When the body is in ketosis, it switches from using carbs as its primary fuel source to using stored fat. This occurs when there is a shortage of glucose in the body. Additionally, you can also induce ketosis through fasting or a low-carbohydrate diet.
How long does it take to reach ketosis when fasting? Your body usually goes into ketosis 12 hours after starting a fast, which can be achieved by intermittent fasting. However, it can sometimes take longer.
Studies show that it can take 24 to 48 hours for the body to enter ketosis. This is due to factors such as body composition, activity level, and diet (before fasting) which affect the speed of ketosis. 
What is Fasting Ketosis?
Fasting ketosis is a metabolic process where your body burns stored fat for energy instead of glucose. This happens during prolonged fasting when the body runs out of glucose.
Prolonged fasting causes a drop in insulin levels, a subsequent decrease in glucose utilization, and an increase in fatty acid metabolism. This results in the breakdown of fatty acids, thereby increasing the level of ketones in the bloodstream, ultimately helping you enter ketosis.
Fasting ketosis has been linked to several health benefits, including improved insulin sensitivity, weight loss, and reduced inflammation.
Can Fasting Help with Ketosis?
Yes! Fasting helps with ketosis. Ketosis occurs when there is a shortage of glucose in the body, which can be induced through fasting.
Fasting can help you attain ketosis if you aren’t already there. When fasting, the body uses stored fat for energy. The stored fat is broken into ketone bodies, which are used for energy. When ketone levels in the blood rise above a certain threshold, the body enters ketosis. 
In that sense, intermittent fasting can help you reach ketosis faster than the keto diet alone. Studies indicate that fasting combined with extended exercise depletes the liver’s glycogen stores and increases ketone production. 
How Long Does It Take To Reach Ketosis When Fasting?
Ketosis is a metabolic state where your body burns fat for energy. This typically occurs when the body has depleted its glucose stores (glycogen) and needs to rely on alternative fuel sources, such as fatty acids.
A typical fasting period of 12–72 hours will induce ketosis in most people as glycogen stores are depleted in this duration and ketone production increases. However, the time it takes to reach ketosis varies depending on factors such as: 
- Individual metabolism: Each person’s metabolism is different; some people may enter ketosis faster than others.
- Type of fast: Different fasts, such as water fasting, juice fasting, or fat fasting, may affect ketone production and the time it takes to enter ketosis.
- Level of physical activity: Being physically active can help deplete glycogen stores more quickly and speed up the transition into ketosis.
In the worst-case scenario, it can take 2-4 days for your body to enter ketosis while fasting.
How To Tell If You Are in Ketosis
One of the most common ways to tell if you are in ketosis is by observing the signs and symptoms accompanying this metabolic state.
- Increased thirst and dry mouth
- Increased urination
- Fruity/bad breath
- Reduced appetite
- Rapid weight loss
- Increased energy
- Better focus and attention
- Changes in bowel habits
While these signs can be helpful, they are not always reliable indicators of ketosis. Measuring your ketone levels is the most reliable way to know if you’re in ketosis.
- Blood ketone meters: These meters measure the concentration of beta-hydroxybutyrate (BHB) in the blood. A small amount of blood is drawn using a lancet, placed on a test strip, and then inserted into the meter. This method provides a highly accurate measurement of ketone levels but can be relatively expensive.
- Urine ketone strips: These strips measure the concentration of acetoacetate in the urine. The strips are dipped in a urine sample, and the color change on the strip is then compared to a color chart to check the level of ketones. This method is inexpensive but not as accurate as blood ketone meters, as it only measures the excreted ketones, not those currently circulating in the bloodstream. 
- Breath ketone meters: These meters measure the acetone concentration in the breath. A person breathes into the meter, and acetone is measured. This method is non-invasive and can be more affordable than blood ketone meters. However, it may not be as accurate, especially at higher levels of ketosis.
Keep in mind that the level of ketones in the body can fluctuate throughout the day and can be affected by factors such as exercise, diet, and hydration levels. Therefore, measuring ketone levels consistently, simultaneously, daily, and under similar conditions is best.
The fasting ketosis timeline can vary depending on the length of the fast and individual factors, such as metabolic rate and exercise habits. However, here is a general timeline of what happens during a fast:
During the first 24 to 48 hours of a fast, the body depletes its glycogen stores and breaks down the stored fats for energy. As a result, the liver starts producing ketones as an alternative energy source. Blood ketone levels may begin to rise during this time, but it is not uncommon for it to take up to 3 days to reach optimal ketone levels.
