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5-Day Fast: Everything You Need To Know About Prolonged Fasting

Fasting for five days may seem like a lot of work, but it is thought to come with many benefits. Some versions of the five-day fast allow you to drink only water, whereas others require you to eat at a reduced-calorie rate instead.

The five-day fast is thought to come with many health benefits and can help you to lose weight and body fat.

What Is a 5-Day Fast Diet?

The five-day fast is, as it sounds, a form of prolonged fasting that lasts for five days. There are a few different versions of the fast, including a fasting-mimicking diet where you eat around 800 calories per day for five days, or water fast, where you only drink water for five days.

The human body can survive only a few days without food and water, but should your water intake be good enough, you can survive a relatively long time without food. Studies vary a lot on how long the human body will survive without food, and your weight and BMI can affect it.

It is thought that women can survive starvation longer than men can, and weight loss will vary depending on the weight you had and the lean muscle mass and body fat you carried, to begin with.

During a prolonged fast, your body enters what is known as starvation mode. This starvation state starts around 48 hours into a fast.

A starvation state is characterized by a decrease in insulin levels (further below that of an average fasting period), and your blood ketone level will continue to rise. These ketone bodies help to fuel the brain.

The kidneys also begin to generate sugar through a process known as gluconeogenesis, and this sugar serves as the main fuel source for the brain. The breakdown of BCAAs in essential amino acids is halted to allow the body to conserve muscle mass.

Before trying long-term fasting, there has to be a period of preparing your body, and it should only be performed under medical supervision.

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Benefits of a 5-Day Fast

Though this fasting method can be dangerous, it is thought to come with weight loss and other benefits. We’ve taken a look at a few of these below.

Weight loss

The intermittent fasting method promotes weight loss, and the five-day fast is just the same. Fasting can help with weight loss in a number of ways.

To begin with, a prolonged state of fasting reduces your caloric intake. If you consume fewer calories than you burn, your body begins to look for other fuel sources to burn for energy.

The body also enters the metabolic state of ketosis around 10–12 hours into fasting. Ketosis is the state in which the body burns stored fat for fuel rather than blood sugar. It can be achieved both by fasting or by eating a low-carbohydrate diet, like the keto diet.

Lastly, fasting may help boost your metabolism which also supports weight loss.

Though prolonged fasting has many weight loss benefits, it can be dangerous. Always seek professional medical advice before trying any kind of prolonged fast to ensure it is safe for you.

Lower insulin

Fasting is well-known for its ability to help manage type 2 diabetes. Long fasts, like the five-day fast, may be able to help you lower your insulin levels, increase your insulin sensitivity, and manage blood glucose levels.

There is evidence to suggest that a longer fast could be beneficial for your insulin levels, but most research has been carried out on shorter fasts, like the 36-hour fast.

Leptin sensitivity

Leptin is a hormone in the body that plays a role in decreasing weight gain and controlling hunger. Long fasts are thought to boost leptin sensitivity, helping you to lose weight. Indeed, leptin resistance may actually play a role in weight gain.

There is evidence to suggest that long periods without food can help to increase your leptin sensitivity and weight loss. However, none of these studies have been carried out on those undertaking a five-day fast.

Lower blood pressure

Many studies have shown intermittent fasting can lower your blood pressure and other markers of poor cardiovascular health.

Some research indicates that medically-controlled periods of fasting may be used to treat those with hypertension.

Dangers To Be Aware of During Prolonged Fasting

Many diets that come with weight loss benefits can also come with dangers. The five-day fast is the same.

Fasting for long periods may have potential benefits for your health but it can also be an incredibly dangerous way of losing weight. Below, we have summarized some of the dangers you risk when trying such a long fast.

Loss of muscle mass

Due to a lack of the macronutrients your body needs, long fasts can actually lead to a loss of muscles. If you’re not getting enough protein, you are in danger of losing muscle.

Lack of regular exercise may also result in a loss of muscle during long fasts, so working out is recommended if it is done safely.

Risk of concentration and memory problems

Though many IF diets boast benefits including improved mental clarity, not eating food for such a long time can actually cause you to lose focus and concentration and have memory problems.

Malnutrition can cause a loss of focus and an inability to concentrate.


Even while monitoring your fluid intake, you’re at risk of becoming dehydrated when trying out long fasts because you actually get a lot of your daily water intake from food. Drinking water is incredibly important when trying any fasting method.

Symptoms of dehydration include:

  • Feeling thirsty
  • Dark yellow or strong-smelling urine
  • Dizziness and lightheadedness
  • Dry mouth and lips
  • Tiredness
  • Urinating fewer than four times a day

Other medical complications

A five-day fast essentially puts your body into a state of starvation. This comes with medical complications which can include:

  • Problems with muscle and bone development, bone weakness, feeling tired
  • Fertility problems
  • Lowered or complete loss of sex drive
  • Cardiovascular problems, including irregular heartbeat, low blood pressure, heart failure, and cardiovascular disease
  • Problems in the brain, including seizures and loss of concentration
  • Kidney problems
  • Bowel problems
  • Weakened immune response
  • Anemia

These medical complications need to be taken seriously. The fat-burning ability of long fasts needs to be weighed against the chances of developing any of these conditions due to long periods without food.

