5 Day Fast: what happens in your body?

Many people see fasting for 5 days mainly as eating less to lose weight. But a structured 5-day fasting protocol goes further than ordinary calorie restriction. It can temporarily put you in a state that supports various health-related mechanisms linked to longevity, endurance performance, gut health and more. Want to know exactly what happens in your body? Then you've come to the right place. 

What is 5 days of fasting?

5 days of fasting is a temporary period where your body gets much less energy than normal. Because of this, it has to adapt to a different availability of fuel.

With classic fasting, this often means no food. With a fasting mimicking approach, such as 5 Day Fast, the body still gets limited nutrition, but in a way that mimics certain effects of fasting.

The main changes are usually:

• lower energy intake

• lower digestive load

• fewer available carbohydrates

• gradual use of glycogen stores

• more use of fat as an energy source

• higher production of ketones as the days progress

The duration of 5 days is important because your body doesn't switch over completely overnight. Day 1 usually feels different from day 4 or 5. In the first phase, your body mainly uses stored carbohydrates and you often lose water weight. Later on, the energy use shifts more towards fat burning and ketosis.

That's why fasting for 5 days is different from skipping a single meal. It gives the body time to go through different metabolic phases.

How does 5 Day Fast work?

5 Day Fast is a structured fasting mimicking programme.

Instead of completely leaving out food, it combines controlled calorie restriction with targeted nutrients and specific supplements. The goal is to mimic certain physiological effects of fasting, without water fasting completely.

The idea behind 5 Day Fast is that you temporarily take in less food. Your body gets less energy than normal, but within a clear structure. Because of this, 5 Day Fast is more practical and easier to stick to for many people than complete fasting.

5 Day Fast consists of:

• controlled calorie restriction

• targeted nutritional intake

• supplements

• daily structure

• guidance just before, during and directly after the 5 days

That structure is important. Without guidance, 5 days of fasting can quickly turn into an experimental approach. 5 Day Fast is scientifically backed and developed to make the process of fasting more controlled, practical and applicable within a broader approach to health.

💡 Caffeine is not allowed during the programme. By not using caffeine for five days, you can better perceive your natural energy levels, fatigue signals and sleep rhythm. Your cortisol levels (stress hormone) also decrease.  For regular coffee drinkers, this can cause headaches, fatigue or less focus in the first few days.

What are the benefits of 5 Day Fast?

The benefits of 5 days of fasting are mainly related to metabolic adaptation. The body temporarily takes in less energy (carbohydrates, fats and proteins), forcing it to deal differently with available fuels.

Possible benefits include support of:

• fat burning

• ketosis

• insulin sensitivity

• metabolic flexibility

• lower digestive load

• cellular repair processes (autophagy)

• gut health

• sleep quality

• endurance performance

Research into fasting and fasting mimicking diets shows that temporary energy and nutrient restriction can influence mechanisms involved in metabolism, healthy ageing and cellular maintenance (1,2,3).

In terms of short-term effects, 5 Day Fast isn't just about losing weight quickly. The goal is mainly qualitatively better weight loss: more use of fat as an energy source, less reliance on quick carbohydrates and better support for muscle preservation.

During the 5 Day Fast protocol, the body gradually uses more fats and ketones for energy. Because of this, the need to use amino acids from muscle tissue as fuel can decrease, allowing you to maintain your muscle mass while still fasting.

Muscle preservation is not automatically guaranteed. That depends on how well you carry out the 5-day programme and limit overly intense workouts. During the rebuilding phase after the 5 days, you'll also want to build up training intensity slowly and ensure sufficient protein intake, sleep and recovery.

For athletes, the potential benefits are mainly relevant after the programme, when normal nutrition, glycogen stores and training have been rebuilt. Then 5 Day Fast can contribute to better fat burning, more efficient energy use and a lower heart rate during the same effort. Although these effects differ per person, we almost always see them in athletes who do 5 Day Fast.

5 days of fasting or a 5-day diet: what's the difference?

Many people look for a "5-day diet" because they want to lose weight quickly. But fasting for 5 days is not the same as a classic crash diet.

A crash diet is usually just about quick weight loss. The goal is to eat as little as possible for a short period. The problem is that this often leads to hunger, poorer recovery, a higher risk of muscle loss and a quick return to old habits.

A structured approach like 5 Day Fast has a different goal. It's not just about eating fewer calories and losing weight. It's about temporarily creating a different metabolic environment, in which fat burning, ketosis, metabolic flexibility, stem cell activation, autophagy, clearing plaque and other recovery processes can be supported.