Between days 2 and 4 of a fast, the body shifts into ketosis, during which period it primarily uses ketones for energy. Blood ketone levels will continue to rise and may reach between 0.5 and 3.0 mmol/L, considered the optimal range for nutritional ketosis. 
After five days of fasting, the body fully adapts to using ketones for energy. Blood ketone levels may be high, and many people report feeling a sense of euphoria, mental clarity, and increased energy. 
Week 2 and Beyond
After the first week of fasting, the body continues to use ketones for energy, and many people report feeling less hungry and more alert. However, it is essential to note that prolonged fasting can lead to nutritional deficiencies and other health complications.
Factors Affecting Ketosis
While fasting is a natural way to induce ketosis, the degree to which an individual enters ketosis can be influenced by several factors. Here are some of the key factors that can impact fasting ketosis:
Length of Fast
The length of the fast is one of the most significant factors influencing fasting ketosis. Generally, the longer the fast, the deeper the state of ketosis. While entering ketosis may take a few days, it typically takes at least 24 hours of fasting for the body to start using stored fat for energy.
Individuals with a higher body fat percentage tend to enter ketosis more quickly and deeply than those with a lower body fat percentage. This is because body fat is broken down into ketones and used as fuel during a fast.
Moderate-intensity exercise has been shown to enhance ketone production during fasting, leading to a deeper state of ketosis. Exercising in a fasted state can help deplete glycogen stores and rapidly increase ketone production. 
Staying hydrated during a fast is vital for maintaining optimal ketone levels. Dehydration can cause blood ketone levels to rise, but it can also lead to electrolyte imbalances, which can have negative health consequences.
Pre-existing Health Conditions
Pre-existing health conditions, such as diabetes, can impact fasting ketosis. People with diabetes may have difficulty entering ketosis due to insulin resistance, which prevents the body from using stored fat for energy.
Consult a healthcare professional before attempting a fast, especially if you have a pre-existing health condition.
Tips for Achieving Ketosis Faster
Achieving fasting ketosis can be challenging but rewarding. Entering ketosis can take time, so be patient and consistent with your efforts. Here are a few tips to help you enter ketosis faster:
- Start with a low-carb diet: A low-carb diet is important for ketosis. So, transition to a low-carb diet before starting your fast. Restrict your carb intake to under 50 grams daily and increase your healthy fat intake. This helps your body transition to ketosis faster.
- Increase your healthy fat intake: Since fats are the primary energy source in a keto diet, add healthy fats to give your body the fuel it needs to produce ketones. Replace processed food with healthy fats such as avocados, nuts, and olive oil.
- Track your carb intake: Tracking your daily carb intake helps you stay within your limit. Use a food diary or, better yet, a calorie-tracking app to keep track of your carb intake and adjust your diet accordingly.
- Experiment with a short fast or fat fast: Short fasting, such as a 24-hour fast, depletes your glycogen stores and forces your body to produce ketones, thereby pushing your body into ketosis. Another approach that helps mimic fasting is the fat fast. The fat fast is when you eat foods high in fat with little to no carbs.
- Increase protein intake: Consuming the right amount of protein helps prevent muscle loss and provides your body with the necessary components to generate ketones.
- Exercise regularly: Regular exercise can help you achieve ketosis faster by depleting your glycogen stores and forcing your body to use stored fat for energy. Aim for at least 30 minutes of moderate-intensity daily exercise, such as brisk walking or cycling.
- Consider supplements: Supplements such as MCT oil are quickly converted into ketones. So you can add it to your coffee or smoothies to support ketone production. Exogenous ketones, produced outside the body, can also help you enter ketosis faster.
Potential Risks and Side Effects of Fasting Ketosis
Despite its numerous health benefits, fasting is not for everyone. Like any other dietary or lifestyle change, fasting has potential risks and side effects, especially when it comes to fasting ketosis.
Here are some of the most common risks and side effects associated with fasting ketosis:
- Electrolyte imbalances: Fasting can lead to electrolyte imbalances, which can cause a range of symptoms, including muscle weakness, fatigue, and an irregular heartbeat. Drinking enough water and consuming electrolytes helps prevent these imbalances and keep your electrolyte levels up.
- Hypoglycemia: Hypoglycemia, or low blood sugar, is a potential risk, especially in individuals with diabetes, where your blood sugar levels drop to dangerous levels. Symptoms of hypoglycemia include dizziness, confusion, and loss of consciousness. If you have diabetes, it pays to monitor your blood sugar levels.
- Keto flu: Some people experience flu-like symptoms when they first enter ketosis, including headaches, nausea, and fatigue. These symptoms typically subside after a few days or weeks but can be uncomfortable.
