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

In endurance sport, body weight can affect your performance, as shifting less mass often takes less energy. That's 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 your power-to-weight ratio becomes important. Still, losing weight isn't automatically good for your performance. If weight loss comes at the expense of energy availability, recovery, muscle mass, or training quality, you might actually end up slower instead of faster.

Why can weight loss improve endurance performance?

In endurance sport, body weight doesn't have the same impact on performance across 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 single step. The heavier your body, the more mechanical work is needed to move forward, climb, accelerate, or hold a pace for a long time.

That's why reducing unnecessary body mass can lower the energy needed per movement. If you can produce the same power or the same speed with less weight, that same effort can become more efficient.

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

• better running economy

• lower energy demands during long efforts

• more efficient climbing

• a better power-to-weight ratio

• less fatigue during sustained efforts

This is particularly relevant in sports like:

• running

• trail running

• cycling, especially when climbing

• triathlon

• hyrox

• team sports where running is key

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

Why does the power-to-weight ratio matter?

Endurance performance isn't just about absolute power. It's also about how much power you can produce relative to your body weight. That's often called the power-to-weight ratio.

In running and trail running, this matters because your body has to shift its own mass with every step.

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

So weight loss can support performance when it improves your power-to-weight ratio without your power, recovery, or training capacity dropping.

If you lose fat mass while maintaining strength, muscle mass, and training quality, your performance can improve. But if weight loss also reduces your power—for example, through muscle loss, causing more fatigue, or making training worse—you might get lighter without actually getting faster.

So the goal isn't simply a lower weight. The goal is to improve the balance between body composition, power, recovery, and available energy.

Why can losing weight actually reduce athletic 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, you might lose more than just weight.

Aggressive weight loss can contribute to:

• lower energy availability 

• harder recovery

• lower training quality

• loss of muscle mass

• more fatigue

• a less robust immune function

• higher injury risk

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

That's why weight loss shouldn't just be judged by the number on the scales. If the process stops you from training well, recovering, or maintaining power, the end result might actually be poorer performance.

Periodising body composition can be useful for athletes, but it needs to be carefully planned so it doesn't disrupt recovery and training quality (1).

What is the risk of having low energy availability?

Energy availability means the energy left over for normal bodily functions after the energy used for training is accounted for.

Put simply: if you train hard but don't eat enough, there might not be enough energy left for other functions, like recovery.

When low energy availability becomes chronic, the consequences can go beyond temporary tiredness or a bad week of training. Over time, this can affect both health and performance. In sports science, this broader issue is known as Relative Energy Deficiency in Sport, or RED-S (2).

Low energy availability can impact:

• recovery

• hormonal function

• bone health

• immune function

• injury risk

• training adaptation

• performance

This doesn't mean athletes should never lose weight. It means weight loss should be planned in a way that protects your ability to train, recover, and adapt.

When can weight loss be worth considering for endurance athletes?

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

It can be useful:

• during a lower-intensity training week

• far enough out from a race or competition

• when there is excess fat mass

• when training quality can be maintained

• when recovery stays stable

• when protein intake is sufficient after a weight loss phase

• when strength training is added or kept up

Athletes also need to keep in mind that performance during a weight loss phase is often not optimal, or might even be disappointing. That's why weight loss is usually better planned during a quieter training block, like a rest week, and not during peak training or the week before an event.

The aim is to accept a controlled, temporary dip in freshness or intensity if needed, in exchange for a potential longer-term benefit once body composition, fueling, and training quality have been built back up.

💡 Remember that the goal isn't 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 a bit lighter is only useful if your body can still perform properly.

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

5 Day Fast is a structured 5-day fasting mimicking protocol. The goal isn't just to drop body weight, but also to temporarily alter your metabolic environment.

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

From a body composition perspective, 5 Day Fast was also developed to help preserve muscle mass better than a traditional prolonged fast. The programme still includes controlled nutrition as well as glycerol, a supplement that fits within fasting mimicking protocols and can serve as a carbon source during an energy deficit. In this context, glycerol helps support muscle retention (3).

That matters, because endurance performance isn't just about dropping weight. The core is improving your power-to-weight ratio: reducing unnecessary fat mass while preserving muscle mass, power, recovery, and training quality as much as possible.

When shouldn't you plan 5 Day Fast around your sport?

It's better not to plan 5 Day Fast when your body needs maximum energy for training, racing, or recovery.

Avoid the programme:

• in the week of your race or competition

• during a heavy training block

• when you're very fatigued

• during an injury or illness

• during a period of severe fatigue

• when your recovery needs are high

• without a clear overall nutrition strategy before and after the programme

During the 5 days, your energy intake is lower. Because of that, your training capacity might temporarily take a hit.

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

💡 If you want to combine 5 Day Fast with a race, start at least 14 days before your race or competition. It's really important to carefully consider the sessions you plan right before and in the two days after 5 Day Fast. Never tried fasting, fasted training, and/or getting into ketosis before and want to see how 5 Day Fast fits into your training? Plan it in a quieter training block first so you can see how it works for you.

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 lowering power, recovery, or training quality.

However, weight loss can also harm performance when it reduces energy availability, muscle mass, immune function, or your ability to train properly.

A programme like 5 Day Fast works within a wider training and nutrition approach, but only when it's timed well and followed by proper, adequate refueling.

Remember: the goal isn't to get as light as possible. The goal is to be light enough, strong enough, and fueled well enough to perform.