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Preparation for a 5-Day Fast

If you’re planning on undertaking a five-day fast, we recommend speaking with a medical professional first to ensure that you are doing it safely.

Below are some of the steps you should take before trying this fasting method.

Decide on your goal

When it comes to weight loss, it is important to set realistic and healthy goals. Healthy weight loss is defined as 1–2 pounds per week and anything more than this can cause medical complications and negative side effects.

Set realistic and achievable goals to ensure that you have the motivation to keep going.

Reduce food gradually

It is not recommended that you throw yourself into a long fast without any preparation. This means you will need to begin gradually reducing your food intake. You could even try less challenging fasting methods to get your body prepared for long periods without eating.

Try out fasting methods like the 12-hour fast, increasing to the 16/8 method, and eventually to the 24-hour fast in preparation. You should always listen to your body and only move on to longer fasts when you are ready to do so.

Gradually reducing your caloric intake should also help to ease any discomfort and food cravings you may experience on longer fasts.

Track your progress

Using a fasting method of weight loss can be made easier by tracking your progress. If you can see how well you’re doing, you’ll be more motivated to stay on track and keep fasting.

Using an app, like the DoFasting app, is a great way to do this. The app provides space for you to track your food intake to ensure you’re getting all of the nutrients you need and will remind you when to start your fast.

You can also find workouts, recipes, and a place to see how much weight you have lost since you started your fasting diet.

Join a fasting community for motivation

Staying connected with other fasters can also help to keep you motivated. Places like DoFasting’s Facebook and Reddit groups can help you to stay motivated, provide a place for tips, and be an easy way to get help when you’re struggling.

What You Can Expect During the 5-Day Fast

As the five-day fasting method is a challenging fasting method, it is likely to take its toll on your body. Below, we’ve summarized how you will experience the five-day fast.

Hunger and cravings

Hunger pangs and cravings are very common with a method like the fasting-mimicking diet you follow on the five-day fast. These feelings may even become overwhelming at times.

You can make them easier by taking an appetite suppressant, like the DoFasting Appetite Suppressant, to make these cravings easier. An appetite suppressant will help to make you feel fuller for longer and can make your brain think you’re less hungry.

Rising ketone levels

Fasting induces a state in the body called ketosis. This essentially means that the number of ketones in your blood has risen and your body is now burning fat for fuel.

A few ways you will know you’re in ketosis include:

  • Fatigue
  • Nausea
  • Feeling cold
  • Alertness and bursts of energy

The transition to ketosis can sometimes be uncomfortable, but there are a few things you can do to ease any negative side effects. You should ensure that you drink enough water, avoid excessive exercise, and supplement with fiber.

A need for electrolytes

Electrolyte deficiency is a danger of such a long fast. Many bodily functions can be impaired if you do not have the required electrolytes, meaning functions like blood clotting and muscle contractions can be affected.

To replenish your electrolytes, consider finding electrolyte mixes or mineral supplements that are fasting-friendly to ensure you don’t experience any negative side effects.

Tips for Sticking With a 5-Day Fast Safely

A five-day fast can be incredibly challenging. While it is best to only undertake such a long fast without medical supervision, you may want to try it at home. Remember to try and minimize any of the risk factors and mentally prepare yourself before starting.

Below are a few further tips we have for a safe five-day fast.

Focus on staying in ketosis

Many of the benefits of fasting come from the prolonged state of ketosis it induces. Ketosis is a fantastic way to lose weight, minimize inflammation and the risks of age-related medical conditions, and boost your insulin sensitivity.

For this reason, the goal of fasting should always be to maintain ketosis, but you don’t need to not eat to remain in this state. Ketosis can also be achieved by eating a very low-carb, high-fat diet, like the keto diet.

Eating this way will break your fast but will help you to remain in ketosis. If fasting becomes too difficult, eating low-carb, high-fat foods is a perfectly fine way to remain in ketosis.

Keep yourself busy

Keeping yourself busy can make your fasting periods much easier. If you’re not thinking about being hungry, you’re much less likely to feel the cravings and pangs that you get during long fasting periods.

Pay attention to your surroundings

Sometimes the wrong environment can ruin a fast. If you’re spending time in places where unhealthy foods are promoted, you might find it difficult to stick to it. Try to avoid places that foster bad habits.

Should You Start 5-Day Fast Now?

Fasting is a challenging dieting method but is generally safe for most people. Some people who should not fast include:

  • People who are pregnant, breastfeeding, or trying to conceive
  • People with type 1 diabetes
  • People with sleep problems
  • Anyone who has a history of eating disorders
  • Anyone with digestion issues
  • Those trying to build muscles or engaging in intensive training
  • People on medications where food is a requirement
  • Anyone with a weak immune system

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The five-day fast is an incredibly challenging fasting method that allows you to reduce your body weight and burn fat. Long fasts like this shouldn’t be carried out without medical supervision.


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This is an evidence-based article that includes scientific citations. DoFasting’s professional writers and editors prepared the content, which a team of medical experts verified to be accurate.

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