- Dehydration: You must stay hydrated during a fast. Watch out for the symptoms of dehydration, namely headaches, dizziness, and fatigue. Ensure that you drink plenty of water and electrolyte-rich fluids during your fast.
- Nutrient deficiencies: A prolonged fast can lead to nutrient deficiencies, especially if you are not consuming a balanced diet. Nutrient deficiencies can have serious consequences, including fatigue, muscle weakness, and hair loss.
- Gastrointestinal issues: Changes in food intake can sometimes cause an imbalance in the gut bacteria, causing gastrointestinal issues such as constipation or diarrhea. A balanced diet rich in fiber helps prevent these gastrointestinal problems.
- Increased cholesterol levels: A diet rich in fat, such as the keto diet, can boost cholesterol levels, increasing the risk of heart disease. 
Frequently Asked Questions
1. Will a 24-hour fast put me in ketosis?
Yes! A 24-hour fast before you start a ketogenic diet can put you in a state of ketosis. By the end of the fast, your body would have burned through its glycogen reserves and started burning fat for fuel.
2. How long after fasting does the body burn fat?
Approximately 12 hours after fasting, your body starts burning fat for fuel, increasing between 16 and 24 hours.
3. Does coffee break ketosis?
Coffee doesn’t interfere with ketosis. In fact, caffeine can help with fasting by increasing insulin sensitivity. But adding creamer, milk, or sugar can break ketosis.
4. What kicks you out of ketosis?
Consuming more than 50 grams of carbohydrates can stop ketosis.
Fasting ketosis, or producing ketones during fasting, has gained popularity recently due to its potential health benefits. When the body is in ketosis, it produces ketones as an energy source. This can lead to numerous health benefits, including weight loss, improved insulin sensitivity, and decreased inflammation.
Factors such as the length of the fast, body composition, exercise habits, hydration, and pre-existing health conditions can impact the level of ketones produced during a fast and hence influence the potential risks and side effects.
Ultimately, the decision to fast and whether or not to pursue fasting ketosis is a personal one that should be made in consultation with a healthcare professional. However, by understanding the potential risks and side effects associated with fasting ketosis and taking steps to manage them effectively, achieving the health benefits of this metabolic state while minimizing potential risks is possible.
- Batch, J. T., Lamsal, S. P., Adkins, M., Sultan, S., & Ramirez, M. N. (2020, August 10). Advantages and Disadvantages of the Ketogenic Diet: A Review Article. PubMed Central (PMC). https://doi.org/10.7759/cureus.9639
- Mattson, M. P., Moehl, K., Ghena, N., Schmaedick, M., & Cheng, A. (2018, January 11). Intermittent metabolic switching, neuroplasticity, and brain health. PubMed Central (PMC). https://doi.org/10.1038/nrn.2017.156
- Collier, R. (n.d.). Intermittent fasting: the science of going without. PubMed Central (PMC). https://doi.org/10.1503/cmaj.109-4451
- Peeke, P. M., Greenway, F. L., Billes, S. K., Zhang, D., & Fujioka, K. (2021, January 15). Effect of time-restricted eating on body weight and fasting glucose in participants with obesity: results of a randomized, controlled, virtual clinical trial. PubMed Central (PMC). https://doi.org/10.1038/s41387-021-00149-0
- Gibson, A. A., Eroglu, E. I., Rooney, K., Harper, C., McClintock, S., Franklin, J., Markovic, T. P., Seimon, R. V., & Sainsbury, A. (2020, June 10). Urine dipsticks are not accurate for detecting mild ketosis during a severely energy restricted diet. PubMed Central (PMC). https://doi.org/10.1002/osp4.432
- Gershuni, V. M., Yan, S. L., & Medici, V. (n.d.). Nutritional Ketosis for Weight Management and Reversal of Metabolic Syndrome. PubMed Central (PMC). https://doi.org/10.1007/s13668-018-0235-0
- Low-carb diets, fasting and euphoria: Is there a link between ketosis and gamma-hydroxybutyrate (GHB)? – PubMed. (2007, January 1). PubMed. https://doi.org/10.1016/j.mehy.2006.07.043
- M. Pinckaers, P. J., Churchward-Venne, T. A., Bailey, D., & C. van Loon, L. J. (2016, July 18). Ketone Bodies and Exercise Performance: The Next Magic Bullet or Merely Hype? PubMed Central (PMC). https://doi.org/10.1007/s40279-016-0577-y
- Kosinski, C., & Jornayvaz, F. R. (2017, May 19). Effects of Ketogenic Diets on Cardiovascular Risk Factors: Evidence from Animal and Human Studies. PubMed Central (PMC). https://doi.org/10.3390/nu9050517