Intermittent Fasting vs. Prolonged Fasting: Differences, Pros and Cons (2023)

Intermittent fasting is all the rage right now, mainly because it is a very efficient method to lose weight. Prolonged fasting is similar to intermittent fasting, but it is said to bring more significant health benefits. I decided to clear the air about both as someone who’s tried them both.

Table of Contents

  • How Are Intermittent Fasting And Prolonged Fasting Different?
  • Intermittent Fasting Pros
  • Intermittent Fasting Cons
  • Prolonged Fasting Pros
  • Prolonged Fasting Cons
  • Instead Conclusion: How Will Your Workout Routine Influence Your Eating Habits?

How Are Intermittent Fasting And Prolonged Fasting Different?

Both intermittent and prolonged fasting encompass going without eating for a certain period. While intermittent fasting means interval eating, i.e., you eat in an eight-hour window and fast the other 16 hours, prolonged fasting means not eating for at least 36 hours. With time, the goal of intermittent fasting is to narrow down the eight-hour window to four hours.

The longest recorded extended fast lasted for 382 days. It was medically supervised and more of an experiment to see what would happen. This person was at no life-threatening risk, and he lost 276 pounds (120 kilos).

It is also said that fasting helps the body heal and repair itself by allowing it to use its own cells as fuel. These are not new concepts to humanity. Even cavemen fasted because food was scarce back then.

Intermittent Fasting vs. Prolonged Fasting: Differences, Pros and Cons (1)

Intermittent fasting is the lower-risk option, and its effects are mostly related to losing weight and generally improved digestion, reduced bloating, and better bowel movement. On the other hand, expanded fasting brings certain risks, such as dehydration, no energy, brain fog, and other health-related problems specific to the person’s health condition.

In my 72 hours of fasting, I drank a lot of water, flavoured sparkling water, and green tea. It really wasn’t that bad. Still, it’s always recommended that you speak to a doctor before doing it.

Extended fasting also brings the benefit of autophagy. It is a state in which the body starts using itself as a fuel, feeding off the so-called “junk cells” or cells you don’t need. Such junk cells can be anything from excess fat cells to cells of a particular disease.

Autophagy is considered to have a potent beneficial effect on diseases such as Crohn’s, Parkinson’s, diabetes, heart disease, liver disease, kidney disease, etc. It has even been connected to cancer treatment, though additional research is needed to confirm this.

Anyway, autophagy kicks in somewhere between 24 and 48 hours of prolonged fasting. It is the third of the five stages of fasting. The first stage is ketosis, which occurs after 12 hours of fasting, and here’s where you burn fat cells. You can reach this stage in intermittent fasting too.

Intermittent Fasting vs. Prolonged Fasting: Differences, Pros and Cons (2)

The second stage is insulin reduction, usually occurring after 16 hours of fasting. This is where your insulin sensitivity increases, allowing your cells to use it better. This 16-hour fasting interval is recommended for people with insulin resistance and type 2 diabetes.

The rest of the stages come in prolonged fasting only.The third stage is autophagy. The fourth stage is the growth hormone stage. It usually kicks in after 48 hours of fasting, and it benefits your muscles and bones.

The last stage is immune cell rejuvenation or “the fountain of youth.” There are studies stating that to start producing new white blood cells to restart your immune system, you need to fast for 30 days. However, this cell rejuvenation has also been shown to occur between 48 and 72 hours of fasting.

Still, there is no sufficient research to clearly state the period in which the last stage takes place, but no research denies its existence.

Intermittent Fasting vs. Prolonged Fasting: Differences, Pros and Cons (3)Source: Robert Born

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After my 72-hour fast, I must say, I felt refreshed overall. My mind was sharper, my body felt lighter, and my energy levels were pretty high. After you finish your fast, start with light food, such as salads and chicken breast, and stay away from bread, sweets, and fatty foods for two-three days.

Now, let’s discuss intermittent fasting. As I said, with intermittent fasting, you reach the first two stages, i.e., ketosis and insulin sensitivity. This makes intermittent fasting great for losing weight.

So, while prolonged fasting brings more health benefits, intermittent fasting is not to be underestimated either. Also, it is easier to fast intermittently, it is low-risk, and it is easier to fit into your daily routine.

If you plan on doing both or either, consult with a doctor first and make sure your body can do it. Also, whether you fast intermittently or prolonged, you won’t get the expected results if you don’t make any dietary changes to your daily nutrition.

