What happens to your weight during a 5-day fast?

Losing weight in 5 days might sound appealing, especially when the scales already start to drop after day one. But that initial drop doesn't tell the whole story. At first, your body weight often changes quickly because of glycogen, water, and gut content, whilst your body only gradually shifts towards fat burning and ketone production. That's why the real question isn't just how much weight you can lose in 5 days, but what kind of weight you're losing, how well you keep your muscle mass, and what you do afterwards to hold onto the result.

Can you lose weight in 5 days?

Yes, your body weight can drop in 5 days. But that doesn't mean you're only losing body fat.

During a short period of calorie deficit, a drop in body weight can be put down to several factors:

• less food volume in your digestive system

• lower carb intake

• lower glycogen stores

• less water retention from glycogen

• more fat burning when energy intake is lower

That's why the scales can move quickly during a 5-day diet or fasting mimicking protocol, but also why we often see differences between people following the exact same programme.

Ultimately, the important question isn't just whether you lose weight. It's also about what kind of weight you lose, and how much of that result you keep afterwards.

And that's precisely where the difference between a crash diet and a structured approach becomes important.

Why does your weight often drop quickly in the first few days?

The initial drop in weight is often down to glycogen, water, and gut content.

Glycogen is the form in which carbohydrates are stored in your body. It's mainly stored in your muscles and liver. Glycogen also binds water, which means that when glycogen stores drop, water weight often drops too.

This can happen quickly when you cut your carb intake.

When your body uses stored glycogen, some of the water bound to that glycogen is released as well. This can temporarily lead to more fluid loss, which is why some people find they pee more often in the first few days of a diet, low-carb phase, or fasting mimicking programme.

At the same time, if you're eating less food volume, there's also less content in your digestive system. Because of that, your body weight can drop further on the scales, even before there's any meaningful fat loss.

So, if you lose weight quickly in the first few days, it doesn't automatically mean it's all fat. Usually, it isn't.

It also doesn't mean the programme has failed if a bit of weight comes back once you start eating normally again. When you eat more carbs again, your glycogen and water stores simply top back up.

When does your body start burning more fat?

When your energy intake—and therefore your carb intake—drops, your body gradually starts moving towards using stored fat.

At first, your body still uses available glucose and stored glycogen. As those stores get lower, fat burning increases and your body can produce more ketones. Ketones are an alternative energy source produced during periods of low carb availability, fasting, or fasting-like conditions (1,2).

Even if your body still isn't burning purely fat, the balance of fuel use shifts. It becomes less dependent on glucose and starts using more fat and ketones for energy.

Around this point, your body usually starts entering a state of ketosis: a metabolic state where ketone production ramps up because carbohydrate availability is low, helping fuel your brain.

During a programme or diet based on fewer calories and lower carb availability, this shift often becomes clearer after about 2 to 3 days, although the timing varies from person to person.

This depends on your starting glycogen stores, your previous carb intake, activity levels, body size, training status, and how closely you stick to the programme.

A structured fasting mimicking approach like 5 Day Fast is designed to support this temporary metabolic shift without cutting out food entirely.

What's the difference between fat loss and water loss?

Your weight on the scales is your total body weight.

It's made up of:

• body fat

• muscle mass

• water

• glycogen

• gut content

• bone mass

• organs and tissues

Fat loss is something else. It refers specifically to a reduction in body fat.

In practice, fat loss happens when your body uses stored fat for energy over time. You need an energy deficit for that, but it also depends on hormones, activity levels, sleep, stress, protein intake, and what happens after the deficit.

Water loss is usually much quicker.

It can happen when glycogen stores drop, salt intake changes, carb intake falls, or food volume gets lower. Because of this, you might look a bit leaner or less bloated without having actually lost a large amount of fat.

That's why someone can lose weight quickly on the scales without losing much fat.

The opposite can happen too: someone might lose fat while the scales barely budge, because water, glycogen, muscle mass, or gut content don't change as much.

So, short-term weight loss can be quite misleading.

A lower number on the scales might feel great, but it doesn't tell the whole story. For better-quality weight loss, the goal is to support fat burning, prevent unnecessary muscle loss, and avoid a massive rebound afterwards.

What does a lower number on the scales mean?

Everyone sees a drop on the scales during 5 Day Fast.

How much that number drops varies a lot from person to person and depends on your starting point, your glycogen stores, and how much water you hold onto. But this range shouldn't be seen as a promise or a set result: there's no single number that applies to everyone.

