Water fasting for 5 days: what happens and how is it different from the 5 Day Fast?

Five days of water fasting can make the scales drop quickly. But that initial result can be misleading: part of the weight lost is often glycogen, water and gut contents, not just fat. So the real question isn't just what happens during 5 days of water fasting, but also whether this strict approach is the best way to lose weight, or if a structured fasting mimicking programme like 5 Day Fast offers a better alternative.

What is water fasting?

Water fasting is the strictest form of fasting. You temporarily eat no food at all and only drink water.

That's different from intermittent fasting, where you still eat within a specific eating window. It's also different from a fasting mimicking diet, where small amounts of specific foods ensure you keep eating, but you're in a controlled energy deficit.

With water fasting, there's no nutrition structure, no set macronutrient balance (fats, proteins and carbs), and no supplement protocol. Some people use it for weight loss, discipline, metabolic reasons, or as a personal reset.

Water fasting can therefore be physically and mentally tough, especially when it lasts several days. It should be used cautiously, for specific reasons, and usually for a short period. For example, some people use a very short fast to give their digestive system a day's rest, or in sports where a certain weight needs to be met before the start of an event or competition, under professional supervision.

What happens during 5 days of water fasting?

During 5 days of water fasting, your body has to switch its fuel source.

At first, it uses up readily available glucose and stored glycogen. Glycogen is the stored form of carbohydrates in the body. It's mainly stored in your muscles and liver.

Glycogen is also stored alongside water. Human research has estimated that each gram of glycogen can be stored with roughly 3 to 4 grams of water (1).

That explains why body weight can drop so quickly during the first phase of fasting.

When your body uses up stored glycogen, some of the water bound to that glycogen is released as well. Part of that initial drop on the scales is therefore often water weight, not fat loss.

As carbohydrate availability drops, your body has to rely more on fat as an energy source: so it burns more fat. The liver will also produce ketones, an alternative energy source meant for the brain, which becomes more important when glucose availability is low.

During a strict water fast, this shift can happen faster than during a fasting mimicking protocol, because you're consuming zero calories or carbs. With longer fasts, ketosis often becomes more apparent after a few days, though the exact timing depends on glycogen stores, activity levels, body size, and the person's metabolic state (2).

But getting into that phase faster isn't automatically better.

Because water fasting removes food entirely, the level of restriction is much higher. As a result, symptoms can be felt more intensely: hunger, weakness, dizziness, irritability, poor concentration, disrupted sleep, or feeling cold can all be much more noticeable.

For some people, this level of restriction can also make the post-fast period harder. A very strict fast can increase the risk of overeating afterwards, especially if you don't have a clear eating plan in place for after the fast.

How much weight do you lose with water fasting?

Body weight can drop quickly during water fasting. But that doesn't mean all the weight lost is fat. It also doesn't mean all the weight came from fat in the first place.

A rapid drop on the scales is often caused by several things at once:

• lower glycogen stores

• less water stored with glycogen

• less food volume in your digestive system

• reduced gut contents

• some fat loss

Because of this, it's hard to know exactly how much fat you lose during a 5-day water fast.

More importantly: if you lose 3 kg during a fast, that doesn't mean you've lost 3 kg of body fat. Part of that weight can return once you start eating normally again, as your glycogen and water stores are replenished.

What does this mean for weight loss?

Water fasting can make the number on the scales go down fast. That's the part a lot of people focus on.

But the cost can be high, both during the fast and afterwards. During the fast, the restriction is severe. After the fast, the results can be hard to maintain if your body and mind respond with intense hunger, cravings, or a rebound in food intake.

Fat loss during water fasting depends on various factors:

• glycogen stores before you start

• body size and body composition

• activity levels during the fast

• length of the fast

• prior carbohydrate intake

• what happens when you start eating again

For most people, a more gradual approach might be more useful: a moderate energy deficit where you still consume limited carbs, fats, and proteins. Combined with movement, strength training where possible, and a clear plan for after the diet.

Water fasting doesn't offer that. It shouldn't just be used as a repeated quick fix for weight loss.

Is 5 days of water fasting safe?

A 5-day water fast completely cuts out food for several days, which can be a major stressor for the body.

Taking extra care is especially important if you:

• are taking medication

• have diabetes

• have an underlying medical condition

• are pregnant

• are breastfeeding

• are under 18

• are underweight

• have a low BMI

• currently have an eating disorder or have had one in the past

• are going through a period of high stress or poor recovery

In these cases, getting medical advice is important before you even consider starting a fasting period.

A strict fast can also affect your relationship with food.

For some people, heavy restriction can create a negative loop: restricting heavily, then feeling out of control, overeating or binging, feeling guilty, restricting again, and repeating the cycle. This can make weight regain more likely and make eating feel more emotional, stressful, or compensatory.

