How Long Is It Safe To Fast For? We Weigh Up The Factors

If you are interested in trying an intermittent fasting diet or a longer water fast, the first logical questions are: “How long should I fast for, or how long do I need to fast to lose weight?”

Knowing the best fasting duration will help you achieve your health and weight loss goals by fasting without exacerbating the side effects.

So, how long should you fast for weight loss? How long can you fast? What is the longest fasting ever? What factors affect how long you should fast for?

In this guide to how long you should fast for weight loss and health, we will discuss factors that affect how long to fast, followed by some guidelines and recommendations to ultimately answer your question, “How long is it safe to fast?“.

We will look at: 

  • How Long Should I Fast? Factors to Consider
  • How Long Is It Safe To Fast?

Let’s get started!

A calendar.

How Long Should I Fast? Factors to Consider

It is important to establish that there isn’t a single best answer as to how long can you fast, or how long should you fast for optimal results that will work universally for everyone.

This is because there are numerous factors that affect the best fasting routine, both in terms of how long you should be fasting and how often you should be fasting.

It is very important to consider all of the factors that affect optimal intermittent fasting schedules and frequency, as well as the best fasting protocols and frequencies for longer fasting durations such as 24-hour fasts or 48-hour fasts.

Here are some factors to consider when deciding how long you should be fasting:

#1: Your Health Status

If you have any chronic or acute medical conditions, you will likely be encouraged to fast for a shorter duration unless you have obesity, diabetes, or a medical condition such as epilepsy in which your healthcare team specifically wants you to be practicing fasting for weight loss or health.

While it is generally safe to follow a time-restricted intermittent fasting diet every day, the longer that you fast at a time, the more important it becomes to speak with a knowledgeable nutritionist or healthcare provider who can make individualized recommendations for the ideal fasting duration based on your health status, and weight loss and health goals.

A health status checkbox list.

#2: Your Weight Loss and Health Goals

People practice intermittent fasting and extended water fasting for different reasons, ranging from weight loss to improved insulin regulation, decreased inflammation, and accelerated autophagy.

Your goal with fasting will not only affect how often you should fast to reach your goals but also how long you should be fasting.

There are different stages of water fasting that we all go through that involve different physiological or biochemical changes due to the cessation of energy intake.

Unlike an “on/off“ light switch, it’s not like these fasting stages have a hard start or stop, but rather they work like dimmer switches in that the physiological or fasting effects peak or ebb and flow with different rhythms and timings as you fast.

Depending on your primary reason to fast, you will want to choose how long to water fast based on when the particular fasting benefit you are seeking reaches its peak or most significant rise.

A person holding up a sign that says weight loss.

In general, fasting for weight loss can be achieved with a general time-restricted intermittent fasting diet or longer fasting protocols, whereas some of the benefits of fasting, such as autophagy, may take up to 24 to 36 hours to even set in, which then means that you will need to be fasting longer for health.

That said, it is also important to keep in mind that everyone has their own unique biochemistry and genetic influences that can affect the timing of the phases of fasting. 

Therefore, these are rough guidelines as to when the various benefits of fasting really set in for most individuals, with plenty of variability on either end of the given fasting timeline for each fasting benefit.

How Long Should I Fast for Weight Loss?

The main consideration with how long you should fast for weight loss is what type of fasting schedule works best for you in terms of controlling your caloric intake without causing you to experience a binge/restrict cycle.

All intermittent fasting diet schedules and even alternate-day fasting can be effective for weight loss, but everyone responds differently in terms of appetite, energy, and how much “confinement“ around your eating window best aids weight loss by controlling caloric intake.

You can learn about different intermittent fasting diet schedules for weight loss here.

A ketosis test.

How Long Do I Need to Fast for Ketosis?

It usually takes a minimum of a 12-hour fast to see the beginnings of ketosis kick in, in which the body runs out of glycogen and starts breaking down and oxidizing fat for energy instead of carbohydrates.

This produces ketones in the liver, which then provide energy.

An 18-hour fast is recommended if your goal is ketosis because unless you generally follow the keto diet and perform a lot of endurance exercise in which you use up your muscle glycogen and then do not replenish it with carbohydrate-rich foods, it will take longer to burn through your glycogen stores to actually enter metabolic ketosis.

If you only fast periodically, you do not generally perform high-intensity exercise (heavy resistance training, HIIT workouts, etc.) or endurance workouts, and you follow a high-carb diet, you may need to fast for at least 18 to 24 hours to experience significant ketosis.

The total number of calories in glycogen storage in the muscles and liver per the Academy of Nutrition and Dietetics for an endurance-trained athlete is about 1,800 to 2,000 calories of fuel.

So, you have to burn through almost all of this glycogen before you really enter metabolic ketosis from fasting.

The word autophagy in wooden blocks.

How Long Do I Need to Fast for Autophagy?

Next, how long should you fast for autophagy?

After a full 24 hours of water fasting, the process of autophagy is said to begin.