During the 5 days, the body can shift towards more fat use, lower digestive load and increased ketone production, while the approach is aimed at maintaining muscle mass (4).

What happens during 5 days of fasting?

A 5-day fasting period is not a static state. Your body changes gradually.

Day 1: glycogen and water loss

On the first day, the body still largely uses stored carbohydrates as an energy source.

We call these stored carbohydrates glycogen. Glycogen is stored along with water. This is why your body weight can drop quickly at the start of a fasting protocol.

That initial drop often consists partly of water and glycogen, not fat.

Common experiences on day 1 are:

• disruption of routine

• lower energy

• altered mental focus

• cravings

• headache or fatigue from caffeine withdrawal

During 5 Day Fast, caffeine is not allowed. Symptoms like headaches, fatigue, irritability or less focus can arise when caffeine is suddenly left out.

That doesn't automatically mean the fasting protocol is "too heavy". Part of the discomfort can come from the temporary caffeine break.

Day 2–3: fat burning and ketosis

As glycogen stores decrease, the body starts to use more fat as an energy source.

Ketone production can also increase. Ketones are alternative energy molecules that are created when the body breaks down fat.

This transition doesn't feel the same for everyone. Some people feel more stable or mentally clear. Others experience less energy, feeling cold, hunger or irritability. You might also feel that your body is 'at work' in certain places. This often feels uncomfortable and sometimes even painful. It's important to realise that this is positive. Your body is busy with repair and maintenance, and that happens automatically in the places where it's needed most.

This is all normal. The body is adapting to a different fuel use.

In this phase, it's important not to expect top performance during workouts. The goal is metabolic adaptation, not maximum output.

Day 4–5: metabolic adaptation

On days 4 and 5, the body can be better adapted to using fats and ketones.

The digestive load remains lower, carbohydrates are limitedly available and the body is in a different metabolic state than normal.

Some people feel more stable in this phase. Others remain fatigued. The experience depends on sleep, stress, activity, previous experience with fasting and individual metabolism.

That's why realistic expectations are important. Fasting for 5 days doesn't feel the same for everyone.

After the 5 days: why rebuilding is important

The phase after the programme is essential.

After 5 days of fasting or fasting mimicking, the body must rebuild. Glycogen stores are replenished. Part of the 'water weight' can return. Protein intake becomes more important again. Workouts must also be built up gradually.

Doing 5 Day Fast periodically contributes to long-term progress anyway. In the period immediately after the 5 days, you can easily adjust bad habits and try to engrain them as 'standard behaviour'.

After the 5 days, it's best to focus on:

• gradual reintroduction of food

• sufficient protein

• nutritious meals

• quality carbohydrates

• healthy fats

• hydration

• movement

• sleep

• avoiding an immediate return to old habits

5 Day Fast vs water fasting: what's the difference?

5 Day Fast is not the same as water fasting.

With water fasting, food is completely left out. 5 Day Fast uses limited nutrition to mimic the effects of fasting, while the programme offers structure and targeted daily intake alongside its scientific backing.

This doesn't mean that 5 Day Fast is risk-free or suitable for everyone. But for many people, it's more structured and easier to follow than complete food deprivation.

The main difference is control. Water fasting is more extreme. 5 Day Fast was developed as a guided, scientifically backed and practical approach.

Losing weight in 5 days: what can you realistically expect?

Many people lose weight during 5 days of fasting. But it's important to understand what that weight loss consists of.

The initial drop on the scales is often partly due to:

• water loss

• glycogen loss

• less food volume in the gut

• fat loss

Fat burning usually increases as the protocol progresses. But long-term fat loss depends on the overall energy balance and what happens after the 5 days.

So fasting for 5 days can support weight loss, but it shouldn't be presented as a quick fix. Instead, it's a repeatable programme that contributes to a fat percentage that is healthy for your body in the long term.

The better question is not just: "How much can I lose in 5 days?" but also: "How much of that result can I keep in the long term?"

💡 Maintaining muscle mass is important to hold onto your results better. Muscle mass plays a role in your resting metabolic rate: how much energy your body burns at rest, and it is also incredibly important for healthy ageing.

Do you lose fat or mostly water during 5 days of fasting?

At the start, a large part of the weight loss is often due to water and glycogen.

When carbohydrate stores decrease, the water stored with glycogen is also released. This can cause the scales to drop quickly.

