5 Day Fast: what happens in your body?

A lot of people see a 5-day fast mainly as eating less to lose weight. But a structured 5-day fasting protocol goes beyond simple calorie restriction. It can temporarily put you into 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’re in the right place. 

What is 5-day fasting?

5-day fasting is a temporary period where your body takes in far 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 at all. With a fasting mimicking approach, like 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

• greater use of fat as an energy source

• higher production of ketones as the days progress

The 5-day duration is important because your body doesn’t completely switch over from one day to the next. 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, energy use shifts more towards fat burning and ketosis.

That’s why 5-day fasting is quite different from just skipping a meal. It gives your body time to move through various metabolic phases.

How does 5 Day Fast work?

5 Day Fast is a structured fasting mimicking programme.

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

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. This makes 5 Day Fast much 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

• a daily structure

• clear instructions just before, during, and right after the 5 days

That structure is key. Without it, 5 days of fasting can quickly turn into a bit of an experiment. 5 Day Fast builds on research into fasting mimicking and was developed to make mimicking a five-day fast more practical.

💡 Caffeine isn’t allowed during the programme. By skipping caffeine for five days, you can better tune into your natural energy levels, fatigue signals, and sleep rhythm. Your cortisol levels (the stress hormone) also drop.  For regular coffee drinkers, this might cause headaches, fatigue, or poor focus in the first few days.

What are the benefits of 5 Day Fast?

The benefits of 5-day fasting are mainly linked to metabolic adaptation. The body temporarily takes in less energy (carbs, fats, and proteins), forcing it to manage available fuels differently.

Research into fasting and fasting mimicking looks at things like:

• fat burning

• ketosis

• insulin sensitivity

• metabolic flexibility

• the load on your digestion

• autophagy, the clean-up process inside cells

• the gut microbiome

• sleep

• endurance performance

Part of that research was done in specific groups or in animal models, so it doesn't say for sure what you'll notice. Results vary from person to person.

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

5 Day Fast isn’t about quick weight loss. The programme focuses on using more fat as an energy source and relying less on quick carbs, with careful attention to protein and movement during and after the 5 days.

During the 5 Day Fast protocol, your body gradually uses more fats and ketones for energy. This can reduce the need to use amino acids from muscle tissue as fuel, helping you maintain muscle mass while you’re fasting.

Muscle retention isn’t automatically guaranteed. That depends on how well you follow the 5-day programme and keep overly intense workouts in check. During the refeeding phase after the 5 days, you'll also want to build up training intensity gently and make sure you're getting enough protein, sleep, and recovery.

For athletes, the potential benefits are especially relevant after the programme, once normal nutrition, glycogen stores, and training have been built back up. How you experience this differs per person. Although these effects vary from person to person, we do see them almost every time with athletes doing 5 Day Fast.

5-day fasting or a 5-day diet: what’s the difference?

Many people search for a “5-day diet” because they want to drop weight fast. But 5 days of fasting isn’t the same as a classic crash diet.

A crash diet is usually just about rapid weight loss. The goal is to eat as little as possible for a short spell. The trouble is, this often leads to hunger, poorer recovery, a higher risk of muscle loss, and quickly bouncing back to old habits.

A structured approach like 5 Day Fast has a different aim. It’s not just about eating fewer calories and losing weight. It’s about temporarily creating a different metabolic environment where your body uses more fat for fuel and can enter ketosis.

During the 5 days, your body can shift towards burning more fat, lowering the digestive load, and ramping up ketone production, all while aiming to preserve muscle mass (4).

What happens during 5 days of fasting?

A 5-day fast isn’t a static state. Your body changes gradually.

Day 1: glycogen and water loss

On the first day, your body is still largely using stored carbohydrates as its energy source.

We call these stored carbs glycogen. Glycogen is stored alongside water. That’s why your body weight can drop quite quickly at the start of a fasting protocol.