That's one of the reasons why water fasting should be approached carefully, especially by people who already struggle with control around food, feelings of guilt, binge eating, or body image.

When should you be extra careful?

You shouldn't start a strict fast if you're not sure whether it's right for your situation.

Symptoms like severe dizziness, weakness, confusion, fainting, heart palpitations, or feeling seriously unwell shouldn't be ignored.

Professional guidance is important if symptoms become severe or if there's any medical risk.

Water fasting also shouldn't be framed as a way to detox the body, cure diseases, or guarantee fat loss. Those claims are too strong and aren't well supported.

A balanced view is more helpful: 5 days of water fasting can lead to metabolic changes, but it's tough, demanding, not suitable for everyone, and can have long-term consequences if it worsens your eating habits or leads to repeated cycles of restriction and rebound. Finally, your body can ‘eat away’ at your own muscle tissue if it doesn't have enough energy available and urgently needs fuel. Losing muscle mass is a major risk with water fasting.

What's the difference between water fasting and 5 Day Fast?

Water fasting and 5 Day Fast are both related to fasting. But they work very differently.

The biggest difference is simple: water fasting means no food at all. 5 Day Fast means eating within a structured fasting mimicking protocol.

Water fasting

With water fasting, you don't eat any food and you only drink water.

There's no meal plan, no controlled food intake, no clear nutritional structure, and no built-in supplement protocol.

For many people, that makes water fasting tricky to plan and hard to combine with work, family life, training, or daily responsibilities. It can also cause a lot of uncertainty.

What happens if you feel weak? Can you exercise? When do you start eating again? How do you reintroduce food afterwards?

Without structure, those decisions are often left entirely up to the person fasting, and it quickly turns into a bit of an experiment.

5 Day Fast

5 Day Fast is a 5-day fasting mimicking programme. You don't stop eating completely.

Instead, you follow a clear structure with an adaptable meal plan, specific foods and portions, supplements, a shopping list, and guidance.

The goal is to mimic certain effects of fasting in an evidence-based and structured way. You still eat, but you're taking in less energy. All within a specific balance of calories, fats, carbs, and proteins.

The programme was developed based on the scientific principles behind fasting mimicking diets, including research by Prof. Valter Longo. This evidence base draws on more than 30 years of scientific research into the effects of fasting on health—including metabolic markers, body weight, and cardiometabolic risk factors (3,4).

Maintaining muscle mass during 5 Day Fast

Holding onto muscle mass is also a key consideration during fasting, both for overall health and for athletic performance.

Research into certain fasting mimicking protocols, which 5 Day Fast is based on, suggests that body weight can decrease without a decline in muscle function or muscle volume. In a randomised trial involving 24 physically active men, muscle strength and muscle volume were maintained after three cycles of a fasting mimicking diet (5).

5 Day Fast also contains glycerol, a compound researched for its ability to support fluid retention and hydration (6). Your body also produces glycerol itself to help protect muscle mass when it enters a fasted state.

5 Day Fast includes movement advice for physical activity across the 5 days as well. On top of that, customer support is available to answer any questions before, during, and after the programme.

This gives 5 Day Fast a structured, scientifically supported approach that sets it apart from a self-guided 5-day water fast.

Why do some people choose 5 Day Fast over water fasting?

For many people, the hardest part of fasting isn't just the hunger. It's the lack of guidance and certainty.

What am I allowed to do? What should I avoid? How long should I stick with the diet? What do I eat afterwards? How do I know if I'm doing it right? What are the consequences?

5 Day Fast takes away some of that guesswork.

It gives you a clear 5-day programme, a meal plan, a shopping list, supplements, and guidance both during and after the programme.

That can make it much more practical for anyone looking to lose weight and/or experience some of the benefits of fasting without having to stop eating completely.

It also makes the whole process easier to repeat in a controlled way.

Instead of relying on a strict, one-off water fast, 5 Day Fast is designed as a structured programme that can be repeated several times a year, depending on your goals, health situation, and suitability.

Conclusion: water fasting or a fasting mimicking approach?

Water fasting and 5 Day Fast both involve fasting.

Water fasting is the strictest version: no food, only water.

It can cause body weight to drop rapidly, mainly because glycogen, water, gut contents, and energy intake are all shifting at the same time. Some fat loss may happen, but the number on the scales won't show you how much of that result is actually fat.

5 Day Fast takes a different approach.

It's a structured fasting mimicking programme with a meal plan, supplements, a shopping list, movement advice, and support before, during, and after the 5 days.

The aim is to experience some of the fasting-related metabolic effects without dealing with the same level of total restriction you get from water fasting.

For many people, this makes 5 Day Fast a more practical long-term alternative. It's easier to stick to, easier to plan, easier to repeat several times a year when appropriate, and it's also backed by over 30 years of scientific research into fasting mimicking diets.