How much faster do you get at running if you lose weight?

Lots of runners wonder how much faster they can get by losing weight. The idea makes sense: if you have less unnecessary body weight to carry around, each step will take slightly less energy, especially over longer distances. But the answer isn't as simple as "lose weight and run faster". The real question is what kind of weight you lose, how much strength and recovery you maintain, and whether your training quality stays high enough to turn a lower body weight into better performance.

How does body weight affect your running speed?

When you run, your body has to absorb impact, generate force, and move forward with every single step.

That means body weight affects the mechanical and energetic cost of running.

If you're carrying more weight, your muscles, tendons, and cardiovascular system have to work harder to maintain the same pace. Over one step, that difference might seem tiny. But over thousands of steps, it really adds up.

That is why body weight is almost always more important in running than in sports where part of your body weight is supported, like cycling and swimming.

If two runners have the same fitness level, running technique, pacing, and strength, the runner with less unnecessary weight might need less energy to run at the same speed. That runner is running more economically.

What is running economy?

Running economy means how much energy you need to run at a certain pace. If two runners are running at the same speed, the runner who uses less energy is more economical.

That's important, because distance running isn't just about how fast you can run. It's also about how long you can sustain a pace before fatigue builds up.

Research shows that running economy is strongly linked to distance running performance, especially among runners with a similar VO2max (1).

Body weight can affect running economy because the body has to move, stabilise, and absorb that weight with every step. When excess fat mass decreases, the energy required to move the body also decreases.

But this only helps if the runner maintains the qualities that make performance possible: muscle function, strength, energy availability, recovery, and training.

So the goal isn't simply to get lighter. The goal is to reduce excess fat mass while keeping the physical qualities that help you run well.

How much time can you save if you lose weight?

The calculator, developed by Koen De Jong, founder of Sportrusten and ProRun and author of several books on running, breathing, and nutrition, makes this question more practical.

The calculator can estimate how much time you could theoretically save if you lose weight. You enter your current body weight, distance, current running time, and expected weight loss.

Then the calculator gives an estimated time difference.

For example: if a runner weighs 65 kg and runs 5 km in 25:00, the calculator estimates a time of 23:05 at 60 kg. That would be a theoretical gain of 1 minute and 55 seconds.

For a 75 kg runner with the same 5 km time of 25:00, the calculator estimates a time of 23:20 at 70 kg. That would be a theoretical gain of 1 minute and 40 seconds.

Why does the result differ per weight?

The same weight loss doesn't always have the same theoretical effect.

Losing 5 kg from 65 kg is relatively a bigger change than losing 5 kg from 75 kg. Losing 5 kg from 65 kg means a decrease of about 7.7% of body weight, whereas losing 5 kg from 75 kg is a decrease of about 6.7%. In theory: the larger the relative drop in body weight, the larger the potential effect on the energy demands of running. That is why the same 5 kg weight loss in this example can show a slightly larger time saving for the lighter runner.

Use the calculator as an estimate

Use the calculator to estimate what weight loss could theoretically mean for your running time. You can test different distances, like 5 km, 10 km, half marathon, or marathon.

Click here to open the calculator.

Just bear in mind that this isn't an exact science: the calculator doesn't know your height, body composition, muscle mass, training history, running technique, fatigue level, fueling, or race conditions.

You'll only run faster if the weight loss lowers the energy cost of running without harming the physical qualities you need to perform.

Why lighter isn't always better for runners

Lots of runners assume that a lower body weight always means better performance. That's too simplistic.

Losing fat mass can be useful if it reduces unnecessary weight. Losing muscle mass, on the other hand, can be harmful.

Muscle mass isn't just extra weight. Muscles help you produce force, stabilise your body, maintain your posture, and keep your running technique efficient as fatigue sets in. In other words: muscles are part of your performance system.

If weight loss is too aggressive, training quality can suffer. You might have less energy, recover slower, sleep worse, or struggle to complete harder sessions.

That is especially risky during periods of high training load.

The risk of too little energy

Runners need enough energy to train, recover, and adapt.

If energy availability stays too low for too long, for example during an aggressive weight loss phase, the body has less capacity to recover from training. Performance can drop, fatigue can increase, and injury risk can also go up.

Weight loss should support your training, or at least not disrupt it.

The goal is to improve your body composition without losing the physical capacities that help you train, recover, and race well.

Losing fat mass without losing muscle mass

Weight loss and fat loss are not the same thing.

Your weight on the scales is made up of fat mass, muscle mass, water, glycogen, gut content, and other tissues.

If you lose weight quickly, it's not automatically fat loss. And it's almost never just fat loss. Part of the drop can come from water and glycogen.

When blood glucose availability drops, the body uses stored glycogen from the muscles and liver. Glycogen is stored alongside water. Research in humans has estimated that each gram of glycogen can be stored with about 3 to 4 grams of water (2).

That explains why weight can drop quickly at the start of a diet, fasting period, or low-carbohydrate phase. It doesn't mean all that weight is fat.

If your approach is too aggressive, some of the weight loss can also come from muscle tissue. This is called catabolism.