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

Common experiences on day 1 include:

• disruption to your routine

• lower energy

• changes in mental focus

• cravings

• headaches or tiredness from caffeine withdrawal

Caffeine isn’t allowed during 5 Day Fast. Symptoms like headaches, fatigue, irritability, or lower focus can pop up when caffeine is cut out abruptly.

That doesn’t automatically mean the fasting protocol is “too tough”. A lot of that discomfort might just come from the temporary break from caffeine.

Days 2–3: fat burning and ketosis

As your glycogen stores run down, your body starts using more fat as an energy source.

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

This transition doesn’t feel the same for everyone. Some people feel more steady or mentally sharp. Others notice lower energy, feeling cold, hunger, or irritability. You might also feel your body ‘at work’ in certain spots. This often feels uncomfortable and sometimes even a bit sore. It’s important to realise this is positive. Your body is busy with repair and maintenance, and that happens automatically right where it’s needed most.

This is all completely normal. Your body is simply adapting to using a different fuel.

In this phase, it’s best not to expect peak performance during workouts. The goal is metabolic adaptation, not maximum output.

Days 4–5: metabolic adaptation

By days 4 and 5, your body can be much better adapted to using fats and ketones.

The digestive load stays lower, carbs are in short supply, and your body is in a different metabolic state than usual.

Some people feel more settled in this phase. Others stay tired. How it goes depends on your sleep, stress, activity, previous fasting experience, and individual metabolism.

That’s why keeping realistic expectations is so important. A 5-day fast doesn't feel the same for everyone.

After the 5 days: why rebuilding matters

The phase right after the programme is crucial.

After 5 days of fasting or fasting mimicking, your body needs to rebuild. Glycogen stores get topped up again. Some of that ‘water weight’ might return. Protein intake becomes important again. And training needs to be eased back into gradually.

Doing 5 Day Fast periodically definitely contributes to long-term progress. Right after those 5 days is the easiest time to tackle bad habits and try to lock them in as your new ‘default behaviour’.

After the 5 days, your focus is best placed on:

• gradual reintroduction of food

• plenty of protein

• nutritious meals

• quality carbohydrates

• healthy fats

• hydration

• movement

• sleep

• avoiding an immediate slide back into 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 cut out. 5 Day Fast uses limited nutrition to mimic certain effects of fasting, using a set menu and a clear daily schedule.

This doesn’t mean 5 Day Fast is completely risk-free or suitable for everyone. But for lots of people, it’s more structured and easier to follow than total food deprivation.

The main difference is control. Water fasting is much more extreme. 5 Day Fast was designed as a practical approach with a set menu, supplements, and a movement schedule.

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

A lot of people lose weight during a 5-day fast. But it’s important to understand what makes up that weight loss.

That first drop on the scales often comes down to:

• water loss

• glycogen loss

• less food volume in your gut

• fat loss

Fat burning usually picks up as the protocol moves along. But long-term fat loss depends on your overall energy balance and what happens after the 5 days.

So, 5 days of fasting can help support weight loss, but it shouldn’t be pitched as a quick fix; rather, think of it as a repeatable programme that helps you maintain a body fat percentage that’s healthy for you in the long run.

A better question isn't just “How much can I lose in 5 days?”, but rather: “How much of that result can I actually keep off long-term?”

💡 Holding onto muscle mass is key to maintaining your results. Muscle plays a big role in your resting metabolic rate—how much energy your body burns at rest—and it's also incredibly important for healthy ageing.

Do you lose fat or mostly water during a 5-day fast?

At the start, a big chunk of the weight loss is often water and glycogen.

When your carb stores drop, the water stored alongside that glycogen gets released too. That’s why the scales can drop so quickly.

That doesn’t mean the result is “fake”. It just means early weight loss isn’t the exact same thing as pure fat loss.

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

How does 5 days of fasting support fat burning?

A 5-day fast can support fat burning through a few different mechanisms.

