeeding, or have a history of an eating disorder.
Common questions
How long does it take to get into ketosis?
For most people it takes about two to four days of eating under roughly 20 to 50 g of carbohydrate a day. It can take longer if you eat a lot of protein or were eating plenty of carbs beforehand.
Is ketosis dangerous?
For most healthy adults, nutritional ketosis is not dangerous in the short term, although the first week can feel rough. Long-term safety studies are limited, and it carries real risks for people with diabetes on certain medicines, so check with your doctor if that applies to you.
Does ketosis burn more fat?
You burn more fat for fuel, but that includes the fat you eat. Fat loss still depends on an overall calorie deficit. Trials comparing keto with other diets at the same calories generally find similar fat loss.
Related
See the keto diet guide for what to eat, low-carb diets for gentler options, and the glossary entries on net carbs, electrolytes and macronutrients. Wondering if it's right for you? Read are online diet plans safe and how to spot a fad diet.