Autophagy is essentially the natural clean-up process for the cells. 

During autophagy, dead and damaged cells and cell fragments are removed and cleaned up around your body. 

Viable cell components and proteins can be recycled and reassembled for new cells or to repair damaged cells, while damaged and misfolded proteins, such as those linked to neurodegenerative diseases like Alzheimer’s, are destroyed.

In this way, autophagy helps protect your body from such diseases and other sources of oxidative damage, inflammation, and premature aging.

One of the benefits of fasting is autophagy, but most studies suggest that you need to fast for at least 24 to 36 hours to experience significant autophagy over normal baseline levels.

Therefore, it is best to try fasting 24 hours once a week or fasting 36 hours once a week or a couple of times per month if you are trying to maximize fasting autophagy benefits.

The words hormone balance.

How Long Should I Fast to Optimize My Hormones?

One of the benefits of fasting is optimizing or changing different hormonal levels.

If your goal is to increase the level of human growth hormone (HGH), you need to fast for about 48 hours to see a peak in the production of HGH from fasting. Studies show that a 48-hour fast will result in about a 5-fold increase in human growth hormone levels.

Therefore, if your goal is maximizing human growth hormone levels, you’ll want to do at least a 2-day fast or consider a 48-hour fast and up to a 72-hour fast.

But, how long do you need to fast to reduce insulin levels?

Research suggests that hormonal benefits of fasting, such as lowering insulin levels, occur at earlier water fasting stages. Insulin starts to drop after fasting for 10-12 hours or so and continues to drop after that.

However, it can take up to 60 hours to see the most significant drops in insulin from fasting.

Therefore, if your goal is fasting for blood sugar regulation and increased insulin sensitivity, or if you are fasting to reverse type 2 diabetes or lower your risk of diabetes, an intermittent fasting diet such as 12/12 intermittent fasting can be a good starting place.

Then, gradually try to build up to 14/10 intermittent fasting and then 18:6 intermittent fasting if you are feeling okay while fasting 12 hours or more and can still meet your nutritional needs.

As you get accustomed to longer fasts, you can progress to fasting 24 hours once a week or try a 48-hour fast once or twice a month.

A napkin that says boost your immune system.

How Long Should I Fast to Improve My Immune System?

Lastly, if you want to fast to support your immune system, you will have to fast for 72 hours or more.

At this point, your body starts destroying old immune cells and generating new, healthier immune cells, which can strengthen your immune system, decrease inflammation, and boost energy and vitality.

It is important to speak with your healthcare provider before attempting a prolonged water fast. Water fasting for an extended period of time may not be safe or advisable for certain individuals.

It is also advisable to build up to extended water fasting protocols with shorter water fasts or intermediary intermittent fasting diet schedules.

A clock, a plate and utensils.

How Long Is It Safe To Fast?

Remember: while most medical and nutrition experts say that it is safe to follow an intermittent fasting diet every day, once you get up to 24 hours of fasting or more, it is imperative that you talk with your doctor or a registered dietitian to ensure that extended fasting is safe for you.

There are numerous contraindications to prolonged fasting, and if you are attempting fasting for weight loss, there are often safer and more effective approaches than a 3-day water fast, 5-day water fast, and so on.

Therefore, we present all of this information with a strong disclaimer that we are not recommending unsupervised fasting longer than 24 hours for anyone, even if you are healthy.

Furthermore, if you have a chronic medical condition, kidney disease, diabetes, a metabolic disorder, a history or current eating disorder, or are pregnant or breastfeeding, you should talk with your healthcare team before engaging in intermittent fasting diets, let alone extended fasting protocols. 

Juice for a detox.

Instead of strict water fasting and trying to do really long fasts, we recommend a more sensible approach to weight loss involving modest caloric restriction and increased physical activity.

Beyond that, during times when you feel like you need more of a true metabolic reset or detox for weight loss, we highly recommend a detox diet that’s actually well thought out, nutritionally balanced and safe, and designed to support your metabolism, decrease inflammation, and help you burn fat.

Of all the detox weight loss programs and cleanses we’ve seen so far, the only one that earns our genuine stamp of approval (and even recommendation!) is the Kroma 5-Day Reset Detox Diet plan.

This is an all-in-one product that you purchase that has five days worth of detox diet smoothies, juices, soups, nutritional drinks, and broths that are supposed to help reduce inflammation, boost your metabolism, help you lose weight, and reset your palate for weight loss moving forward.

Plus, the Kroma 5-Day Reset Detox is high in fiber, and fiber is one of the few nutrients that may aid the liver in detoxing the body, and there’s lots of online support.

Oh, and the longest fast ever? The fasting world record is credited to Angus Barbieri, who is said to have fasted for 382 days in 1971. Crazy (and NOT something we recommend!).

Interested in trying a prolonged water fast? Check out our guide to 3-day water fasting here.

A person drinking a glass of water,
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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