Updated: October 2026
In short: The body starts producing ketones after around 18 hours without food. Pronounced ketosis usually takes longer, often two days or more. With 16/8 or 14/10, you usually don't reach true ketosis, because the fasting pause is too short. As of: October 2026.
Many people want to get into ketosis with intermittent fasting. We show what happens in the body over the hours, what the sources say and why "more hours" isn't automatically better.
What does ketosis mean when fasting?
Ketosis is a metabolic state in which the body uses fat instead of sugar as its main energy source. The liver produces ketones in the process. The Cleveland Clinic describes it like this: the body uses fat as its main fuel, and ketones become the main energy supplier.
The distinction from ketoacidosis is important. In ketosis, there are ketones in the blood, but not so many that the blood becomes acidic. Diabetic ketoacidosis, on the other hand, is life-threatening and a medical emergency. If you have diabetes, you should therefore not aim for ketosis on your own.
After how many hours of fasting does ketosis begin?
According to Healthline, ketone production begins somewhere between 18 hours and two days without food. The time spans are rough and vary with diet, exercise and metabolism.
| Time without eating | Phase according to Healthline | What happens |
|---|---|---|
| 0 to 3 hours | Fed State | Blood sugar and insulin rise, glycogen is stored |
| 3 to 18 hours | Early fasting | Glycogen from the liver is used up |
| 18 hours to 2 days | Fasting phase | Ketone production starts, transition takes time |
| from 48 hours | Long-term fasting | Ketone levels (BHB) rise steadily, medical supervision necessary |
The timings apply to people who eat normally. If you previously ate a low-carb diet, you reach the state faster.
Do I reach ketosis with 16/8?
Usually not. Healthline writes that most forms of intermittent fasting such as 16/8 alternate between the fed state and early fasting and don't reach true ketosis. Windows of 12 to 18 hours a day often aren't enough for it.
That's not bad news. The effect of 16/8 rests, according to the evidence, more on less eating time and a possible calorie deficit, not on ketosis. A Cochrane analysis sees no clear weight advantage for intermittent fasting overall compared with a normal diet. You'll find an overview in Intermittent Fasting Methods Compared. If you want to lose weight, also read Intermittent Fasting and Weight Loss: Why Isn't It Working?
Ketosis is a state, not a goal in itself. In the long run, what counts is what you can keep up over weeks and months and what agrees with you. For weight loss too: it's the week's calorie balance that decides, not the colour of the test strip.
How do I get into ketosis faster?
For the dietary route, the Cleveland Clinic cites about 20 to 50 g of carbohydrates a day, with which ketosis usually sets in within two to four days, sometimes only after a week or longer. The timeframe depends on age, metabolism, sleep and stress.
Important for you: we don't recommend long-term ketosis as a goal for everyone. If you want to try it anyway, you should do so with medical supervision. According to the Cleveland Clinic, typical side effects are bad breath, constipation, nausea, headaches, tiredness, sleep problems and dehydration. Over a longer course, it also lists kidney stones, lower bone density and raised cholesterol levels.
What does the timeline look like in everyday life?
A worked example: you eat dinner at 7 pm and fast afterwards. The markers from the Healthline table then fall at these times:
| Time | Fasting hours | Assessment |
|---|---|---|
| 10 pm | 3 | End of the digestive phase |
| 11 am next day | 16 | End of a 16/8 fast, early fasting phase |
| 1 pm next day | 18 | According to Healthline, ketone production begins here |
| 7 pm the day after next | 48 | Long-term fasting, only under medical supervision |
You see: with a normal 16/8 rhythm, you're in early fasting at 11 am. Ketone production would only start two hours later. For most goals, you don't need a longer pause than 16 or 18 hours. Instead, observe how you feel: concentration, sleep and energy in the afternoon say more about the right length than any table. If you feel dizziness, shaking or palpitations, eat something and shorten the window next time. Especially at the start, not every body tolerates the long pauses, and if you also exercise, you often need more energy than you think. In that case, give yourself a rest day and a balanced meal instead of pushing through. That also protects your motivation for the coming weeks.
If you're planning fasting pauses beyond 24 hours, you shouldn't try that on your own. Healthline stresses that longer fasting should only take place under medical supervision.
Do a Juice Cleanse and ketosis go together?
No. A Juice Cleanse is not ketogenic. Depending on the variant, the frank. Juice Cleanse provides 97 to 160 g of sugar and 108 to 174 g of carbohydrates per day, many times the 20 to 50 g cited for ketosis. That's an observation, not a drawback: a Juice Cleanse follows a different concept, namely fruit and vegetables in liquid form at a reduced calorie count. You'll find more context in Why You Should Drink Vegetable Juices While Fasting and under Juice Cleanses.
What misconceptions are there about ketosis and fasting?
We come across four assumptions again and again:
- After 12 hours I'm in ketosis: According to Healthline, 12 to 18 hours is usually too short for that.
- Ketosis automatically means fat loss: What remains decisive is how much energy adds up over the week.
- The longer, the better: After 48 hours the risks rise, such as the muscle mass breakdown that Healthline mentions.
- Ketosis is the same as ketoacidosis: Both terms sound similar, but medically they are very different.
If you're interested in preserving muscle, read Do You Lose Muscle When Fasting?
Who is ketosis not a good idea for?
During pregnancy, breastfeeding, with eating disorders, underweight, type 1 diabetes and with medication that affects blood sugar or blood pressure, ketosis should only be pursued after medical consultation. Healthline names the same groups for intermittent fasting generally. Women whose period stops should end a fast and get advice. During the fasting period, you drink water or unsweetened tea. Milk, sugar and juices end the pause. Ideas for the evening are in Tea for Fasting.
If you have persistent nausea, severe discomfort or confusion, seek medical help, especially with diabetes.
Pairs well with frank.
Unsweetened drinks are part of the fasting period. In our Organic Teas you'll find loose herbal and green tea blends, from 15 EUR per 100 g (as of October 2026).
You might also like
- Boost Your Autophagy Through Fasting!
- What Is Intermittent Fasting and What Are the Rules?
- What Is the Optimal Fasting Duration?
Frequently asked questions about intermittent fasting and ketosis
After how many hours of fasting are you in ketosis?
According to Healthline, ketone production begins somewhere between 18 hours and two days. Pronounced ketosis tends to take longer.
Is 16/8 enough for ketosis?
Usually not. Healthline suggests that windows of 12 to 18 hours generally don't produce true ketosis.
Is ketosis dangerous?
Nutritional ketosis has to be distinguished from ketoacidosis. The latter is life-threatening in diabetes. If you have pre-existing conditions, talk to your doctor first.
How long does it take to get into ketosis?
At about 20 to 50 g of carbohydrates a day, the Cleveland Clinic cites two to four days, sometimes longer.
What side effects does ketosis have?
Bad breath, constipation, nausea, headaches, tiredness and sleep problems are mentioned.
Note: This article is for general information and does not replace medical or therapeutic advice. If you have symptoms or pre-existing conditions, are pregnant or take medication, please speak to your doctor first.
