Losing weight and endurance sport: can weight loss improve your performance?

In endurance sport, body weight can affect your performance because moving less mass often costs less energy. That is especially true for running, trail running, and other sports where you carry your own body weight, but also for cycling and triathlon when climbing or the power-to-weight ratio becomes important. Yet, losing weight is not automatically good for your performance. If weight loss comes at the expense of energy availability, recovery, muscle mass, or training quality, an athlete can actually become slower instead of faster.

Why can weight loss improve endurance performance?

In endurance sport, body weight does not have the same influence on performance in every discipline.

In sports where you carry your own body weight, such as running, trail running, climbing, or hiking, your body has to move its own mass with every step. The heavier the body, the more mechanical work is needed to move forward, climb, accelerate, or keep up a pace for a long time.

Therefore, lowering unnecessary body mass can reduce the energy needed per movement. If an athlete can deliver the same power or speed with less weight, the same effort can become more efficient.

In practice, well-planned weight loss can help with:

• better running economy

• lower energy requirements during long efforts

• more efficient climbing

• better power-to-weight ratio

• less fatigue during prolonged effort

This is especially relevant in sports such as:

• running

• trail running

• cycling, especially during climbing

• triathlon

• hyrox

• team sports where running is important

The quality of the weight loss remains important though. Losing muscle mass, lowering energy availability, or reducing training quality can actually harm your performance.

Why does the power-to-weight ratio matter?

Endurance performance is not just about absolute power. It is also about how much power you can deliver in relation to your body weight. That is often called the power-to-weight ratio.

In running and trail running, this is important because your body has to move its own mass with every step.

In cycling, body weight plays a smaller role on flat roads because the bike supports the weight and air resistance often becomes the main limiting factor. But as soon as the road goes up, gravity becomes much more important. At that point, less body mass can improve the power-to-weight ratio and make climbing more efficient.

Weight loss can therefore support performance when it improves the power-to-weight ratio without decreasing power, recovery, or training capacity.

If an athlete loses fat mass while maintaining strength, muscle mass, and training quality, performance can improve. But if weight loss also reduces power, for example due to muscle loss, causes more fatigue, or makes training worse, the athlete can become lighter without becoming faster.

The goal is therefore not simply a lower weight. The goal is to improve the relationship between body composition, power, recovery, and available energy.

Why can losing weight actually reduce sports performance?

If weight loss is poorly planned, it can quickly backfire.

Your body needs enough energy to train, recover, and adapt. If the energy deficit is too large or lasts too long, an athlete may lose more than just weight.

Aggressive weight loss can contribute to:

• lower available energy 

• harder recovery

• lower training quality

• loss of muscle mass

• more fatigue

• weaker immune function

• higher injury risk

Muscle mass remains important for endurance athletes too. It supports force production, stability, recovery, and fatigue resistance.

Therefore, weight loss should not be judged solely on the number on the scales. If the process makes an athlete less able to train, recover, or maintain power, the end result can actually be worse performance.

Periodisation of body composition can be useful for athletes, but must be carefully planned so as not to disrupt recovery and training quality (1).

What is the risk of having too low available energy?

Energy availability means the energy left over for normal bodily functions after the energy for training has been taken into account.

Simply put: if an athlete trains hard but does not eat enough, there may be too little energy left for other functions, such as recovery.

When low energy availability becomes chronic, the consequences can go beyond temporary fatigue or a poor training week. In the long term, this can affect both health and performance. In sports science, this broader problem is described as Relative Energy Deficiency in Sport, or RED-S (2).

Low energy availability can affect:

• recovery

• hormonal function

• bone health

• immune function

• injury risk

• training adaptation

• performance

This does not mean athletes should never lose weight. It means weight loss must be planned in such a way that the ability to train, recover, and adapt remains protected.

When can weight loss be interesting for endurance athletes?

Weight loss can be relevant for endurance athletes when it happens at the right time and for the right reason.

It can be useful:

• during a lower-intensity training week

• far enough from a competition/race

• when there is excess fat mass

• when training quality can be maintained

• when recovery remains stable

• when protein intake is sufficient after a phase of weight loss

• when strength training is added or maintained

Athletes should also keep in mind that performance during the weight loss phase is often not optimal, or can even be disappointing. That is why weight loss is usually better planned during a quieter training block, during a rest week, not during peak training or in the week before a race.

The goal is, if necessary, to accept a controlled temporary drop in freshness or intensity in exchange for a potential longer-term benefit, once body composition, fueling, and training quality have been rebuilt.

💡 Remember that the goal is not to reach the lowest possible body weight. The goal is to find the best balance between body composition, power, recovery, and energy availability. For endurance athletes, being slightly lighter is only useful if your body can still perform well.

How does 5 Day Fast fit into a sports strategy focused on performance?

5 Day Fast is a structured 5-day fasting mimicking protocol. The goal is not only to lower body weight, but also to temporarily change the metabolic environment.

During a fasting-like period, the body can shift towards mechanisms focused on fat use, ketone production, and metabolic flexibility. These mechanisms are relevant for endurance sport because they contribute to more efficient energy use.

From a body composition perspective, 5 Day Fast is also developed to retain muscle mass better than a classic prolonged fasting protocol. The program still contains controlled nutrition and also glycerol, a supplement that fits within fasting mimicking protocols and can serve as a carbon source during an energy deficit. In this context, glycerol contributes to supporting muscle retention (3).

That is important, because endurance performance is not just about losing weight. The core is improving the power-to-weight ratio: reducing unnecessary fat mass, while maintaining muscle mass, power, recovery, and training quality as well as possible.

When should you not plan 5 Day Fast around sports?

You had better not plan 5 Day Fast when your body needs maximum energy for training, racing, or recovery.

Avoid the program:

• in the week of your competition or race

• during a heavy training block

• in case of high fatigue

• in case of injury or illness

• during a period of great fatigue

• when your recovery needs are high

• without a clear total nutrition strategy before and after the program

During the 5 days, your energy intake is lower. As a result, your training capacity may temporarily decrease.

Here, the goal is to apply the right stimulus at the right time, then recover well and only later make room for potential benefits.

💡 For optimal sports performance improvement, we see that you get the best effects if you start 5 Day Fast 14-15 days before your competition or race. It is important here to take very good account of the training sessions you plan immediately before and in the two days after 5 Day Fast. Have you never done anything with fasting, training on an empty stomach, and/or getting into ketosis and do you want more certainty about the effects of 5 Day Fast on your sports performance? Then plan it in an earlier training block once and experience what it does to your performance 8-10 days after you finish 5 Day Fast.

Conclusion: can losing weight improve your endurance performance?

Yes, weight loss can improve endurance performance.

It helps when it improves running economy, climbing efficiency, or power-to-weight ratio without decreasing power, recovery, or training quality.

But weight loss can also harm performance when it reduces energy availability, muscle mass, immune function, or the ability to train well.

A program like 5 Day Fast works within a broader training and nutritional approach, but only when it is well timed and followed by refuelling in the right way and sufficiently.

Remember: the goal is not to get as light as possible. The goal is to be light enough, strong enough, and well-nourished enough to perform.