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 quickly learn how to switch burned fats to energy.

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

Even studies have proven that long periods of fasting can stimulate weight loss. One research that was 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].

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Prolonged fasting can be considered the most effective fasting type, 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 need to condition your body to adapt to fat burning and become the source of energy. 

When doing prolonged fasting for five days, you may lose an average of 5.5 pounds or over three per cent of your body weight. The percentage of weight loss can range from a little 1.5 per cent to nearly 7 per cent 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-hour fasting, your body’s energy is mobilised from adipose tissue stores that are approximately two times that 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 and so as 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]. 


Fasting can activate autophagy due to the absence of nutrients for days. Autophagy is a process of recycling or cleaning up old and damaged cells and replacing them with healthier ones. Compared with other fasting types, prolonged fasting can activate more autophagy. 


Lowered insulin levels can signal your liver to begin fat burning and then it produces 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]. 

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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 minimise 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 rises 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, realising that hunger is only a signal to hunker down.

Prolonged fasting: Is it good for weight loss?

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.

Food cravings can be triggered by external cues, such as smelling or seeing food. 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 keep yourself away from 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 rising cortisol levels of the body when doing prolonged 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 to your usual eating habits. Fatigue may occur because your body transitions from burning readily available sugars to mobilising 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–a condition that 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. 


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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