Prolonged fasting: Benefits, side-effects, uses and how to get started

In recent years, fasting has garnered considerable attention for its potential health benefits beyond weight management. At the moment, prolonged fasting, or fasting for 12 to 36 hours at regular intervals, is all the rage. 

In contrast, when you pass the 36-hour mark, you enter the world of prolonged fasting. Also called periodic fasting, humans have gone long without food throughout history [1].

In most cases, longer fasts can be beneficial and safe. Despite this, prolonged fasting isn’t easy for everyone. 

How does prolonged fasting work?

A prolonged fast is an extended period, typically over 36 hours, without eating. During this time, water, electrolytes, and noncaloric beverages are allowed.

You can consider it as a more powerful variation of intermittent fasting. Due to long periods without food, the effects are more or less the same, but they are amplified. 

Fasting affects your metabolism or energy consumption primarily. As a result of fasting, you switch from sugar-burning (or fat-storing) to fat-burning mode. The body stores fat for this very reason: to provide energy during times of scarcity. 

Despite what you might think, you can fuel your body for a shockingly long time with enough body fat. For instance, a 1970s research studied a morbidly obese man who fasted for 382 days [2]. His prolonged fast, fueled by body fat, allowed him to lose 180 pounds without suffering severe side effects.

Featured product offer
ProLon® 5-Day Meal Program
  • The only nutrition program designed and tested by 14 renowned universities worldwide.
  • Clinically shown to reduce fine lines and wrinkles after 3 cycles (15 days total).
  • Each kit contains various soup flavors, snacks, teas, and supplements.

Exploring intermittent fasting

Compared to prolonged fasting, this type of fast requires less commitment. This involves alternating between eating and fasting within one day or over several days and can therefore be practiced without restriction.

Crucially, it is not a conventional diet as it does not restrict what you eat during fasting periods. Intermittent fasts come in many forms, like alternate-day fasting, 5:2, and one-meal-a-day fasting (OMAD).

TRE also alternates between periods of fasting and eating within a day, such as the 12:12, the 16:8, and the more intensive 20:4 [3]. Incorporating TRE into any lifestyle is easy by extending this natural fast during sleep.

IF health benefits

Ketosis begins between 12-16 hours into a fast, so even the shortest intermittent fasts can result in fat burning [4]. Compared to prolonged fasting, intermittent fasting avoids severe feelings of hunger and can be practiced daily. 

Fasting becomes easier and has less impact on other areas of life, such as socializing. By practicing it daily, you can re-calibrate your relationship with food and reduce feelings of hunger.

Since autophagy begins at least 16 hours into a fast, it becomes more difficult to achieve its cellular renewal benefits with short intermittent fasts than with prolonged fasts.

Potential side effects of intermittent fasting

Intermittent fasting comes with minimal risk compared to prolonged, with mild side effects including headaches and fatigue. However, some users may unconsciously reduce their physical activity and increase energy intake before, during, and after a fasting period to compensate for the caloric restriction, reducing the benefits of the fast [5].

A way to stop this is by avoiding overeating and partaking in light exercise – which is not recommended during prolonged fasts.

Both intermittent and prolonged fasting are effective weight loss methods, and while prolonged has the added benefit of autophagy, its intensive calorie restriction makes intermittent fasting an easier option for long term weight loss.

What are the benefits of prolonged fasting?

Losing excess fat is one reason prolonged fasting is good for you, but why do you practice prolonged fasting? Take into consideration the following advantages:

Long periods of fasting lead to weight loss. One year of prolonged fasting was followed by 1,422 volunteers in a study published in 2019. The participants lost a significant amount of weight by the end of the study [6].

Prolonged fasting: Benefits, side-effects, uses and how to get started

Early on in the fast, most weight loss will be water weight, especially if you are not fat-adapted. As a result, you will likely gain some weight when you continue regular eating. As your metabolism adapts to burning fat for energy, more of this weight loss will be fat loss. 

A reduction in blood sugar and insulin levels: An estimated one in ten Americans suffer from type 2 diabetes, indicated by high blood sugar and insulin levels. There are different types of fasting, but longer fasts lower blood sugar levels faster. Using intermittent and prolonged fasting protocols, Dr Jason Fung (author of The Complete Guide to Fasting) helped his patients reverse type 2 diabetes.