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Intermittent Fasting Pros

Take a glimpse at the intermittent fasting pros:

  • Weight loss
  • Increased insulin sensitivity
  • Feeling lighter
  • Improved bowel movement
  • Feeling refreshed
  • More energy in the long run

Note that adjusting takes some time.

Intermittent Fasting Cons

Below are some of the most common side effects of intermittent fasting:

  • Feeling hungry
  • Feeling irritated
  • Feeling tired until you adjust
  • Headaches
  • Brain fog
  • Insomnia
  • Nausea until you adjust

You may not experience all of the cons, and the ones you experience will likely subdue with time.

Prolonged Fasting Pros

Take a look at the prolonged fasting pros:

  • Lower cholesterol and blood pressure
  • Lower cancer risk
  • Lower risk of cardiovascular disease
  • Increased insulin sensitivity
  • Weight loss
  • Slower ageing
  • System reset

Prolonged Fasting Cons

Please see the prolonged fasting cons below:

  • Generally riskier than intermittent fasting, so talk to a doctor before doing it
  • Severe hunger
  • Risk of dehydration
  • Risk of constipation after you are done fasting
  • Headaches
  • Insomnia
  • Irritability
  • Brain fog
  • Nausea

During your prolonged fasting, make sure you hydrate and take vitamins such as vitamin C and magnesium. Drink green tea and mineral water to boost your energy levels and suppress hunger.

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Instead Conclusion: How Will Your Workout Routine Influence Your Eating Habits?

You have probably noticed that working out impacts how you eat. But do you know why? It is absurd that the more people eat, the hungrier they are, but that actually makes perfect sense.

A sedentary lifestyle is often accompanied by a lot of food and a tough time controlling hunger. This happens because you dull up your brain by creating more “white noise” inside your head. This white noise is basically how clear your head is, and the less white noise, the better.

Intermittent Fasting vs. Prolonged Fasting: Differences, Pros and Cons (4)

So, when you sit all day, you find yourself reaching for snacks. This is because you don’t scan your body about what you need and do things mechanically, which is why getting stuck in that routine is so easy.

With each muscle contraction, your body receives a signal from outside, bringing it back to the present moment and reducing this white noise. So, a regular workout routine makes you better aware of what and when you need to eat.

Working out activates the hypothalamus which clears the white noise. You become more aware of what you need at a given moment and scan your body to see if you really need to eat. This way, you end up eating only as much as you need to be functional.

One of the best pieces of advice I read was to fuel my workout before training and maximize my recovery with a well-balanced meal. It helped me to focus on what my body needs instead of focusing on what my mind thinks it needs.

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Intermittent Fasting vs. Prolonged Fasting: Differences, Pros and Cons? ›

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 is the disadvantage of long fasting? ›

For some, fasting may cause problems. For example, studies have found that people who regularly fast more than 16 or 18 hours a day have a higher risk of gallstones. They're also more likely to need surgery to remove the gallbladder.

What are the disadvantages of intermittent fasting? ›

Potential Drawbacks of Intermittent Fasting

If you're not already used to skipping meals or not eating for an extended period of time, the hunger and fatigue that can occur when starting IF can be difficult to handle. Other side effects can include insomnia, irritability, headaches, and nausea.

What are 5 cons of intermittent fasting? ›

  • Hunger and cravings. It may be no surprise that hunger is one of the most common side effects related to intermittent fasting. ...
  • Headaches and lightheadedness. ...
  • Digestive issues. ...
  • Irritability and other mood changes. ...
  • Fatigue and low energy. ...
  • Bad breath. ...
  • Sleep disturbances. ...
  • Dehydration.
Apr 23, 2021

Why don t doctors recommend intermittent fasting? ›

Intermittent fasting is safe for many people, but it's not for everyone. Skipping meals may not be the best way to manage your weight if you're pregnant or breast-feeding. If you have kidney stones, gastroesophageal reflux, diabetes or other medical problems, talk with your doctor before starting intermittent fasting.

What does prolonged fasting lead to? ›


In addition, prolonged fasting leads to an increase in the concentration of growth hormone glucagon and a decrease in the blood levels of thyrotropin and T3/T4 [19]. The release and turnover of serotonin will increase during a prolonged fasting period [20].

Who should not do long fasting? ›

Skipping meals and severely limiting calories can be dangerous for people with certain conditions, such as diabetes. People who take medications for blood pressure or heart disease also may be more prone to electrolyte abnormalities from fasting.

