Prolonged fasting: Is it good for weight loss?

There are a bunch of fasting types that you can try to help you reduce your weight. One of them is prolonged fasting, which is the most extreme calorie-restriction diet.

The idea is to fast for two to five days, but some dieters can take it up to two weeks with only water, plain tea, or coffee consumption in between. You only need to follow the number one rule – do not eat.

Weight loss during prolonged fasting

Prolonged fasting is a great and effective weight loss strategy. By starving yourself, you help your body to use your stored fat as fuel. Fasting is a way for your body to learn how to switch burned fats to energy quickly.

Within 18 to 25 hours of fasting, your body starts to rely heavily on breakdown for energy. This is great if you have unnecessary fats, as you could lose them while preserving lean muscle mass.

Even studies have proven that long periods of fasting can stimulate weight loss. One research conducted in 2019 looked at over 1,422 volunteers on an extended fasting program within a year. At the end of the study, the participants were found to lose significant weight [1].

Prolonged fasting can be considered the most effective, especially if you want to lose a few pounds that fast. In the early days of fasting, only the water weight is lost; hence, you will likely regain some weight when normal eating is resumed.

With prolonged fasting, you must condition your body to adapt to fat burning and become the energy source. 

When doing prolonged fasting for five days, you may lose an average of 5.5 pounds or over three percent of your body weight. The percentage of weight loss can range from a little 1.5 percent to nearly 7 percent within five days.

Insulin and glucose drops 
Photograph: indypendenz/Envato

Insulin and glucose drops 

One of the many things that may happen to your body when you do prolonged fasting is that your insulin and glucose drop. Your insulin levels may drop to nearly as low as they can reach during 50 hours and more of prolonged fasting; however, it can actually go low quickly, even within just 24 hours.

Hence, you can still significantly lower your insulin levels even if you decide to fast only for a day. What happens if you have high levels of insulin? You become prone to obesity, type 2 diabetes, and cardiovascular disease, which all can decrease your life expectancy [2]. 

After 12 hours of fasting, your body’s energy is mobilized from adipose tissue stores that are approximately two times what is available from glucose production.

Meanwhile, fasting for 72 hours can release energy from adipose tissue, which is sixfold greater than that generated from gluconeogenesis–a process by which your liver turns amino acids, for example, into glucose if needed [3].

As your insulin levels lower during prolonged fasting, so do your glucose levels. Having low blood glucose is like telling your body that it is a good time to grow molecules in the cells, bypass cellular recycling, and do protein cleanup processes.

In fact, observational studies found that fasting glucose levels usually start to lower significantly between 24 and 36 hours of fasting and can reach a stable lowered level around 50 hours [4]. 

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Fasting can activate autophagy due to the absence of nutrients for days. Autophagy is a process of cellular self-eating, where old or damaged cells are broken down and recycled into new and healthy ones. Compared with other fasting types, prolonged fasting can activate more autophagy


Lowered insulin levels can signal your liver to burn fat, producing ketones. In short, high ketone levels are a sign that your body is burning fat and using fatty acids while it produces more ketones.

As you progress beyond a 16-hour fast, your ketone levels start to slowly rise above baseline, especially noticeable when measured through blood or breath. Your ketone levels will also jump up progressively every morning while on prolonged fasting. 

Moreover, ketones are considered a highly efficient alternative energy source for the brain. Research reveals that higher ketone levels can be associated with improved mental performance in older adults. Ultimately, prolonged fasting can incite ketosis, which can better cognitive function [5]. 

Having high levels of ketone provides a range of other health benefits as well. Ketones can serve as signaling molecules, like hormones, initiating cellular processes and even changing gene expression, which can result in becoming more resilient to stress. For instance, ketones have the ability to activate genes that help in fat metabolism. 

In addition, the ketogenic diet or a regular cycle of high ketone levels–which can be obtained as well through prolonged fasting–has been found to enhance healthspan and memory in aging mice [6]. 

Potential benefits of prolonged fasting for weight loss

Prolonged fasting has garnered increasing attention as a potential strategy for weight loss, prompting curiosity and debate within the health and wellness community.

Advocates tout its ability to induce metabolic changes, reduce calorie intake, and regulate hunger hormones, possibly contributing to shedding excess pounds. However, amidst the enthusiasm, questions linger about its safety, efficacy, and suitability for long-term weight management.

Let’s see the potential benefits of prolonged fasting for weight loss, exploring the physiological mechanisms that underlie its effects and examining the evidence supporting its role in achieving a leaner physique.

Metabolic changes and fat burning

Prolonged fasting triggers the body to enter a state of ketosis, predominantly burning fat for energy instead of carbohydrates. This can lead to accelerated fat loss.

Fasting stimulates lipolysis, breaking stored fat into fatty acids and glycerol, which are then utilized for energy production.

Research suggests that prolonged fasting can improve insulin sensitivity, allowing cells to absorb glucose better and reducing the risk of insulin resistance, a common factor in weight gain.

Reduction in calorie intake

By abstaining from food for an extended period, individuals naturally consume fewer calories, leading to a caloric deficit necessary for weight loss.

Prolonged fasting encourages mindful eating habits and can help individuals become more attuned to their hunger cues, potentially reducing overall calorie intake beyond fasting.

Regulation of hunger hormones

Prolonged fasting has been associated with decreased levels of ghrelin, the hormone responsible for stimulating hunger. This suppression may lead to reduced feelings of hunger and fewer cravings.