Ketosis: As mentioned above, fasting decreases insulin levels. The liver then starts burning fat and producing ketones as a result. The primary function of ketones is to provide energy to the brain. One study found that older adults with higher ketone levels performed better mentally [7]. Thus, fasting induces ketosis, which is likely to improve cognition.

Autophagy: During a fast, your cells activate an autophagy recycling program in the absence of nutrients. As an anti-aging mechanism, autophagy eliminates old, damaged cell parts and replaces them with fresh ones.

The longer the fast, the more autophagy is activated. While autophagy cannot be felt and measured in humans, it certainly plays an important role in maintaining cells’ health and vitality.

The process of prolonged fasting

It’s best to start slow when you’re new to fasting. Fasting for two days shouldn’t be your first step. Slowly work your way up to fasts of two or more days by initiating overnight fasts. 

It has been shown that an overnight fast of 14 to 16 hours lowers blood sugar, improves insulin function, and improves your sleep-wake cycle [8]. Typically, this type of fasting involves two meals per day, one at 12 pm and one before 8 pm or one at 9 am and 5 pm.

If you have been fasting overnight for two weeks, you can progress to intermittent fasting for 18 to 24 hours after that.

Your body fat adapts to these shorter fasts, making prolonged fasts, which generally range from two days to about a month, much easier. 

You can also eat a ketogenic diet before and after you fast. In the same way that fasting lowers insulin levels, keto helps your cells burn fat instead of sugar.

Breaking your prolonged fast

Your stomach shrinks when you fast for days at a time. To avoid indigestion, don’t gorge yourself on your first meal back. Instead, it would help if you thought small. 

Start with a small meal, such as a protein shake or lean meat with a few hundred calories. As well as being easier to digest than fat, protein also stops muscle catabolism (breaking down of complex molecules). 

After your mini-meal, give yourself about an hour before eating a regular meal containing healthy fats like olive oil, avocado oil, or coconut oil. You will stay in ketosis if you consume these fats.

Lastly, eating iodine-rich foods (such as seaweed, shrimp, tuna, and eggs) following a fast will support the creation of thyroid hormones T3 and T4 [9]. This is significant for women, who generally need more thyroid support while fasting.

Differences in fasting between men and women

Regular fasting can benefit weight loss, health, and longevity, but its effects differ between men and women. Physiological differences between men’s and women’s bodies after fasting can be traced back to prehistoric hunter-gatherer gender roles.

In the past, men hunted while women gathered, which may explain why men and women have different body sizes, metabolisms, and muscle and fat compositions. Over time, modern-day’s toxic diet culture has primarily focused on women. 

Research in this area is limited, but some evidence suggests that men are better adapted to caloric restriction and may benefit more from fasting.

Generally, fasting is safe, and most women who fast are satisfied with the results, but some women have reported side effects like binge eating, metabolic disruption, and changes in their menstrual cycles.

This extreme response may be explained by the intricate balance of hormones that controls both men’s and women’s essential functions. Changing caloric intake or timing of caloric intake can disrupt hormonal functioning in women, affecting ovulation, metabolism, and mood [10].

Kisspeptin, which stimulates the production of sex hormones such as gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH), may be involved. Other hormones that affect hunger and fullness, such as leptin, insulin, and ghrelin, can influence kisspeptin levels.

There is more kisspeptin in women than in men, which may make them more sensitive to changes in energy intake that may affect their hormonal cycle [11]. A hormonal imbalance can affect more than just reproduction, and disruptions in sex hormones, such as estrogen, can spread throughout the body.

As an example, imbalances in estradiol, a type of estrogen, may increase appetite and promote fat storage [12].

Featured product offer
Future Kind+ Plant-Powered Meal Shake
  • Contains 16 g of protein to promote fullness.
  • 100% vegan and gluten-free.
  • Combines organic protein powders, probiotics, fiber mix, super greens and digestive enzymes.

Optimising your fast regardless of sex

In light of these variations, it is important to optimize yours fast according to your sex. Even though women’s bodies can react differently to fasting, it should not prevent them from completing their fasts.