That doesn't mean the result is "fake". It just means that early weight loss is not the same as pure fat loss.

As the body starts to use more fat, fat loss can become a bigger part of the result. The exact amount depends on body size, physical activity, metabolism, energy intake and the phase after the programme.

How does 5 days of fasting support fat burning?

Fasting for 5 days can support fat burning through various mechanisms.

Think of:

• lower energy intake

• fewer available carbohydrates

• lower glycogen stores

• more use of fat as fuel

• higher ketone production

• improvement of metabolic flexibility

Metabolic flexibility means that your body can switch better between different fuels, like carbohydrates and fats.

That is relevant for body composition, metabolic health and endurance performance. At the same time, it doesn't mean that fasting for 5 days once automatically guarantees long-term fat loss. Your overall lifestyle and periodically repeating the 5 days of fasting are the key to longer-term health improvement.

How do you limit muscle loss during a 5 Day Fast?

A structured fasting mimicking protocol contributes much better to muscle preservation than complete fasting because it still contains controlled nutrition and supplements. This means your body doesn't have to break down muscle tissue out of necessity to use as an energy source.

Ketones can also help to reduce the reliance on amino acids (muscles) as an energy source. Still, the phase after the programme remains important.

To support muscle preservation, the focus is on:

• sufficient protein after the protocol

• gradual return to intensive training

• strength training when appropriate

• sleep and recovery

• avoiding repeated aggressive food restriction

For people focusing on body composition, the goal isn't simply to lose weight. The goal is mainly to lose fat while maintaining muscle mass.

5 Day Fast and gut health: what is the potential link?

A 5-day fasting mimicking protocol can also affect your digestive system.

When your food intake changes, what is available to the bacteria in your gut also changes. A lower digestive load can affect how your gut feels during the protocol. Your digestive system gets a rest and much more time for repair work. Changes in eating patterns can also affect the gut-brain axis, the connection between digestion, stress, mood and sleep.

Potential links include:

• lower digestive load and room for repair work

• change in available nutrition for gut bacteria

• influence on the gut-brain axis

• indirect effects on sleep and recovery

• more awareness around eating habits after the protocol

The effects differ per person. Anyone with active digestive issues very often notices an improvement during the 5 days. If you are under treatment by a specialist, you should always be careful and ask for advice before starting.

5 days of fasting and healthy ageing

Fasting is often discussed in relation to longevity and healthy ageing.

Key mechanisms include autophagy, mTOR, insulin sensitivity and cellular maintenance processes.

Autophagy is a cellular cleanup process. mTOR is a nutrient-sensitive signaling pathway involved in growth and repair. During periods of energy restriction, the body can temporarily focus less on growth and more on maintenance processes.

That doesn't mean more fasting is always better. Healthy ageing isn't about constant restriction. It's about the right balance between temporary stress and good rebuilding. If you're in good health, fasting for 5 days two to four times a year is perfect for you. Is your health more out of balance? You can then safely do 5 Day Fast up to once a month for maximum effect.

5 Day Fast for athletes: when is it interesting?

For athletes, 5 Day Fast can be particularly interesting when the goal is metabolic flexibility, fat oxidation or body composition.

This applies to most team sports and, of course, especially to endurance athletes and hybrid athletes.

Potentially relevant goals include:

• better fat burning

• support of body composition

• more awareness around lifestyle

• sports performance improvement

• support of long-term health

• better recovery after the rebuilding phase

Timing is crucial here.

You don't end a 5-day fasting protocol right before a race. During the protocol, energy availability is lower, training quality can drop and the need for recovery is higher. If you're experienced with 5 Day Fast, you can plan it so that you finish 9-15 days before your race or competition. If you're still inexperienced, plan it a bit further away first.

The sports performance benefits lie mainly after the protocol, when normal nutrition, training and recovery have been rebuilt.

💡 During 5 Day Fast, you can keep doing low-intensity workouts. In fact, they are essential to support the transition into ketosis.

Sportvasten or 5 Day Fast: what's the difference?

Sportvasten is a well-known concept in the Netherlands. It is often linked to fasting, movement and metabolic switching.

5 Day Fast is not the same.

Both approaches deal with fasting and metabolism, but they differ in structure, nutritional approach, positioning and context of use.

Sportvasten is often presented as a method combining food restriction and movement. 5 Day Fast is a fasting mimicking programme with more than 30 years of scientific backing that revolves around controlled nutrition, supplements and guidance.