Catabolism means the body breaks down tissue or molecules to produce energy. During a severe energy deficit, especially if protein intake, training, and recovery aren't properly adjusted, the risk of losing fat-free (muscle) mass can increase.

A runner who loses weight but also loses strength, energy, and recovery capacity won't perform better. In some cases, performance might actually get worse.

How do you use weight loss without sabotaging your training?

The goal is better quality weight loss.

For runners, that means: reducing excess fat mass while maintaining muscle mass, recovery capacity, energy levels, and training quality as much as possible.

This depends on several factors:

• protein intake

• strength training or muscle-preserving stimuli

• high-quality sleep

• timing of the weight loss phase

• fueling around and during key sessions

• gradual rebuilding after restriction

A practical protein range during weight loss can be around 1.2 to 1.6 grams of protein per kilogram of body weight per day, depending on the runner, training load, and total energy intake. Research in athletes shows that protein intake during energy restriction can affect the preservation of fat-free mass (3).

Timing is also important.

A weight loss phase creates stress. Training also creates stress. If you combine aggressive weight loss with your heaviest training block, both compete for the same energy demands and recovery capacity.

That is why weight loss usually fits better in a quieter training phase. The quality of the weight loss determines whether losing weight can actually support your running performance.

How much time should you leave before a race?

As a practical safe rule, it's better not to plan a conscious weight loss intervention too close to race day.

If the approach includes a short but structured restriction period, leave at least 13 to 14 days before a major race.

This gives you time to rebuild your nutrition, restore glycogen, normalise training intensity, and recover from the energy restriction.

If it's your first attempt, more margin is better. Two to four weeks before a major event is usually more comfortable, as you don't yet know exactly how your body will react.

The closer you get to race day, the less room there is for experimentation.

Practical guidelines

Use weight loss smarter by:

• choosing a quieter training phase

• avoiding aggressive restriction during peak weeks

• keeping easy runs genuinely easy

• protecting protein intake

• prioritising sleep and recovery

• fueling around and during key sessions

• rebuilding carbohydrate intake before key workouts

• leaving at least 13 to 14 days before a major race

• not using restriction as compensation for overeating

Also pay attention to warning signs.

Low energy, poor sleep, irritability, slower recovery, heavy legs, or recurring niggles can all suggest the strategy is too aggressive.

In that case, the goal is to adjust the strategy.

When can 5 Day Fast be interesting for runners?

5 Day Fast can be interesting for runners who want to work on body composition and metabolic flexibility in a structured way.

It is a 5-day fasting mimicking programme. You don't stop eating completely, but follow a controlled structure that tries to mimic certain effects of fasting.

During the programme, energy intake and carbohydrate availability are temporarily lower. This can support a shift towards more fat utilisation and ketone production.

For runners, this can be relevant for two reasons.

Firstly, better fat utilisation and metabolic flexibility are relevant for energy efficiency during longer efforts. Secondly, a short structured intervention can give runners a clearer framework to reduce excess fat mass than a vague or undefined crash diet.

But this doesn't mean 5 Day Fast directly improves running performance. The potential value depends on timing, recovery, and what happens after the 5 days.

What happens to your muscle mass?

For runners, muscle preservation is important.

5 Day Fast is designed to preserve muscle mass through the structure and supplements that are part of the programme.

The programme also contains glycerol. Glycerol is involved in energy metabolism and is used in fasting mimicking diet protocols as part of a broader strategy aimed at limiting muscle mass loss.

Research on fasting mimicking diet cycles has shown drops in body weight and fat-related markers, while one study comparing fasting mimicking diet cycles to a Mediterranean diet found that loss of lean mass during follow-up occurred in the Mediterranean diet group, but not in the FMD group (4).

Of course, training design, protein and energy intake after the programme, sleep, recovery, and the return to normal nutrition remain important.

When do you plan it?

5 Day Fast shouldn't be done during race week.

It also shouldn't be planned during your heaviest training weeks, when your body needs more energy to train and recover.

For runners, it fits better in a quieter training phase, recovery week, or period where peak performance isn't the immediate goal.

After the programme, normal nutrition, protein intake, glycogen stores, and training intensity need to be rebuilt.

A good margin is to start 5 Day Fast no later than 13 to 14 days before your race.

This gives you time to restore glycogen stores, rebuild carbohydrate intake, return to normal training, and assess how your body feels.

Conclusion: running faster by losing weight is possible, but only if the quality is right

Losing weight can theoretically help you run faster.

If you carry less unnecessary body weight, it can take less energy to maintain the same pace.

Koen's calculator is useful because it turns this idea into a concrete estimate. It helps runners see how weight loss can theoretically affect a time over 5 km, 10 km, half marathon, or marathon.

But the calculator should be used as a tool, not a promise.

Body composition, muscle preservation, recovery, fueling, training quality, pacing, and race conditions all count towards performance.

Weight loss can only support running performance if the right weight is lost and training, recovery, and muscle mass are preserved.

Use the calculator to understand the potential effect. Then build your strategy around the part that matters most: losing weight without losing performance.