Can 5 Day Fast support your marathon prep?

Training, nutrition, carbohydrates, glycogen in your muscles, fat burning, body weight, and recovery all influence your marathon performance and how well you can hold your target pace over 42.2 kilometres. For marathon runners, the goal isn't just to feel energetic, but to use that energy efficiently and save enough glycogen for when the race gets tougher. 5 Day Fast might help with that, provided you use it at the right moment in your marathon prep.

Why nutrition and energy use matter in marathon preparation 

Because a marathon is a long endurance effort, it’s not just about having enough energy. It’s mainly about being able to use the right fuel efficiently for long enough.

Your body primarily uses two fuels during a marathon: carbohydrates and fat.

Carbohydrates are the body's fastest energy source. During a marathon, the general rule is: the higher the intensity gets, the more the body relies on carbohydrates, because they can supply energy faster than fat.

The problem is that carbohydrate stores are limited. We call stored carbohydrates glycogen. As soon as glycogen stores run low, fatigue sets in quickly.

Fat stores are much larger. Even lean athletes have plenty of energy stored as fat. But fat is converted into usable energy more slowly. Because of this, the body can use fat well during long efforts, but not always quickly enough to support higher intensities.

That's why it's important for marathon runners to be able to use fat efficiently while sparing glycogen: we call this metabolic flexibility.

Metabolic flexibility is your body's ability to use different fuels efficiently, depending on what it needs. For marathon runners, better metabolic flexibility can help burn fat better at a higher intensity, while keeping glycogen in reserve for tougher moments.

This doesn't mean carbohydrates become optional. For marathon performance, glycogen stores still need to be topped up before race day, and carbs remain important before and during the marathon.

The goal isn't to replace carbs with fat. The goal is to use fat better at a higher intensity, so limited glycogen stores are used more efficiently. And that's exactly where 5 Day Fast might help.

Fat burning and running 

During 5 Day Fast, your energy intake is temporarily lower and you also take in fewer carbohydrates. This genuinely changes the available fuel entering the body through food.

Initially, the body still uses available glucose and stored glycogen. As you take in less energy over a longer period, the body has to rely more on fat as an energy source in two ways.

Firstly, the body can use more fat directly as fuel, especially at low and moderate intensities. We call this fat oxidation (1).

Secondly, the liver starts converting some of the fat into ketones. Ketones are the alternative energy source for the brain once carbohydrate availability gets too low (1). The brain can't get energy from fat oxidation. Converting fat into ketones is effectively a survival mechanism of the body.

So fat oxidation and ketone production are part of the same metabolic shift: when less glucose is available, the body becomes more dependent on energy derived from fat (2).

For marathon runners, this is interesting because fat is a massive energy store. Being able to use that energy efficiently during long efforts is one way to delay fatigue and slowing down.

This doesn't mean 5 Day Fast directly improves marathon performance.

The potential benefit depends on timing and what happens after the programme. Once normal nutrition, glycogen stores, recovery, and marathon-specific training are built back up, better fat utilisation and metabolic flexibility help support energy efficiency during long efforts.

Sparing glycogen: why that might be relevant for marathon performance

Sparing glycogen means you use less glycogen to produce the same effort.

For marathon runners, that's important because glycogen is limited, yet remains crucial when the body needs faster energy that fat can't provide efficiently enough: for example, on a climb, when fatigue or perceived exertion increases, or when you want to hold your pace late in the marathon.

If your body can produce more energy from fat better and at higher intensities, it doesn't need to rely as heavily on glycogen right from the start. That helps save carbohydrate stores for later in the race.

Here too, 5 Day Fast can be a useful tool. By temporarily offering less energy and fewer carbs, it supports the body's capacity to use more fat and ketones as an energy source. This leads to better metabolic flexibility: the ability to use more fat when the effort allows, while keeping glycogen saved for moments when faster energy is needed.

💡 Keep in mind that sparing glycogen doesn't mean carbs aren't needed anymore. Marathon runners still need plenty of carbohydrates before and during race day.

