Extended fasting is a great to tool overcome stagnant weight loss, help your cells clean up themselves through autophagy, and fight risk factors for cancer and neurodegenerative diseases. But how long should your extended fasts be, how often should you do them, and what precautions should you take? We give you an overview of extended fasting below, how extended water-fasting and modified fasting compare, the benefits of both, and some safety tips for doing fasts of 5 or more days.

A brief history of extended fasting

Obesity treatments in the 60s and 70s consisted of long water-fasts (also known as zero-calorie diets) lasting 60 days and longer. But this approach was almost entirely discontinued due to complications, including low blood pressure, anemia, heart problems and death. Since then, water-fasting has taken less extreme forms, including, for instance, alternate-day fasting, which consists in alternating between a day of no calorie consumption and a day of normal food intake.

Most of what we know about the benefits of water-fasting has come from studies of alternate-day fasting and other fasting methods, such as time-restricted eating and the 5:2 diet. Although these shorter fasts are beneficial, there are health reasons for wanting to go beyond 48 hours with no or minimal calories. These include reducing heart disease risk factors, breaking through a weight loss plateau, rejuvenating your immune system, or tapping into autophagy to rid your body of damaged cell components.

Women holding a glass cup of tea and sipping on it
Extended fasts can help you reduce heart disease risk factors, break through a weight loss plateau, rejuvenate your immune system, and tap into autophagy

Modified fasting vs water-only fasting

Most studies of fasts lasting 5 days and longer allow subjects to eat some calories in the form of low-protein, low-carb foods. This usually means vegetable juices and vegetable soup, averaging a total calorie intake of 200–250 kcal and 25–35 g of carbohydrates per day.  Modified extended fasting has been shown to reduce body weight, reduce circulating blood glucose and blood lipids, improve emotional and physical well-being, reduce chronic painincrease healthy gut microbes and reduce blood pressure.  The most common side effects that have been reported during 4-21-day modified fasts include sleep problems, fatigue, dry mouth, back pain, hunger, bad breath, headache, muscle pain, bloating, diarrhea, and sensitivity to cold.

Studies of strict water-fasting are limited. Extended water-only fasting (5+ days) has been shown to reduce body weight and oxidative stress, improve blood pressure, improve lower tract urinary function, and improve quality of life during chemotherapy. These types of fasts are generally well tolerated. Some expected side- effects that have been reported with water-only fasts lasting from 2-22+ days include mild-to-moderate fatigue, nausea, insomnia, headache, dizziness, indigestion and back pain. Patients in these studies were under the supervision of a doctor, and underwent comprehensive physical and psychological tests before participating.

Water-fasting and modified fasting seem to have similar health benefits and both cause some of the same side effects, although this has not been directly compared. For some, having a calorie “crutch” makes extended fasting easier.  Whether you’d like to embark on a modified or water-only extended fast, there are some safety principles that apply to both:

Safety tips for extended fasting

Keep it seasonal

Extended fasts can be a stressor on the body, and just like with anything in life, more is not always better. Long term studies of repeated cycles of extended fasting (modified or water-only) are lacking and it’s unknown what the long term effects of restricting calories for extended periods could be. Doing up to 1 extended fast per season is enough for you to reap the benefits of autophagy and immune cell regeneration 4 times per year!

screenshot of the LIFE Fasting Tracker showing a 7 day fast
Up to one extended fast per season is enough to rip the benefits of extended fasting. We recommend limiting these long fasts to 7 days.

Keep your fast under 7 days

Although humans seem to tolerate much longer fasts, this doesn’t mean you should do them. It’s not clear that fasts longer than 5-7 days are better. For example, ketosis seems to peak 5 days into a 21-day fast!

From animal studies, we know that fasting or restricting calories for extended periods can be costly. For example, studies in fruit flies have shown increased mortality and reduced reproduction after returning to a standard diet following a long period of calorie restriction. Similarly, worms subjected to extended fasting live longer, but have a delayed reproductive schedule, lay smaller eggs, and produce smaller, shorter-lived offspring. Although we are not fruit flies or worms, it’s best to proceed with caution until more studies in humans become available.

