Updated: October 2026
In short: When fasting, the body first uses up its glycogen stores; from roughly 18 hours to two days it increasingly switches to fat and ketone bodies. How fast that happens depends on diet, exercise and the person. From about day four, ketosis is detectable in most people who fast. Longer fasting belongs under medical supervision. As of: October 2026.
Fasting feels completely different from day to day. Here you can see roughly what happens in the body over the first seven days and which of it is well documented.
What happens in the first hours without food?
Your body first uses the glucose from your last meal. According to Healthline, blood sugar rises after eating, and excess glucose is stored as glycogen. About three to four hours after the meal, blood sugar and insulin fall, the body breaks glycogen down into glucose and starts tapping into fat reserves too.
That's the normal state between two meals and not a sign of deficiency. A short overnight fast therefore doesn't bring about a switch yet.
What happens after 18 hours to two days?
The glycogen stores in the liver run low, and the body produces more ketone bodies from fat. Healthline describes this phase as the start of ketosis, but explicitly says that the transition doesn't have to happen immediately. Shorter fasts of 12 to 18 hours often don't reach it.
How quickly you get into ketosis depends on what you ate beforehand and how much you move. The Cleveland Clinic gives less than 50 g of carbohydrates per day as the limit for staying in ketosis. During the transition, headaches, tiredness and dizziness are possible. Details are in the article Juice Cleanse Side Effects.
How does a fast go from day 1 to day 7?
Roughly like this, though every body reacts differently. The table summarises documented key points and typical experiences. The "Experience" column is our own assessment, not a measurement.
| Day | Metabolism | Typical experience |
|---|---|---|
| 1 | Glycogen is used up, insulin falls | Hunger, habit-driven urge to eat |
| 2 | Ketone production begins, ketosis not yet certain | Headache, tiredness possible |
| 3 | Ketone levels rise, muscle breakdown may begin | Often calmer, hunger eases |
| 4 | Ketones detectable in urine in over 95% of people fasting | Clear head, but less strength |
| 5 to 7 | Ketone levels keep rising | More even, feeling cold possible |
The figure for day 4 comes from a study of 1,610 people in a fasting clinic. There, energy intake was 75 to 250 kcal daily from juice, soup and honey. Ketonuria was detectable from day 4 in over 95%, and blood ketone levels rose on average to around 4 mmol/L by about day 12.
When does fat get broken down while fasting?
Fat breakdown already happens between meals, but on a larger scale only once glycogen is used up. According to Healthline, that happens from about 18 hours. Fat is processed into ketone bodies, which the brain and muscles use as energy.
Important for context: weight loss while fasting doesn't consist of fat alone. In the study, people with stronger ketosis lost 4.44 kg on average, and those with weaker ketosis 3.77 kg, over 4 to 21 days of fasting. The difference was small; the numbers show averages and not a guarantee for you. You can read more about losing weight with juices in the article Losing Weight With a Juice Cleanse.
Does the body break down muscle while fasting?
With longer fasting, yes, at least partly. Healthline writes that muscle breakdown can begin from about 48 hours, because the liver has to produce glucose from amino acids (gluconeogenesis). The longer and lower in calories you fast, the more important protein intake when breaking the fast and light exercise become.
Our article explains how this plays out in practice Do You Lose Muscle When Fasting? Short cleanses of one to five days are less critical in this respect than weeks of fasting.
Is ketosis also reached on a Juice Cleanse?
Usually not. A Juice Cleanse isn't zero-calorie fasting, but provides energy and carbohydrates. Depending on the variant, the frank. Juice Cleanse contains between approx. 650 and 930 kcal per day. Per cleanse day, that's 108 g of carbohydrates (of which 97 g sugar) for Pro, 137 g for Medium and 174 g for Starter (of which 160 g sugar).
| Variant | kcal/day | Carbohydrates | of which sugar |
|---|---|---|---|
| Starter | 934 | 174 g | 160 g |
| Medium | 799 | 137 g | 124 g |
| Pro | 680 | 108 g | 97 g |
All three are well above the 50 g carbohydrate limit that the Cleveland Clinic gives for ketosis. A Juice Cleanse is therefore more of a low-calorie relief than ketosis fasting. What you can expect is in the article Juice Cleanse Guide.
What can you do to help your body adjust more easily?
Give your body fluids, rest and a gentle preparation. The consumer advice centre (Verbraucherzentrale) recommends drinking at least 2.5 litres of calorie-free drinks such as water or unsweetened tea daily, gradually reducing the amount of food for two to three days beforehand and building back up slowly afterwards. Complete fasting shouldn't last longer than a week, even for healthy people.
- Plan the first fasting days on days with few appointments.
- Light exercise such as walking helps more than intense training.
- Keep warm, because feeling cold is a common phenomenon.
- Go to bed early if you feel weak.
A fasting diary with the time, hunger on a scale of 1 to 10 and mood shows you after a few days when your personal low point comes. Where your low point lies is individual. With the diary, you can counteract it deliberately next time, for example by scheduling more calmly on exactly that day, taking a walk or having a cup of tea at your usual hunger time.
Who shouldn't fast?
People with diabetes, eating disorders, underweight, pregnant and breastfeeding women and people who regularly take medication. The Cleveland Clinic points out that ketoacidosis, a state with too many ketones, is life-threatening in diabetes and must be treated immediately. It is clearly distinct from normal ketosis during fasting.
Healthline stresses that extended fasting should be medically supervised. Stop and seek medical advice if you feel dizzy, get a racing heart or can no longer think clearly.
Pairs well with frank.
If you'd like a gentle experience of the transition, the Juice Cleanse is a variant with energy instead of zero food: six juices per day, from 99.75 EUR for three days (as of October 2026).
You might also like
- Do You Lose Muscle When Fasting?
- Boost Your Autophagy Through Fasting!
- Juice Cleanse Guide: How Do I Fast Properly?
Frequently asked questions about what happens in the body when fasting: day 1 to 7 explained
How long does it take for the body to burn fat when fasting?
Fat is already used between meals. It becomes stronger after about 18 hours, when the liver's glycogen stores run low.
When are you in ketosis when fasting?
That depends on diet and the person. Healthline gives 18 hours to two days. In the fasting clinic study, ketones were detectable in the urine in over 95% from day 4.
What's the difference between ketosis and ketoacidosis?
Ketosis is a normal state with moderate ketone levels. Ketoacidosis occurs in diabetes due to insulin deficiency and is life-threatening.
Is only fat broken down when fasting?
No. With longer fasting, muscle tissue can also be broken down from about 48 hours. Part of the weight loss can also be water.
Is fasting the same for every body?
No. Age, sex, weight, exercise and previous diet play a role. That's why the timings are guidelines.
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.