Why is 16 hours the magic number for fasting? ›

Insulin sensitivity helps in fat loss and protects the body from diseases such as diabetes, high blood pressure, and heart diseases. During the 16-hour fast, your body undergoes a process known as autophagy, a process where the body destroys old or damaged cells in the body.

How long intermittent fasting is safe? ›

As it stands now, the current evidence supports intermittent fasting as a weight loss method for up to 12 months. Some people are also probably not right for intermittent fasting. It's not recommended for anyone who is pregnant, has diabetes or is under 18.

Why am I not losing weight on 16 8 fasting? ›

Your Fasting Window is Too Short

This means it won't burn stored fat, and there won't be significant weight loss. As one study concluded, a fasting window of 18-19 hours is more effective in terms of weight loss than those between 12-17 hours [11].

Is it OK to intermittent fast everyday? ›

Longer periods without food, such as 24-, 36-, 48- and 72-hour fasting periods, are not necessarily better for you, and may be dangerous. Going too long without eating might actually encourage your body to start storing more fat in response to starvation.

Is fasting hard on your heart? ›

One word of caution, though: Fasting can lead to an electrolyte imbalance. This can make the heart unstable and prone to arrhythmias. “So whenever we prescribe certain diets, including a very low calorie diet and protein-sparing modified fast diet, these require medical supervision.

What happens to your body when you fast for 16 hours? ›

If you don't eat for 10–16 hours, your body will go to its fat stores for energy, and fatty acids called ketones will be released into the bloodstream. This has been shown to protect memory and learning functionality, says Mattson, as well as slow disease processes in the brain.

What is the best fasting for weight loss? ›

The 16/8 intermittent fasting plan is one of the most popular styles of fasting for weight loss. The plan restricts food consumption and calorie-containing beverages to a set window of 8 hours per day. It requires abstaining from food for the remaining 16 hours of the day.

Does 12 hours count as intermittent fasting? ›

Twelve-hour fasting is a form of intermittent fasting (IF) also known as 12:12 that many people try for weight loss and health benefits, including improved digestion, increased energy, and improved sleep.

What is the longest you should fast? ›

There is no set time that water fasting should last for, but medical advice generally suggests anywhere from 24 hours to 3 days as the maximum time to go without food. Throughout history, people have undertaken fasts for spiritual or religious reasons.

Is it safe to do a 72 hour fast every week? ›

Water fast (24–72 hours)

Most people drink two to three liters of water per day during a water fast. The water fast lasts for 24–72 hours. You should not water fast for longer than this without medical supervision because of health risks.

What is the optimal prolonged fasting time? ›

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!

What not to do after a long fast? ›

Breaking your fast with foods that are especially high in fat, sugar, or even fiber can be difficult for your body to digest, leading to bloating and discomfort. Foods and drinks that can be even more shocking to your system after a fast include those like a greasy cheeseburger, slice of cake, or soda.

Does lemon water break a fast? ›

The truth about lemon water and intermittent fasting

Long story short - the answer to the question “Does lemon water break a fast?” is no, lemon water does not break a fast. Lemon water contains almost no calories and zero sugars, it doesn't raise insulin levels, which means it will not break your fast (1).

What are the things not to do when fasting? ›

What Not to Do When Fasting Intermittently

Why 16 hour fast is better than 12? ›

Benefits of 12 hour Fasting. Benefits of 16 Hour Fasting. Benefits Comparison Chart.
Benefits Comparison Chart.
Benefit12 hour fast16 hour fast
Improved digestionyesyes
Improved mental focusyesyes
Aids with weight lossyesyes
Autophagy (cellular repair)noyes
3 more rows
Oct 12, 2022

Does coffee break your fast? ›

The bottom line. You can drink moderate amounts of black coffee during fasting periods, as it contains very few calories and is unlikely to break your fast. In fact, coffee may enhance the benefits of intermittent fasting, which include reduced inflammation and improved brain function.

What is the 8 hour eating window with a 16 hour fast? ›

The practice of 16/8 intermittent fasting involves limiting your intake of foods and calorie-containing beverages to a set window of 8 hours per day. You abstain from food for the remaining 16 hours, though you're still allowed to drink water and other no-calorie beverages, like plain coffee or tea.