There are endless fasting variations to suit everyone – for females, it may be prudent to avoid extreme forms of fasting such as the OMAD, or one meal a day fast, while male bodies tend to be more resilient to most types of fasting.

Beginners of both sexes should gradually extend their natural fast into daylight, for example, by gradually extending it during sleep. A time-restricted eating plan (TRE) alternating between fasting and feasting within a day may suit women. The 12:12 is a good place to start for beginners. 

Gradually, as your body adapts to fasting, this can be extended to the 16:8 or even the more intensive 20:4 best suited to experienced fasters.

While gender can lie on a spectrum, certain physiological differences between people are more fixed, and our bodies respond differently to fasting. However, females can still unlock the health and longevity benefits by being aware of these differences and matching their fasts accordingly.

Several tips for prolonged fasting success

You can succeed in fasting over 36 hours if you follow these tips:

Take electrolytes: Insulin levels fall during a fast, causing an increase in sodium loss through urine [13]. While fasting, potassium, magnesium, and phosphorus can also help prevent refeeding syndrome, a condition in which your body depletes vital minerals to rebuild glycogen, protein, and fat.

You can drink bone broth instead of electrolytes if you don’t want to supplement them. This superfood is electrolyte-rich.

Bone broth can be enjoyed twice a day during longer fasts. Although it has a bit of protein, it shouldn’t interfere with your fast.

  • Hydrate: Fasting has a diuretic effect, meaning you expend fluids quickly. Rather than drinking those fluids, try noncaloric, unsweetened, nondairy alternatives like coffee, tea, bone broth, and water.
  • Ride out the hunger: Hunger doesn’t persist the whole time you fast, but rather it ebbs and flows. An extended fast usually makes people hungrier on the second day. After that, it gets easier.
  • Track your fast: Make a fasting plan, track key metrics, and stay accountable using an app.
  • Fast with others. Staying motivated is easier if you do this. Additionally, sharing metrics like fasting hours, ketone levels, and glucose ketone index (GKI) is fun.
  • Monitor ketone and glucose levels: In the course of a fast, you should be able to see a rise in ketone levels and a decrease in glucose levels. A properly functioning metabolism is indicated by this. Use an accurate at-home device like the Keto-Mojo meter to monitor your ketones, your glucose, and your GKI
  • Take time to plan your first meal: If you don’t break your fast wisely, your gut won’t be happy.

Side effects of prolonged fasting

Sustained calorie restriction is associated with constant hunger, an obvious drawback of prolonged fasting. Since most prolonged fasts only allow water, tea, and coffee to be consumed, this method requires serious commitment and motivation.

According to one study, 93 percent of subjects reported not feeling hungry after a meal [14].

Fasting for a prolonged period of time is generally safe, but it can cause mild side effects such as dehydration, electrolyte imbalance, fatigue, and electrolyte imbalance. When the body transitions into ketosis, users may experience irritability and low energy.

By increasing your water intake, adding 12-1 teaspoons of sea salt to your drinking water, and resting regularly, you can prevent electrolyte depletion [15].

Safety precautions to keep in mind

  • Make it seasonal: Extended fasts can be a stressor on the body, and as with anything in life, more is not always better. Repeated cycles of prolonged fasting (modified or water-only) haven’t been studied long-term, and its long-term effects are unclear. The benefits of autophagy and immune cell regeneration can be reaped four times a year by doing one prolonged fast per season.
  • Fast for no more than 7 days: Although humans seem to be able to tolerate longer fasts, this does not mean you should. Longer fasts are not necessarily better than shorter ones. A 21-day fast, for instance, seems to peak 5 days in. 

Animal studies have shown that fasting or restricting calories for long periods can be costly. After returning to a standard diet after a long period of caloric restriction, fruit flies showed increased mortality and reduced reproduction [16]. 

Long-fasted worms live longer, lay smaller eggs, and produce smaller, shorter-lived offspring. They also have a delayed reproductive schedule. Even though we are not fruit flies or worms, we should proceed with caution until more human studies have been conducted.