Why weight can matter for marathon runners

Marathon runners carry their body weight with every single stride.

That means body weight can influence how much energy it takes to run at a certain pace. If two runners have the same fitness level, the same running economy, and the same muscle strength, but one carries less unnecessary body weight, that runner faces less mechanical load over thousands of strides.

For marathon runners, losing fat mass can therefore be useful if it reduces unnecessary weight. But losing muscle mass, which is fairly common with weight loss programmes, can actually be harmful, because you really need your muscles during the marathon. 

How much weight you lose and keep off varies from person to person. A short, structured fasting period is quite different from prolonged fasting or an aggressive crash diet.

5 Day Fast also contains glycerol. Your body naturally produces glycerol when it enters a fasted state, and that's why it's used in these kinds of fasting mimicking protocols as part of the broader strategy to prevent muscle loss (3).

Of course, maintaining muscle still depends on the whole picture. The moment you are in, or want to enter, a fasted state, it's essential on the one hand to keep moving. After all, you want to use the glycogen from your muscles and get into ketosis. On the other hand, you don't want to demand too much energy too quickly, because then your body might choose to start using protein from your muscles as an energy source. And then you're effectively eating away your own muscles. So definitely keep moving plenty, but keep it at a moderate intensity.

For marathon runners, it’s not about the number on the scales after 5 days. What matters most for runners is keeping your muscle mass and training quality intact. Sufficient protein and a proper rebuild after the programme are key here.

For body composition and performance, you can also read our article on weight loss in endurance sports.

When should you plan 5 Day Fast before a marathon?

5 Day Fast shouldn't be scheduled in the final days before a marathon, when topping up glycogen stores, getting enough nutrition and sleep, race-specific training, and minimising unnecessary stress take centre stage.

5 Day Fast fits better earlier in your preparation, during a lower-intensity week or a recovery block.

For most marathon runners, starting 13–14 weeks before the race is optimal.

That gives your body enough time to:

• rebuild your nutrition

• restore glycogen

• return to normal training

• recover from the energy restriction

• and still complete your final race-specific sessions afterwards

If you include 5 Day Fast in your marathon prep, finish the programme at least two weeks before your race. 

That said, this shouldn't be seen as a rigid rule.

If you're doing 5 Day Fast for the first time, you could also choose to allow a bigger buffer, or try 5 Day Fast when you're still, say, 12 weeks out from your race. For some, it's nicer not to find out how your body responds too close to race day.

How do you train during 5 Day Fast as a marathon runner?

During the 5 days, movement remains part of the process.

Energy intake is lower, carb intake is lower, and your body is adapting to a different fuel mix. This isn't the right time for hard intervals, heavy long runs, or intense strength training.

A simple rule of thumb is the talk test. If you can't chat comfortably during the session, the intensity is probably too high.

Training during the 5 days should support the shift towards fat burning, not create loads of extra stress. Plenty of easier movement can help the body use stored glycogen and move towards more fat oxidation and ketone production.

Don't judge your fitness straight away the day after the programme. Nutrition, protein intake, glycogen stores, and recovery need time to rebuild.

For marathon runners, the phase before and after the programme is just as important as the 5 days themselves. Bear your training schedule in mind on the days immediately before and after 5 Day Fast. Trying to do too much or pushing the intensity too hard too close to it will only backfire.

Conclusion: 5 Day Fast as a tool in your marathon preparation

5 Day Fast is no replacement for training, fueling, carb loading, or recovery.

However, it might be relevant as a structured tool in your marathon prep. The potential value lies in mechanisms like fat utilisation, ketone production, metabolic flexibility, glycogen management, body weight, and muscle retention.

Timing is essential. 5 Day Fast should be scheduled during a lower-intensity phase, not right before your race. You'll notice the benefits in the phase after the programme, once normal nutrition, glycogen stores, and recovery are back to normal and marathon-specific training has resumed.

Want to understand the broader endurance perspective? Read more about Fasting and endurance performance.