A common misperception about fasting is that the body quickly starts to break down muscle for new amino acids during periods without food. This would actually be pretty dumb on the part of our bodies – to tear down valuable muscles and other proteins when another source of energy, fat, is so plentiful. In reality, we see very little muscle loss during the first few days of a fast. However, if you fast long enough, eventually your body will have no other energy resources to tap into and you will start to lose muscle mass. This is another reason that we recommend that you keep your fasts shorter than seven days unless you’re under the close supervision of a physician or other healthcare professional.

Drink plenty of water

It’s a good idea to stay hydrated during extended fasting. Aim for a minimum of 2–2.5 L  (8-10 glasses) of water or fluids per day. You can also take a multivitamin to provide your body with some micronutrients.

Consider eating some calories 

Enjoying under 500 calories of low-protein, low-carb foods won’t hinder your progress or kick you out of ketosis during extended fasting (You can always measure your blood or breath ketones to know for sure). Having some salt content in these fasting “meals” can also help you maintain your electrolyte balance during extended fasting, which can prevent headaches and fatigue. Participants in studies of modified extended fasting typically drink fruit or vegetable juice and veggie soup averaging around 250 calories. You can also go for the prolon fasting-mimicking diet, or do what LifeOmic’s CEO does during his modified 5-day fasts. He eats 2 cups of mixed greens with olive oil and a handful of almonds for a total of 250 calories.

a bottle of olive oil next to a bowl of mixed greens sitting on a table.
Our CEO’s modified fasting ‘meal’. 2 cups of mixed greens, 1 tablespoon of olive oil and a handful of almonds, for a total of 250 calories.

Replenish your body after extended fasting

How to break an extended fast? Refeeding with the right foods is critical to the benefits of extended fasting, including cellular renewal and improved brain function. For fasts 5 days and longer reintroduce calories gradually. For example, you can start with some veggie or bone broth, or some fermented vegetables to get your gut ready to receive food again. Wait about 30 minutes to have a small, low-fat meal with vegetables and a low-fat cut of fish or chicken.

In studies of prolonged fasting, calories are reintroduced gradually, starting with 800 calories and progressively increasing to 1600 over 4 days. Participants usually follow an ovo-lacto- vegetarian diet consisting of grains, fruits and vegetables, legumes, seeds, nuts, dairy and eggs.

To minimize uncomfortable side effects, consider taking a laxative before a modified extended fast

In studies of modified extended fasting, participants typically take a laxative, usually  20–40 g Na2SO4 (sodium sulfate) in 500 ml water according to body weight. This helps with the transition into fasting mode, minimizes feelings of hunger, removes food remnants from your intestines, and gets rid of bile secretions that can cause nausea, indigestion, headaches, and fatigue.  Purging is recommended for those prone to headaches and indigestion, and those with low blood pressure.

Listen to your body

A little bit of hunger that you can mentally work through is ok, but you shouldn’t deal with feeling dizzy or faint. It’s ok to end your fast before your desired goal! You’ve still earned a ton of health benefits in the form of autophagy, gut rest and bringing your blood glucose levels down.

Doing an extended fast? Share your experiences with us on Instagram and Facebook  with the hashtags #SeizeLIFEFast #LIFEfastingapp.

Download the LIFE Fasting Tracker app here.

Luisa Torres, PhD

I'm a science communications manager at LifeOmic and the editor of this blog. I am a neuroscientist and science writer interested in covering topics related to aging, metabolism, and brain health. I have written for NPR's blogs 'Shots', 'Goats and Soda', and 'The Salt'.

LifeOmic® is the software company that leverages the cloud, machine learning and mobile devices to improve healthspans – from prevention and wellness to disease management and treatment.



Contact Us

Privacy Preference Center