Losing fat without losing muscle mass: what does the research say?

Whether you lose fat around a short fasting period without sacrificing muscle mass mainly comes down to protein, training, and recovery. Unlike traditional calorie restriction, your body shifts more towards burning fat, while certain mechanisms that support muscle retention stay active. However, these benefits depend heavily on how you follow the programme and, especially, on the period after it.

What happens to fat burning and muscle mass during fasting?

During 5 Day Fast, your body gradually shifts from a metabolism that mainly relies on glucose and carbohydrate stores to a state of ketosis.

In this state, fats become the main source of energy, and your body also starts producing ketones as fuel for your brain.

This metabolic shift leads to several effects:

• a clear increase in burning your own body fat

• less reliance on glucose

• a lower reliance on protein as an energy source

In practice, the distribution of energy sources changes: your body gets more energy from fat stored in your body and less from carbohydrates and protein.

That doesn't mean the use of protein stops completely.

Your body keeps making glucose via gluconeogenesis, mainly from amino acids, because certain tissues still need glucose. However, that need decreases thanks to the greater use of fats and ketones (1).

Why can fat loss happen without muscle loss?

Under normal circumstances, your body mainly uses glucose to produce energy.

When less glucose is available:

• your body first uses glycogen, the quick energy store kept in your muscles and liver

• gluconeogenesis is then activated, producing glucose from other substrates, mainly amino acids from protein

• the use of protein increases, which can encourage muscle breakdown

That explains why muscle loss often happens with traditional (crash) diets.

During the 5 Day Fast programme:

• you keep moving/training, but at a lower intensity than usual

• you eat high-quality fresh produce, albeit in limited amounts

• ketones and body fat provide an alternative energy source

• meaning your body doesn't need to break down muscle mass for energy

• which helps prevent muscle breakdown (2)

💡 The 5 Day Fast pack also contains Glycerol. Glycerol is a substance your body also releases naturally when it breaks down fat.

Fat burning and metabolic efficiency: what changes?

One of the main effects of the programme is an improvement in metabolic efficiency. In other words: how efficiently your body uses energy sources.

Your body becomes more efficient at using fat as fuel. And unlike carbohydrates, fat stores are much larger.

💡 By way of comparison: a 70 kg adult with around 15% body fat carries an average of 10 to 12 kg of fat mass. Since 1 kg of fat represents roughly 7,500 to 7,700 kcal, that equals about 75,000 to 90,000 kcal of stored energy.

Energy production becomes steadier, and big swings in energy levels decrease.

For people with excess body fat, this supports a sustainable, lasting reduction in body fat percentage.

In ketosis, your body uses far more fat as fuel, which encourages genuine fat loss rather than just overall weight loss—which often includes muscle and water loss on traditional diets. Before you enter ketosis, you first lose a lot of water that's 'held' by glycogen. Because you always use up your glycogen first, you always drop water first too. That's always the initial weight loss and where many people/diets stop. 5 Day Fast actually pushes through and guides you safely into a state of Ketosis. 

These metabolic adaptations have been observed across various fasting and energy restriction protocols, including improved fat oxidation (being able to use body fat as an energy source) and metabolic flexibility (being able to switch between carb and fat burning) (1)(3).

When does a short fasting period suit your goals (and when doesn't it)?

During five days of lower energy intake, the number on the scales often drops right from the start, mainly because glycogen stores empty out along with the water bound to them.

However, that initial weight loss doesn't automatically mean you're mainly losing fat.

What really counts is how your body adapts during the programme and how well you maintain those results afterwards.

During the programme:

• the energy deficit causes rapid weight loss, mostly water at first

• fat burning gradually increases

• your body switches more and more to using fat as fuel

After 5 Day Fast, even when you start eating normally again, your body can keep using fat more efficiently… How long that lasts is up to you.

In other words: 5 Day Fast trains your body to get better at using body fat as an energy source.

After the programme, your diet, enough protein, and training determine what you maintain.

💡 Muscle mass plays a key role in your resting metabolic rate. The more muscle mass you have, the more energy your body burns at rest.

How do you minimise muscle loss around a fasting period?

To make sure weight loss comes mainly from fat mass:

• after 5 Day Fast, pick one or two things you want to change—your 'low-hanging fruit'—and avoid overcompensating after the programme (overeating or rebound eating)

• make sure you get enough protein after 5 Day Fast (around 1.6 to 2.2 g/kg of body weight per day)

• gradually build up intense training again over a period of 5–10 days

Without this approach, some of the lost weight can quickly return.

When can it lead to muscle loss?

Muscle loss can increase when:

• a large calorie deficit is kept up for too long after 5 Day Fast

• protein intake isn't high enough after 5 Day Fast

• training is too heavy during or straight after 5 Day Fast (too much intensity or sessions that are too long and hard)

Available energy then drops while physiological stress rises, which can lead to muscle breakdown.

Who is this relevant for?

5 Day Fast is worth considering for:

• people wanting to improve their body fat percentage

• athletes looking to boost their fat-burning engine and lose fat without losing muscle mass

• people who already live a healthy lifestyle, but are looking for a reset or even further improvement

On the other hand, it's less recommended:

• when you have low energy availability or during times of high stress

• during periods of severe fatigue

• in situations with higher recovery needs, such as injury or illness

• for women who are pregnant or breastfeeding

Can you lose weight without losing muscle mass?

5 Day Fast helps people with excess body fat achieve a sustainable, lasting reduction in body fat percentage. If your body fat percentage is already balanced, it helps you maintain that balance and 'trains' your body to be metabolically flexible.

What makes the difference:

• you teach your body to get better at using body fat as an energy source

• which lasting changes you can keep up after 5 Day Fast

• doing 5 Day Fast again when you feel ready for it (and might have slipped back into a few old bad habits)

When used properly, it can maximise fat burning while preserving muscle mass. When done the wrong way, it can create too much stress that your body struggles to handle.