A commonly held misperception about fasting is that the body quickly begins to break down muscle in search of new amino acids during periods without food. It would actually be quite foolish of our bodies to destroy valuable muscle and other proteins when another source of energy, fat, is so plentiful. In reality, we see very little loss of muscle mass during the first few days of fasting. 

If you fast for a long time, your body will eventually run out of energy resources, and you will begin to lose muscle mass [17]. As a result, we recommend that you keep your fasts shorter than seven days unless a doctor closely supervises you.

  • Stay hydrated: It’s a good idea to stay hydrated during a prolonged fast. Try to consume a minimum of 2 to 2.5 L (8 to 10 glasses) of water or fluids per day [18]. Micronutrients can also be provided by taking a multivitamin.
  • Try eating a few calories: Enjoying less than 500 calories of low-protein, low-carb foods won’t hinder your progress or knock you out of ketosis during prolonged fasting (you can always test your blood or breath ketones to be sure). Having some salt in these fasting “meals” can also help you maintain electrolyte balance during prolonged fasting, which can prevent headaches and fatigue.

Participants in modified prolonged fasting studies generally drink fruit or vegetable juice and vegetable soup, averaging about 250 calories. Two cups of mixed greens with olive oil and a handful of almonds provide 250 calories.

  • Listen to your body: 

A little hunger that you can mentally overcome is fine, but you shouldn’t deal with feeling dizzy or faint. If you don’t reach your goal, ending the fast is okay. Still, it has reaped a ton of health benefits, including autophagy, gut rest, and lowering blood sugar.

Prolonged fasting and longevity

Even those who are already at a healthy weight can lose weight, reduce belly fat, and lower blood pressure by fasting for a prolonged period of time [19]. For long-term weight loss, it should be repeated regularly, for example, once a month, in order to see the benefits.

Prolonged fasting has an advantage over short intermittent fasts in that it induces autophagy [20].

As well as being used during nutrient stress to balance energy sources for survival, it can also be triggered by calorie restriction. In response to nutrient deprivation, nutrient-sensing pathways repress the TOR kinase protein, thereby inducing autophagy.

Autophagy may prevent age-related diseases like neurodegeneration, cardiomyopathy, diabetes, and cancer, as well as increase longevity. In short intermittent fasting methods, autophagy is more difficult to achieve since it peaks around day two of a prolonged fast.

Is prolonged fasting recommended for everyone?

A prolonged fast is unlikely to cause significant problems for the majority of people. Fasting study participants who were not obese experienced fewer than 1 percent adverse effects. 

However, these fasts were conducted under medical supervision. Consult your primary care provider before making drastic dietary changes.

Those with type 2 diabetes may benefit from supervision in some cases. If diabetes medications aren’t adjusted properly, insulin and metformin can cause dangerously low blood sugar levels during a fast.

Prolonged fasting is not suggested for the following groups:

  • Children
  • People who are underweight
  • Anyone suffering from an eating disorder

Finally, hunger is normal during fasting. If you feel shaky, dizzy, or weak, you may have hypoglycemia (low blood sugar). That’s a signal to break the fast. 

To help your body adapt to using body fat for energy, try starting with shorter fasts and work your way up. After breaking your fast, eat a small protein-rich meal, then wait 60 minutes before eating a regular meal with healthy fats.

In order to succeed during longer fasts, you can take electrolytes, drink bone broth, and track your fast. In conclusion, prolonged fasting is safe for most people, but some groups (like type 2 diabetics) should be cautious.

No matter what, it is always important to consult with your primary care provider before making a drastic change to your diet [21].

Featured product offer
ProLon® Intermittent Fasting Bars
  • The recipe is a unique blend of macronutrients and micronutrients that has been researched for over 20 years at USC.
  • Created with proprietary formula made up of high-quality ingredients that don't activate the body's nutrient-sensing system.
  • Each box contains 12 bars--all plant-based, non-GMO, soy-free, dairy free, and gluten-free.


Image by user14908974 on Freepik
The information included in this article is for informational purposes only. The purpose of this webpage is to promote broad consumer understanding and knowledge of various health topics. It is not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your physician or other qualified health care provider with any questions you may have regarding a medical condition or treatment and before undertaking a new health care regimen, and never disregard professional medical advice or delay in seeking it because of something you have read on this website.