5 Day Fast experiences: what can you expect?

Experiences with 5 Day Fast differ per person.

Some participants mainly notice weight loss and more structure in their eating behaviour. Others notice changes in energy, digestion, focus or sleep.

Common experiences are:

• hunger and cravings in the first few days, later feeling very stable

• lower energy in the first few days, later a whole new energy

• changes in digestion

• uncomfortable feeling in the place where your body is 'at work'

• less stress

• better sleep

• weight loss

• feeling lighter

• more awareness around eating habits

• changing bad habits afterwards

• temporary adjustment of training intensity and volume during the 5 days

• sports performance improvement afterwards

Some people feel good throughout the 5 days. Others find the first few days particularly hard.

The exact results depend on your starting point, preparation and what you do during and after the 5 days.

Because caffeine is not allowed during 5 Day Fast, regular coffee drinkers may temporarily experience headaches, fatigue, irritability or less focus. These complaints usually improve after the first few days.

Who is 5 days of fasting suitable for?

A structured 5-day fasting protocol can be relevant for people who want to support their metabolic health, sports performance, body composition or long-term lifestyle and health.

It can be suitable for:

• people with an interest in metabolic health

• people who want to reset their eating habits in a structured way

• people who want to support fat loss

• people focused on healthy ageing

• experienced athletes wanting to improve their performance strategy

• people who prefer structure over complete fasting

It's not about "gritting your teeth" for 5 days. It's about using a structured protocol with a clear goal.

When should you be careful with 5 days of fasting?

5 days of fasting is not suitable for everyone.

Don't start if you are:

• pregnant

• breastfeeding

• suffering from severe fatigue

• under high stress

• actively ill

• dealing with a short-term physical injury

Be careful if you have:

• specific medication use

• a history of eating disorders

• a temporary increased need for recovery (for whatever reason)

If you use medication or have a medical condition, always consult your doctor or specialist first.

A fasting protocol is a stressor. For the right person, at the right time, that stressor can be useful. For the wrong person, or at the wrong time, it can actually have the opposite effect.

What do you do after 5 Day Fast to maintain your results?

What you do after 5 Day Fast is an important part of the process.

If you immediately return to bad habits, the effects won't disappear overnight. But the long-term result will be limited.

Some of the weight can return when glycogen and water stores are replenished. That is normal.

To hold onto your results better, you'd best focus on:

• gradual rebuilding of nutrition

• sufficient protein

• nutritious meals

• quality carbohydrates

• healthy fats

• regular exercise

• sleep

• stress management

• changing 1 or 2 old habits that are low-hanging fruit for you

Frequently asked questions about 5 days of fasting

Is 5 days of fasting healthy?

A programme like 5 Day Fast was developed to influence various health-related mechanisms, including fat burning, ketosis, metabolic flexibility, insulin sensitivity and cellular repair processes. These mechanisms can contribute to benefits like weight loss, gut health, endurance performance and more.

Can you lose weight in 5 days?

Yes, many people notice weight loss during 5 days of fasting. The initial drop often consists partly of water and glycogen. Fat loss depends on energy expenditure, the individual and what happens after the protocol.

Do you gain weight again after 5 days of fasting?

Some of the weight can return when glycogen and water stores are replenished. That is normal. Long-term results depend on nutrition, exercise, sleep and habits after the 5 days.

Is 5 Day Fast the same as water fasting?

No. With water fasting, food is completely left out. 5 Day Fast is scientifically backed, containing controlled nutrition, supplements and structure, with the goal of mimicking certain effects of fasting.

What's the difference between 5 Day Fast and sportvasten?

Both deal with fasting-related strategies, but they differ in scientific backing, structure, duration, nutritional approach and goal. 5 Day Fast is a fasting mimicking programme, while sportvasten is usually positioned around fasting combined with exercise.

Can you work out during 5 Day Fast?

Low-intensity workouts are essential. Heavy/intensive training is usually not wise during the protocol. The available energy is lower, so your duration and intensity will be a bit lower than you're normally used to.

What do you eat during 5 Day Fast?

5 Day Fast contains controlled nutrition and supplements during the 5-day protocol. The exact structure is part of the programme. The goal is to mimic certain effects of fasting without completely leaving out food.

How often can you do 5 Day Fast?

The frequency depends on the person, goal, health status, training load and recovery. If you're in good health, do 5 Day Fast 2 to 4 times a year. If your health is more out of balance, you can safely do 5 Day Fast once a month for maximum effect.