Fat burning and stamina: what does fasting do?

5 Day Fast supports mechanisms linked to fat oxidation, ketone production and metabolic flexibility (1,2). For endurance athletes, that's relevant, because making better use of fat as an energy source can help spare glycogen during long efforts. This article explains when this might support your endurance, and why timing, nutrition and recovery are so important along the way.

Why is fat burning important for endurance sports?

During endurance exercise, your body mainly uses two fuels: carbohydrates and fat.

Carbohydrates provide quick energy. They're useful when the going gets tougher, like on climbs, during surges or in moments of higher intensity. The snag is that your carbohydrate stores, called glycogen, are limited. As soon as your glycogen stores drop low, fatigue can set in quickly.

Fat works differently. The body stores far more energy as fat, but fat is slower to use. It's useful for long, steady efforts, but can't completely replace carbs when the intensity picks up (3).

As glycogen stores drop, your body has to use more fat. Still, that doesn't mean the body always does this efficiently. Some people are better at using fat for fuel than others, especially during long efforts at low to moderate intensity.

For endurance athletes, better fat burning helps support:

• sparing glycogen stores

• steadier energy during long efforts

• delaying metabolic fatigue

• handling fluctuating energy demands more efficiently

This doesn't mean carbs become redundant. The goal isn't to ditch carbs altogether, but to use fat better when the effort is steady, keeping glycogen tucked away for moments when you need quicker energy.

What happens to fat burning during fasting?

During 5 Day Fast, energy intake is lower and less carbohydrate energy is available.

That changes the fuel available in the body. At first, the body still relies on stored carbs. As glycogen availability drops, the body shifts towards using more fat as an energy source. Ketone production increases (1,2).

Ketones are produced when glycogen stores run low and the body breaks down fat for energy. They can be used as an alternative energy source, especially when carb intake is low. Ketones are specifically intended as fuel for the brain and are automatically produced by your body from fat the moment your glycogen stores deplete.

Put simply, 5 Day Fast helps the body use more fat and ketones as an energy source.

Possible effects include:

• increased fat burning

• higher ketone production

• less reliance on glucose

• a better ability to switch between fuels

That's why 5 Day Fast can be relevant for endurance athletes. Not because you'll get faster during those 5 days, but because it may support mechanisms involved in energy use during long efforts.

What is metabolic flexibility and why does it matter for athletic performance?

Metabolic flexibility is the body's ability to efficiently produce energy from different fuels, especially carbohydrates and fats.

For endurance athletes, the goal is usually to sustain a steady effort for as long as possible. That's where metabolic flexibility plays an important role.

If the body can use more fat at low and moderate intensities, it doesn't have to rely as heavily on glycogen at that same pace. That can help keep carb stores in reserve for later in the session, when fatigue usually ramps up.

Put simply: better metabolic flexibility can help the body produce energy for longer, with less reliance on limited glycogen stores.

Carbs don't become any less important because of this. They remain essential for higher intensities, surges and race-specific efforts. But a more flexible metabolism can help endurance athletes manage their energy more efficiently during long sessions.

5 Day Fast can indirectly support this by temporarily making less carbohydrate energy available and nudging the body more towards using fat and ketones.

Can better fat burning improve your endurance?

Better fat burning supports endurance. The main reason for that is glycogen sparing.

During long efforts, glycogen depletion is one of the biggest limiting factors. If the body can produce more energy from fat at that same low or moderate intensity, it might use less glycogen early on in the session (3,4). That helps save carb stores and can delay fatigue.

In practice, better fat burning can help endurance athletes to:

• keep up a steady effort for longer

• stave off energy dips during long sessions

• save glycogen for harder moments

• improve fuel efficiency at low and moderate intensity

However, this only works within certain limits. Fat provides energy more slowly than carbohydrates. At higher intensities, carbs remain essential.

So, better fat burning doesn't mean you can ignore carbs, train less, or automatically perform better. It means the body can become more efficient at using the right fuel at the right time.

Research in endurance athletes therefore looks at fat oxidation (using fat as an energy source), metabolic flexibility and glycogen sparing after a fasting period. Results vary from person to person.

When should you schedule 5 Day Fast around your training?

The potential benefits usually aren't expected during the 5 days themselves.

During the programme, energy intake is lower. Carbs are less available. Training can feel tougher. Your energy might dip, especially before the shift towards using more fat and ketones. That's all part of the process.

The potential benefit comes afterwards, once your body has access to:

• normal nutrition

• glycogen stores

• training intensity

• recovery

• plenty of available energy

This is the moment when you can really feel the adaptations in fat burning and metabolic flexibility.

💡 For many athletes, it makes sense to schedule 5 Day Fast during a lower-intensity training week. Don't do it right before an important race or during a heavy block of training: finishing 5 Day Fast nine to sixteen days before a race is a solid margin. Allowing more time, say 2 to 4 weeks, is sensible if it's your first time doing 5 Day Fast.

When can a 5 Day Fast actually hurt your athletic performance?

5 Day Fast can work against athletic performance if it's done at the wrong time.

That can happen when it's used:

• too close to a race, like in the week leading up to it

• during a heavy training week

• when you're really fatigued or low on energy

• when carrying an injury or needing extra recovery

• without properly building back up from day six

• without a gradual return to intense training

The reason is simple: 5 Day Fast is still a stressor. Useful, yes, but still a stressor.

If your body is already under too much stress, extra energy restriction can drag down training quality, slow recovery and worsen performance.

For athletes, the aim is to apply the right stress at the right time, recover properly afterwards, and let the potential benefits shine through on race day.

How do you schedule 5 Day Fast around training or racing?

For most athletes, 5 Day Fast should be scheduled well away from important races and heavy training weeks.

A practical approach is to do it during a lower-intensity training week. During the 5 days, the goal is maintenance, light movement and recovery, not performance.

General guidelines:

• pick a low-intensity training week

• avoid hard interval sessions during the 5 days

• keep movement light and controlled

• gradually build carbs and fats back up afterwards

• restore protein intake after the programme

• ease back into intense training gradually

• allow enough time to replenish glycogen and rebuild your training rhythm

When it comes to timing:

• if you're experienced with 5 Day Fast: finish the programme about 9 to 16 days before a race or performance-focused event

• if you've never done 5 Day Fast before: finish the programme 2 to 4 weeks before a race or performance-focused event

The main principle is simple: don't combine severe energy restriction with excessive training stress.

Conclusion: fasting, fat burning and endurance sports

By lowering energy and carbohydrate availability, a fasting mimicking programme like 5 Day Fast is partly designed to nudge the body towards using more fat, producing ketones and increasing metabolic flexibility.

For endurance athletes, that's relevant, because better fat use can help spare glycogen, keep energy levels steadier and improve fuel efficiency during long sessions.

However, the effect comes down to timing, training, nutrition, recovery and how you manage the phase after the programme.

You shouldn't expect the benefits during the programme itself. They come afterwards, once your normal nutrition, glycogen stores and training intensity are back on track.

Planned well, 5 Day Fast fits into a training year, as long as you take timing, nutrition and recovery seriously. Used poorly, it simply becomes extra stress.

Want to understand the broader link between 5 Day Fast and athletic performance? Have a look at our guide on 5 Day Fast and athletic performance.