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

Losing weight in 5 days might sound appealing, especially when the scales start to drop after the very first day. But that initial drop doesn't tell the whole story. In the beginning, your body weight often changes quickly due to glycogen, water and bowel contents, while the body only gradually shifts more 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're preserving muscle mass and what you do afterwards to maintain the results.

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.

In a short period of calorie deficit, a drop in body weight can be explained by several factors:

• less food volume in your digestive system

• lower carbohydrate intake

• lower glycogen stores

• less water retention from glycogen

• more fat utilisation 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 same programme.

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

And that's exactly 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 bowel contents.

Glycogen is the form in which carbohydrates are stored in the 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 carbohydrate intake is reduced.

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

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

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

It also doesn't mean the programme has failed if some weight comes back once you eat normally again. When you eat more carbs again, glycogen and water stores are replenished.

When does your body start using more fat?

When energy intake and thus carbohydrate absorption drop, the body gradually starts to 'move' more towards stored fat.

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

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

Around this point, the body usually starts to enter a state of ketosis: a metabolic state in which ketone production increases because carbohydrate availability is too low to supply the brain with energy.

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

This depends on your starting glycogen stores, your previous carbohydrate intake, activity level, body size, training status and how closely you follow the programme.

A structured fasting mimicking approach like 5 Day Fast is designed to support this temporary metabolic shift without taking away food completely.

What is the difference between fat loss and water loss?

Your weight on the scales is your total body weight.

It consists of:

• body fat

• muscle mass

• water

• glycogen

• bowel contents

• bone mass

• organs and tissues

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

In practice, fat loss occurs when your body uses stored fat as energy over time. That requires an energy deficit, but it also depends on hormones, activity level, sleep, stress, protein intake and what happens after the deficit.

Water loss is usually quicker.

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

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

The opposite can also happen: someone can lose fat while the scales change less than expected, because water, glycogen, muscle mass or bowel contents change less drastically.

So, short-term weight loss can be misleading.

A lower number on the scales can be motivating, but it doesn't tell the whole story. For better quality weight loss, the goal is to support fat utilisation, prevent unnecessary muscle loss and avoid a big rebound afterwards.

How much weight can you realistically lose with 5 Day Fast?

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

In practice, we often see weight loss somewhere between 2 and 6 kg in people who have already done 5 Day Fast. But this range shouldn't be seen as a promise or fixed 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

• carbohydrate intake before the programme

• activity level

• how closely you follow the protocol

• sleep and stress

• bowel contents

• what you do after the 5 days

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

Muscle mass can also influence the initial drop on the scales.

Someone with more muscle mass can often store more total glycogen, as glycogen is mainly stored in muscles and the liver. Since glycogen is stored along with water, someone with larger glycogen stores can lose more water weight when those stores drop.

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

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

The more important question is whether the 5 days help you towards a 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 quickly. It often has little structure, poor nutritional quality and no clear plan for what happens next.

As a result, it can be hard to sustain and might become 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 guidance

• food, as it isn't a complete water fast

Here, the goal is to create a temporary metabolic environment that supports fat utilisation, ketone production and greater awareness of 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 preservation is one of the most important parts of responsible weight loss.

When energy intake gets lower, 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 when you ask too much of your muscles during a period of low energy intake.

This is important, because muscles aren't just about looks.

Muscles help maintain strength, movement quality, posture, glucose metabolism and resting metabolic rate. If too much muscle mass is lost, your daily energy expenditure can drop, performance can suffer and it can become easier to put weight back on afterwards.

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

5 Day Fast is designed to support a more structured form of short-term weight loss.

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

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

Muscle preservation still depends on:

• the duration of the protocol

• avoiding overly heavy training/exercise 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 general 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 can come back after the 5 days. That is completely normal.

When you eat more carbs again, glycogen stores are replenished. Since glycogen holds water, some water weight can also return. This doesn't mean fat loss has been reversed.

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

To make the transition easier:

• gradually build up your food intake again

• prioritise protein

• eat vegetables and fibre-rich foods

• choose minimally processed foods

• avoid overeating straight after the 5 days

• gradually return to training

• 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 next determines whether that opportunity turns into a longer-term result.

Conclusion: you can lose weight in 5 days, but the approach counts

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 bowel contents. As carbohydrate intake drops, the body can shift more towards fat burning and ketone production. This is when fat loss happens.

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: maintaining muscle mass, keeping a suitable energy balance, avoiding aggressive rebound eating and consistently making better food choices.