Think of things like:

• lower energy intake

• fewer available carbohydrates

• lower glycogen stores

• using more fat for fuel

• higher ketone production

• improving metabolic flexibility

Metabolic flexibility means your body is better at switching between different fuels, like carbs and fats.

That’s really handy for body composition, metabolic health, and endurance performance. At the same time, it doesn’t mean doing a 5-day fast once will automatically guarantee long-term fat loss. Your overall lifestyle and repeating the 5-day fast periodically are the real keys to improving your health over time.

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

A structured fasting mimicking protocol supports muscle retention much better than total fasting, because it still provides controlled nutrition and supplements. This helps stop your body from desperately breaking down muscle tissue to use as an energy source.

Ketones can also help reduce the reliance on amino acids (muscle) for fuel. Still, what you do after the programme remains just as important.

To support muscle retention, you’ll want to focus on:

• getting enough protein after the protocol

• a gradual return to intense training

• strength training when appropriate

• sleep and recovery

• avoiding repeated, aggressive calorie restriction

If you're focusing on body composition, the goal isn't simply losing weight. The main goal is losing fat while keeping your muscle mass intact.

Fasting and gut health: what do we know?

A 5-day fasting mimicking protocol can also have an impact on your digestive system.

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

Possible links include:

• a lower digestive load and space for repair work

• shifts in available nutrition for gut bacteria

• an effect on the gut-brain axis

• indirect effects on sleep and recovery

• greater awareness around eating habits after the protocol

The effects vary from person to person. If you're dealing with active digestive issues, you’ll very often notice an improvement during the 5 days. If you're currently under a specialist's care, you should always be cautious and seek advice before starting.

5-day fasting and healthy ageing

Fasting often comes up in conversations about longevity and healthy ageing.

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

Autophagy is a cellular clean-up process. mTOR is a nutrient-sensing 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 striking the right balance between temporary stress and proper rebuilding. 5 Day Fast is meant to be repeated periodically. Got a medical condition or take medication? Check with your doctor first to see if and how often 5 Day Fast suits you.

5 Day Fast for athletes: when is it worth considering?

For athletes, 5 Day Fast can be especially interesting when your goal revolves around metabolic flexibility, fat oxidation, or body composition.

That goes for most team sports, and naturally even more so for endurance athletes and hybrid athletes.

Potentially relevant goals include:

• working on metabolic flexibility

• focusing on body composition

• building better lifestyle awareness

• a deliberate rest period in your training plan

• a structure you can repeat periodically

• a conscious rebuilding phase after the 5 days

Timing is everything here.

You wouldn't want to wrap up a 5-day fasting protocol right before a race or match. During the protocol, available energy is lower, training quality can dip, and you need more recovery. If you're experienced with 5 Day Fast, you can plan it so you finish 9–15 days before your race or competition. If you’re new to it, schedule it a bit further out first.

The sports performance perks mainly kick in after the protocol, once normal nutrition, training, and recovery are back up and running.

💡 During 5 Day Fast, you can keep doing low-intensity workouts. They're actually essential for supporting the shift into ketosis.

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

Sportvasten is a well-known concept in the Netherlands. It’s often linked with fasting, exercise, and making a metabolic switch.

5 Day Fast is not the same thing.

Both approaches touch on fasting and metabolism, but they differ in their structure, nutritional approach, positioning, and how you use them.

Sportvasten is often pitched as a method that combines dietary restriction with exercise. 5 Day Fast is a fasting mimicking programme backed by over 30 years of fasting research, complete with a set menu, supplements, and an exercise plan.

5 Day Fast reviews: what can you expect?

Experiences with 5 Day Fast differ from person to person.

Some people mainly notice weight loss and more structure in their eating habits. Others spot shifts in their energy, digestion, focus, or sleep.