Fasting may improve sensitivity to leptin, the hormone that signals satiety. By enhancing leptin sensitivity, individuals may feel fuller faster and be less inclined to overeat.

Some proponents suggest that prolonged fasting can reset taste preferences and diminish cravings for unhealthy, calorie-dense foods, promoting a shift towards healthier dietary choices conducive to weight loss.

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What to expect during prolonged fasting? 

Prolonged fasting may be effective in weight loss, but it is not a piece of cake. You have to endure your urge to eat, which can be extremely difficult for people with bad eating habits. Before you start doing prolonged fasting, you should know the following factors to set your expectations. 


You will surely feel hungry, and that’s for sure! Prolonged fasting can’t minimize your hunger hormone levels over time, similar to regular intermittent fasting or alternate-day fasting. 

Unfortunately, the hunger dilemma is much more intense for some women who have higher fasting levels of the hunger hormone called ghrelin [7]. Ghrelin is a byproduct of the stomach that increases one’s appetite and food consumption.

It can be observed by an individual’s circadian rhythm, cycling up and backing down on a daily basis. This hunger hormone typically increases its levels all throughout the day, particularly in the early to late afternoon, and slowly wanes as you prepare for sleep.

Fortunately, ghrelin slowly decreases on average over a multi-day fast; hence, it is quite challenging to fast during the first few days, around two to five days. However, eventually, as your body begins to adjust, your hunger sensations slowly decrease. 

Although prolonged fasting may not physically help lower hunger hormones, it is a good way to help build a strong mental relationship with your eating habits.

You can become conscious of your hunger and food cravings as you try to fight your urge to eat something. Prolonged fasting can also help with mindful meditation and a better mindset towards hunger, realizing that hunger is only a signal to hunker down.

food cravings

Food cravings

If you think hunger is the best enemy during prolonged fasting, you better think about food cravings, too! When you fast for days, you might find yourself actively in need of specific foods, making you salivate while starving.

External cues, such as smelling or seeing food, can trigger food cravings. The external cues can activate your brain’s reward system; it actually becomes really tricky by then as it usually engages with calorie-dense foods. 

The best way to deal with this is to avoid scrumptious foods while on a prolonged fast. You can also opt to empty your refrigerator of food except for your low protein and low carb fasting “crutch” calories before you begin prolonged fasting so that there are no temptations around. 

Alertness and energy boosts

Normally, people who do prolonged fasting feel more alert and energetic, especially after 18 to 24 hours of fasting. The explanation relies on the body’s rising cortisol levels when fasting. 

According to a data report, the cortisol or stress levels cycle every day but rise throughout a prolonged fast. It reaches low points when your ghrelin levels are highest and high points in the evenings when ghrelin levels are lowest. Then, the cortisol levels slowly rise over the course of a multi-day fast as your hunger slowly dampens [8].

Fatigue or tiredness

During prolonged fasting, you may definitely experience fatigue or tiredness and feelings of weakness, lightheadedness, slowness, or lack of motivation, most especially in the first few days. Fatigue is actually normal and usually experienced by the majority of dieters who do prolonged fasting. 

You may experience fatigue or tiredness as the body adapts to the period of low-calorie intake, which is apparently different from your usual eating habits.

Fatigue may occur because your body transitions from burning readily available sugars to mobilizing and burning fats. It is somewhat like a “carbohydrate withdrawal.” 

Fatigue from prolonged fasting may include health signs, such as low blood sugar or hypoglycemia, which can directly affect you while fasting.

Hypoglycemia can be experienced by people who are accustomed to frequently consuming high amounts of sugars and have insulin resistance or liver issues like fatty liver. 

What is the practical advice for fasting?

While the potential benefits of prolonged fasting are intriguing, it’s essential to approach this practice with caution and mindfulness.

Let’s check some practical recommendations to help individuals safely incorporate prolonged fasting into their weight loss journey.

  1. Consultation with a professional: Before embarking on a prolonged fasting regimen, individuals should consult with a healthcare provider, especially if they have underlying health conditions or are taking medications.
  2. Health assessment: Assessing one’s overall health status, including any nutrient deficiencies or metabolic issues, can help determine if prolonged fasting is appropriate and safe.
  3. Gradual approach: Start with shorter fasting durations and gradually increase the fasting period to allow the body to adapt and minimize discomfort.
  4. Monitoring symptoms: Pay attention to how your body responds to fasting and be mindful of any adverse symptoms such as dizziness, fatigue, or extreme hunger. If experiencing severe symptoms, break the fast and seek medical attention if necessary.
  5. Mindful breaking: Eat small, nutrient-dense meals to prevent digestive discomfort and avoid overeating when breaking a prolonged fast.
  6. Regular check-ins: Monitor key health indicators such as blood pressure, blood sugar levels, and hydration status during prolonged fasting.
  7. Professional supervision: Consider working with a qualified healthcare provider, nutritionist, or fasting expert who can provide personalized guidance, monitor progress, and address any concerns that may arise.
  8. Individualized approach: Recognize that fasting protocols may vary in effectiveness and safety depending on individual factors such as age, gender, medical history, and lifestyle. Tailor fasting regimens to suit individual needs and preferences.


Prolonged fasting is a promising weight loss strategy backed up by much research. You can try to incorporate prolonged fasting into your healthy habits, but of course, with a go-to signal from your physician. Consulting with your doctor is necessary to avoid any health backlash. 

While trying to lose weight, you are also unintentionally prolonging your lifespan with fasting. Many research studies have proven that it can help with slowing down aging and extending longevity. Isn’t it amazing? 

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