Why am I gaining weight on intermittent fasting? ›


You'll be so hungry, you may start eating and not stop. As well, the body stores food to protect itself. Your body will sense the need to stock up on reserves and may store those extra pounds as fat instead of lean muscle.

Can you live longer with intermittent fasting? ›

Yes, you can do intermittent fasting for longevity every day. Research suggests that daily intermittent-fasting practices and longer-duration periodic fasting are safe strategies to improve your healthspan and lifespan.

Will I lose weight if I fast 16 hours everyday? ›

According to some researchers, fasting for 10–16 hours can cause the body to turn its fat stores into energy, which releases ketones into the bloodstream. This should encourage weight loss. This type of intermittent fasting plan may be a good option for beginners.

How many pounds can you lose with 16 8 fasting? ›

Research shows that people who try intermittent fasting typically lose about 3 to 8 percent of their body weight— which, for a 200-pound person, is about 6 to 16 pounds.

How quickly will I see results with 16 8 intermittent fasting? ›

Additionally, it will take your body a while to get used to this new eating schedule. So don't expect results right away. You may need to wait between 2 and 4 weeks to see or feel any results.

How many days a week should I do intermittent fasting? ›

Eat a normal diet five days a week and fast two days per week. Eat normally but only within an eight-hour window each day. For example, skip breakfast, but eat lunch around 11 a.m. and dinner by 7 p.m.

Is it better to intermittent fast everyday or every other day? ›

“Generally evidence appears to suggest that alternate day fasting is as beneficial as more traditional methods of weight loss, such as creating a daily calorie deficit. Interestingly, of all intermittent fasting regimes, such as time-restricted eating, alternate day fasting appears to have a slight advantage.”

Does intermittent fasting slow metabolism? ›

Metabolic switching through intermittent fasting results in improved metabolism, increased health span, and increased longevity through multiple processes [16,30].

Why does LDL go up when fasting? ›

Intermittent fasting affects cholesterol levels due to the change in metabolism from glucose to ketones. When this occurs, the body begins using lipids rather than storing them. Lipids move out of the cells, travel in the bloodstream, and go into the liver to be made into ketones.

Is fasting good for inflammation? ›

Researchers say intermittent fasting can reduce inflammation in the body. Inflammation is one way the body fights infection, but if there is too much inflammation that can lead to various diseases.

Is fasting good for cholesterol? ›

Cardiovascular disease: Intermittent fasting promotes weight loss and appears to improve blood pressure, cholesterol and other indicators of cardiovascular health, says Mattson.

How do you know when autophagy starts? ›

A key sign of autophagy is reduced appetite. It's likely due to changed levels of hormones like glucagon and insulin. Specifically, levels of glucagon tend to increase during autophagy. Glucagon helps manage your blood sugar levels and has been shown to suppress appetite ( 6 , 10 , 11 ).

How much should I eat after 16 hour fast? ›

It's pretty simple. Basically, during your 16-hour "fasting" period, you limit yourself to black tea, coffee, or diet soda. Then, for the next eight hours, you can eat whatever you want (no calorie counting needed). That's it.

What are the 5 stages of fasting? ›

The 5 Stages of Intermittent Fasting with the LIFE Fasting Tracker app: 1) Ketosis and heavy ketosis, 2) Autophagy, 3) Growth hormone, 4) Insulin reduction, 5) Immune cell rejuvenation!

What is the most famous intermittent fasting? ›

Many people find the 16/8 method to be the simplest, most sustainable and easiest to stick to. It's also the most popular. There are several different ways to do intermittent fasting.

What is the most popular intermittent fasting method? ›

Some of the most common ways are:
  • The 16/8 method: Only eating between 10 a.m. and 6 p.m.
  • The 14/10 method: Only eating between 9 a.m. and 7 p.m.
Mar 3, 2022

How can I drop 20 pounds fast? ›

Here are the 10 best ways to quickly and safely drop 20 pounds.
  1. Count calories. ...
  2. Drink more water. ...
  3. Increase your protein intake. ...
  4. Reduce your refined carb consumption. ...
  5. Start lifting weights. ...
  6. Eat more fiber. ...
  7. Follow a sleep schedule. ...
  8. Set reasonable goals and stay accountable.
Oct 19, 2022

Does fasting help with belly fat? ›

Nutrition experts say that switching back and forth between fasting and eating again can bring many health benefits, including weight loss, reducing belly fat. However, to lose belly fat and get the nutrients you need, you should eat healthy, limiting sugary desserts and processed foods.

