Can 5 Day Fast support your marathon prep?

Training, nutrition, carbohydrates, muscle glycogen, fat burning, body weight, and recovery all affect your marathon performance and how well you can maintain 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 those moments when the race gets tougher. That's where 5 Day Fast can help, as long as you use it at the right time in your marathon preparation.

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.

During a marathon, your body mainly uses two fuels: carbohydrates and fat.

Carbohydrates are the body's fastest source of energy. During a marathon, the 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 your glycogen stores run low, fatigue sets in quickly.

Fat stores are much larger. Even lean athletes have plenty of energy stored as fat. However, 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 the body's ability to use different fuels efficiently, depending on what your body needs. For marathon runners, better metabolic flexibility can help you burn fat better at a higher intensity, while saving glycogen for the tougher moments.

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

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

How 5 Day Fast can support fat burning in running 

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

In the beginning, the body still uses available glucose and stored glycogen. As you get 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 intensity. We call this fat oxidation (1).

Secondly, the liver will convert some of the fat into ketones. Ketones are the alternative energy source for the brain once carbohydrate availability is too low (1). The brain cannot get energy from fat oxidation. Converting fat into ketones is essentially 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 from fat (2).

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

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

The potential benefit is linked to the timing and what happens after the program. Once normal nutrition, glycogen stores, recovery, and marathon-specific training are built up again, better fat use and metabolic flexibility help to support energy efficiency during long efforts.

Sparing glycogen: why that might be relevant for marathon performance

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

For marathon runners, that's important because glycogen is limited, but remains important when the body needs quicker energy that fat cannot supply efficiently enough: for example, during a climb, when fatigue or perceived exertion increases, or when you want to maintain your speed late in the marathon.

If the body can produce more energy from fat better and at a higher intensity, it won't have to rely as heavily on glycogen from the start. That helps to save carbohydrate stores for later in the race.

Here too, 5 Day Fast can be a powerful tool. By temporarily offering less energy and carbohydrates, it supports the body's capacity to use more fat and ketones as an energy source. This ensures better metabolic flexibility: the ability to use more fat when the effort allows, while saving glycogen for moments when quicker energy is needed.

💡 Bear in mind that sparing glycogen doesn't mean carbohydrates are no longer needed. Marathon runners still need plenty of carbohydrates before and during race day.

Why weight can be important for marathon runners

Marathon runners carry their body weight with every step.

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, running technique, and muscle strength, but one of them carries less unnecessary body weight, that runner will have less mechanical stress over thousands of steps.

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

In terms of weight loss, 5 Day Fast helps you in a sustainable, safe, and healthy way with a better/lower body fat percentage and therefore with weight loss. This is a big difference from classic long-term fasting or an aggressive crash diet.

5 Day Fast also contains glycerol. Your body produces glycerol itself when it enters a fasting state, and it is therefore used in these types of fasting-mimicking protocols as part of the broader strategy to prevent muscle loss (3).

Of course, muscle preservation still depends on the whole picture. When you are in, or want to enter, a fasting state, it is essential on 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 the proteins from your muscles as an energy source. And that means you are essentially eating your own muscles. So definitely move a lot, but at a moderate intensity.

For marathon runners, the value of 5 Day Fast isn't simply the number on the scales after 5 days. The real value lies in the fact that the program supports higher-quality weight loss: less unnecessary body weight (fat), while preserving your muscle mass to train and race well.

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

When do you plan 5 Day Fast before a marathon?

5 Day Fast shouldn't be planned in the final days before a marathon, when topping up glycogen stores, getting enough food, sleep, race-specific training, and limiting unnecessary stress are central.

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

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

That gives the body enough time to:

• rebuild nutrition

• restore glycogen

• return to normal training

• recover from the energy restriction

• then do the final race-specific sessions

So if you want 5 Day Fast to optimally contribute to your marathon performance, start 13-14 days before your race. 

By the way, this shouldn't be seen as a hard rule.

If you are doing 5 Day Fast for the first time, you can also choose to give yourself more margin, or to try 5 Day Fast when you are, say, 12 weeks away from your race. For some, it's nice not to discover how your body reacts 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, carbohydrate intake is lower, and your body is adapting to a different fuel use. This is not the right time for hard intervals, heavy long runs, or intense strength training.

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

Training during the 5 days should support the shift to fat burning, not create too much extra stress. Moving a lot, but more gently, can help the body use stored glycogen and move towards more fat oxidation and ketone production.

Don't assess your fitness the very day after the program. Nutrition, protein intake, glycogen stores, and recovery need time to rebuild.

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

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

5 Day Fast does not replace training, fueling, carb loading, or recovery.

But it can be relevant as a structured tool in your marathon preparation. Its potential value lies in mechanisms like fat use, ketone production, metabolic flexibility, glycogen management, body weight, and muscle preservation.

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

Want to understand the broader endurance perspective? Then read more about 5 Day Fast and endurance performance.