Most people get into ketosis within 2 to 4 days of cutting carbohydrates to about 20 to 50 g a day, but it can take a week or longer for some. Harvard’s Nutrition Source explains that when carbohydrate is very low for 3 to 4 days and stored glucose is used up, insulin levels fall and the body begins to use fat as its main fuel, and that how soon ketosis happens varies from person to person. Your carb intake, activity level, body size, metabolism and how much protein you eat all affect the timing. Feeling fully adapted, when energy and exercise performance settle, usually takes a few weeks.
What happens when you cut carbs
- Days 1 to 2: your body uses up glycogen, the carbohydrate stored in your liver and muscles. Glycogen holds water, so you often lose 1 to 2 kg of water weight early on.
- Days 2 to 4: as glycogen runs low, your liver starts making ketones from fat. Many people reach nutritional ketosis during this time.
- Weeks 1 to 2: some people experience “keto flu” symptoms, such as headache, tiredness, irritability and constipation, as the body adjusts and loses water and salt. Our guide to keto flu explains how to ease it.
- Weeks 2 to 6: the body becomes more efficient at using fat and ketones for fuel. Energy, focus and exercise tolerance often improve.
What affects how fast you get into ketosis
- How low your carbs are. Staying under about 20 to 30 g of net carbs speeds things up; eating closer to 50 g may take longer.
- Physical activity. Exercise uses up glycogen faster.
- Protein intake. Very high protein can slow ketosis a little, because the body can turn some protein into glucose. Keto is meant to be moderate in protein. Our guide to protein on keto explains how much you need.
- Hidden carbs in sauces, dressings, drinks and “low-carb” snacks.
- Individual differences in metabolism, body size and insulin sensitivity.
- Fasting. Some people combine keto with time-restricted eating, which can speed up ketosis. Our guide to keto and intermittent fasting covers the pros and cons.
How to tell if you’re in ketosis
Signs and symptoms
- Reduced appetite.
- Increased thirst and urination in the first days.
- A fruity or metallic taste or breath (“keto breath”).
- Temporary tiredness or headache, then steadier energy.
These signs are not reliable on their own.
Measuring ketones
- Blood ketone meters are the most accurate. Nutritional ketosis is generally considered to be a blood ketone level of around 0.5 to 3 mmol/L.
- Urine strips are cheap and show ketones in early keto, but become less reliable as your body adapts.
- Breath meters measure acetone and are less precise.
You don’t need to measure ketones to lose weight on keto. Many people simply track their carbs. Our guide on how to track macros on keto explains how.
Ketosis is not the same as ketoacidosis
Nutritional ketosis in healthy people is different from diabetic ketoacidosis (DKA), a dangerous condition that mainly affects people with type 1 diabetes and sometimes type 2 diabetes, in which ketone levels become very high and the blood becomes acidic. The NHS explains that in people with diabetes, a blood ketone level of 1.6 to 3 mmol/L means a risk of DKA and over 3 mmol/L means you may have DKA and need urgent care. If you have diabetes, especially if you take insulin or certain tablets, only follow keto with medical supervision and know the warning signs: vomiting, stomach pain, fast breathing, confusion and very high ketones.
Tips to get into ketosis more smoothly
- Keep net carbs low, around 20 to 30 g a day for the first week or two.
- Plan meals in advance. Our 7-day keto meal plan is a good starting point.
- Eat plenty of non-starchy vegetables for fiber and nutrients.
- Add salt and drink water, as you lose more sodium in the first weeks. Our guide to keto electrolytes explains how much.
- Be patient. Don’t judge results in the first few days, when much of the change is water.
- Stay active with walking and light exercise; hard training may feel tougher for a week or two.
Getting back into ketosis after a break
If you’ve eaten more carbohydrate, for example on holiday or at a celebration, it usually takes a day or two of low-carb eating to return to ketosis, sometimes a little longer after several days off. You may notice some keto flu symptoms again, and your weight will often drop back quickly as the extra water from glycogen is lost. To make it easier:
- Return to your usual low-carb meals straight away rather than “detoxing” with very low calories.
- Drink plenty of water and add a little salt.
- Do some light exercise, such as walking, to use up glycogen.
- Keep meals simple and plan ahead for the first few days.
Common questions
Does exercise help you get into ketosis faster?
Yes, exercise uses up stored glycogen, which can speed up the switch to burning more fat and making ketones. Light to moderate exercise is best while you adapt.
Why am I not in ketosis after a week?
You may be eating more carbs than you think, from sauces, drinks or snacks, or a lot of protein. Track your intake for a few days to check.
Can I get out of ketosis quickly?
Yes. Eating a meal high in carbohydrates can take you out of ketosis within hours. It usually takes a day or two to return once you go back to low carbs.
Is ketosis safe?
For most healthy adults, short-term nutritional ketosis is generally considered safe. Long-term effects are less certain, as our guide on whether keto is safe long term explains.
Safety note: Keto isn’t suitable for everyone. If you have diabetes, kidney or liver disease, take regular medication, or are pregnant or breastfeeding, talk to your doctor before cutting carbs this far (why it matters).
This article is general information, not medical advice. Talk to your doctor or a registered dietitian before making big changes to how you eat, especially if you have a health condition, take regular medication, or are pregnant or breastfeeding.