Common experiences include:

• hunger and cravings in the first few days, followed by feeling really steady

• lower energy at first, followed by a brand new burst of energy

• changes in digestion

• an uncomfortable feeling right where your body is ‘hard at work’

• lower stress

• better sleep

• weight loss

• feeling lighter

• greater awareness of eating habits

• changing bad habits afterwards

• temporarily dialling back training intensity and volume during the 5 days

• improved sports performance afterwards

Some people feel great right across the 5 days. Others find the first couple of days quite a bit tougher.

Your exact results will come down to your starting point, preparation, and what you do during and after the 5 days.

Since caffeine isn't allowed during 5 Day Fast, regular coffee drinkers might temporarily run into headaches, tiredness, irritability, or brain fog. These symptoms usually clear up after the first few days.

Who is 5-day fasting suitable for?

A structured 5-day fasting protocol can be a great fit for anyone wanting to support their metabolic health, sports performance, body composition, or long-term lifestyle and health.

It can be a good option for:

• people interested in metabolic health

• people wanting a structured reset for their eating habits

• people looking to support fat loss

• people focusing on healthy ageing

• experienced athletes wanting to level up their performance strategy

• people who prefer structure over total fasting

It’s not about just “gritting your teeth” for 5 days. It’s about following a structured protocol with a clear goal in mind.

When should you be cautious with 5-day fasting?

A 5-day fast isn’t suitable for everyone.

Do not start if you have/are:

• pregnant

• breastfeeding

• severe fatigue

• high stress

• active illness

• a recent physical injury

Be careful if you have:

• specific medication use

• a history of eating disorders

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

If you take medication or have an underlying medical condition, always check with your doctor or specialist first.

A fasting protocol is a stressor. For the right person at the right time, that stressor can be really beneficial. For the wrong person, or at the wrong moment, it can completely backfire.

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

What you do after 5 Day Fast is a huge part of the whole process.

If you jump straight back into bad habits, the benefits won’t vanish overnight, but your long-term results will definitely take a hit.

Some weight might come back as your glycogen and water stores top up again. That’s completely normal.

To lock in your results, it’s best to focus on:

• gradually reintroducing food

• plenty of protein

• nutritious meals

• quality carbohydrates

• healthy fats

• regular movement

• sleep

• stress management

• swapping out 1 or 2 old habits that feel like easy wins for you

Frequently asked questions about 5-day fasting

Is 5-day fasting healthy?

During a programme like 5 Day Fast, your body starts using more fat as fuel and can enter ketosis. 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 a 5-day fast. The initial drop is often partly water and glycogen. Fat loss depends on your energy expenditure, you as an individual, and what you do after the protocol.

Do you put the weight back on after a 5-day fast?

Some of the weight can return as your glycogen and water stores replenish. That’s totally normal. Long-term results come down to your diet, exercise, sleep, and habits after the 5 days.

Is 5 Day Fast the same as water fasting?

No. With water fasting, you cut out food completely. 5 Day Fast builds on fasting mimicking research and includes a set menu, supplements, and a daily schedule, designed to mimic certain effects of fasting.

What’s the difference between 5 Day Fast and Sportvasten?

Both involve fasting-related strategies, but they differ in their scientific backing, structure, duration, nutritional approach, and goals. 5 Day Fast is a fasting mimicking programme, whereas Sportvasten is usually built around fasting combined with exercise.

Can you exercise during 5 Day Fast?

Low-intensity workouts are essential. Heavy or intense training usually isn't wise during the protocol. You've got less available energy, 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 includes controlled nutrition and supplements throughout the 5-day protocol. The exact structure is part of the programme. The aim is to mimic certain effects of fasting without cutting out food altogether.

How often can you do 5 Day Fast?

How often you do it depends on you, your goals, your health, training load, and recovery. If you’re in good health, doing 5 Day Fast 2 to 4 times a year is great. If your health is a bit more out of balance, you can safely do 5 Day Fast once a month for maximum effect.