How much weight you lose depends on:

• starting weight

• muscle mass

• glycogen stores before starting

• carb intake before the programme

• activity levels

• how closely you follow the protocol

• sleep and stress

• gut content

• what you do after the 5 days

Someone starting with higher glycogen stores or more food volume might see a faster drop in the first few days. Someone who already eats low-carb or has a lower body weight might see a smaller change.

Muscle mass can also affect that initial drop on the scales.

Someone with more muscle mass can often store more total glycogen, as glycogen is mainly stored in your muscles and liver. Because glycogen is stored alongside water, someone with larger glycogen stores might lose more water weight when those stores go down.

This can partly explain why a more muscular or larger person sometimes sees a faster initial weight loss than someone with a smaller frame.

But again: that first drop isn't necessarily fat loss. Part of it is often glycogen and water. That's why the programme shouldn't just be judged by the number on the scales after 5 days.

The more important question is whether the 5 days help you move towards better body composition, better habits, and a more structured post-programme phase.

Is losing weight in 5 days healthy?

That depends on the approach.

A crash diet usually focuses on eating as little as possible to lose weight fast. It often has little structure, poor nutritional quality, and no clear plan for what happens next.

As a result, it can be tough to stick to and potentially unhealthy.

A structured 5-day fasting mimicking approach is different. 5 Day Fast includes:

• a clear 5-day structure

• a controlled and personalised menu

• a supplement protocol

• a shopping list

• practical instructions

• actual food—it's not a complete water fast

Here, the goal is to create a temporary metabolic environment that supports fat burning, ketone production, and greater awareness around eating habits through a fasting mimicking programme. That makes the weight loss healthier and more efficient in the long run.

Can you lose weight without losing muscle mass?

Muscle retention is one of the most important parts of sensible weight loss.

When energy intake drops, your body still needs to produce energy. If the deficit is too aggressive in too short a time, muscle tissue can be converted into energy, especially if you demand too much of your muscles during a period of low energy intake.

This matters, because muscles aren't just about how you look.

Muscle helps maintain strength, movement quality, posture, glucose metabolism, and resting metabolic rate. If you lose too much muscle mass, your daily energy expenditure can drop, performance can suffer, and it can become easier to regain weight afterwards.

This is one of the reasons why very aggressive weight loss can lead to a rebound effect.

5 Day Fast is designed to make five days of fasting mimicking structured and practical.

During a fasting mimicking protocol, your body can shift towards more fat burning and ketone production. This may reduce the need to use muscle tissue as an energy source (1,2).

5 Day Fast also contains glycerol, a supplement linked with fasting that can serve as a carbon source during a calorie deficit. It's used in fasting mimicking diet protocols to help preserve muscle tissue (3).

Holding onto muscle still depends on:

• the duration of the protocol

• keeping workouts/exercise light during the 5 days

• protein intake after the programme

• gradual refeeding

• training after the programme

• sleep and recovery

5 Day Fast is therefore designed to support the overall strategy of limiting muscle loss during a short period of energy restriction.

How do you stop the weight from coming straight back?

Some of the weight might return after the 5 days. That's completely normal.

When you start eating more carbs again, your glycogen stores replenish. Because glycogen holds water, some water weight might come back too. This doesn't mean your fat loss has been undone.

The post-programme phase determines how much of the result you keep.

To make the transition easier:

• build your food intake back up gradually

• prioritise protein

• eat vegetables and high-fibre foods

• choose minimally processed foods

• avoid overeating straight after the 5 days

• get back into training gradually

• get enough sleep

• use the 5 days as a reset, not as compensation

This phase is important because the 5 days create an opportunity. What you do afterwards decides whether that opportunity turns into a longer-term result.

Conclusion: losing weight in 5 days is possible, but your approach matters

Yes, losing weight in 5 days is possible.

But not all weight loss is fat loss. In the first few days, body weight often changes due to glycogen, water, and gut content. As carb intake drops, your body can shift more towards fat burning and ketone production. That's when fat loss takes place.

A structured approach like the 5 Day Fast programme can make this process clearer and easier to follow than a random crash diet. It's a healthier way to support short-term weight loss and help you move in the right direction for longer-term results.

However, the real value doesn't just come from the 5 days.

What counts afterwards is the long-term effort: preserving muscle mass, maintaining a suitable energy balance, avoiding aggressive rebound eating, and consistently making better food choices.