How long do you have to fast before you start burning fat? ›

Fat burning typically begins after approximately 12 hours of fasting and escalates between 16 and 24 hours of fasting.

What is the 4 4 12 rule? ›

The 4-4-12 method has a shorter fasting window—12 hours—but eliminates snacking between meals. The goal here is to fast for at least four hours between breakfast and lunch, four hours between lunch and dinner, and at least 12 hours between dinner and breakfast the next day.

Is it better to fast 12 14 or 16 hours? ›

She says, “Not a minute more than 14 hours of fasting is recommended for women, as it can delay periods and increase the risk for anxiety and depression.” For men, she recommends that they can fast up to 16 hours.

Is 12 noon to 8pm intermittent fasting? ›

One of the most common ways to do this is by skipping breakfast and only eating from midday-8pm, so you are fasting for 16 hours per day (between 8pm and midday the next day). Other variations of the diet involve 6-hour eating periods or shorter.

Is it safe to fast for 14 days? ›

The results from 1422 subjects showed for the first time that Buchinger periodic fasting lasting from 4 to 21 days is safe and well tolerated.

Is it OK to fast for 7 days? ›

Fasting for a week may result in adverse health and metabolic changes such as dehydration, a loss of lean muscle mass, hyperuricemia, hyponatremia, protein-sparing, sodium, and potassium-sparing, decreased serum calcium and magnesium levels, and acidic urine.

What happens after 40 hours of fasting? ›

When you fast for 36 hours or more, you enter into the realm of extended fasting. The benefits of this practice include weight loss, lower blood sugar, ketosis, and enhanced autophagy. To help your body adapt to using body fat for energy, start with shorter fasts and work your way up to longer fasts.

What happens when you fast for 40 days? ›

The 40 Days of Prayer is inspired by Matthew 4:1-11, where Jesus fasted for forty days and forty nights and cited Scripture to resist the temptations of Satan. The forty days serve as a time of fasting and prayer to draw closer to God and strengthen your prayer life.

Can I safely fast for 30 days? ›

Because of all the risks, health professionals don't recommend fasting for a period longer than 1-3 days. Remember that it is dangerous to stick to this dietary pattern for a longer period without medical supervision.

What is the safe length of fasting? ›

To keep yourself safe, especially if you are new to fasting, consider limiting your fast periods to 24 hours or fewer and keeping a snack on hand in case you start to feel faint or ill. If you do become ill or are concerned about your health, make sure you stop fasting straight away.

What's the hardest day of fasting? ›

No matter how many times I fast, day one is never easy and day two is always the most difficult (ghrelin, the hunger hormone, is said to peak on day 2 of a fast).

What happens if you don't eat for 4 days but drink water? ›

Although water fasting may have some health benefits, it comes with many risks and dangers. For example, water fasting could make you prone to muscle loss, dehydration, blood pressure changes, and a variety of other health conditions.

How long does it take to lose 20 pounds fast? ›

5:2 Intermittent Fasting

The research revealed that it takes four to six months to lose 20 pounds with no change in diet. Most intermittent fasting studies surrounding the 5:2 diet protocol or minimal calorie fasting days report significant weight loss.

Does coffee break a fast? ›

The bottom line. You can drink moderate amounts of black coffee during fasting periods, as it contains very few calories and is unlikely to break your fast. In fact, coffee may enhance the benefits of intermittent fasting, which include reduced inflammation and improved brain function.

What's better a 24 or 48 hour fast? ›

Fasts of 24-48 hours may be more beneficial for blood sugar control than shorter fasts, and they increase insulin sensitivity so that your body works more efficiently (1).

How long did Jesus fast for? ›

At that time Jesus was led by the Spirit into the desert to be tempted by the devil. He fasted for forty days and forty nights and afterwards was hungry. The tempter approached and said to him, “If you are the Son of God, command that these stones become loaves of bread.

Why you shouldn't fast for 48 hours? ›

The main drawback of 48-hour fasting is severe hunger, although many people claim that this feeling is temporary. In one study in 768 people fasting for at least 48 hours, 72% of participants experienced side effects, including hunger, fatigue, insomnia, and dizziness.

Why do Greeks fast for 40 days? ›

According to the Orthodox Church tradition, physical fasting, i.e. refraining from certain kinds of food for a certain period of time, is a necessary tool in the believer's effort to limit sin – as far as possible and with the grace of God.


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