Does Running Make You Lose Muscle? 7 Key Facts

Running does not automatically burn muscle. But high mileage, underfuelling, too little protein and dropping strength work can make muscle loss more likely. Here is what the evidence actually says.

Running does not automatically burn muscle. But the combination of high mileage, insufficient food, limited strength training and poor recovery can make muscle loss more likely.

You start running alongside your gym sessions. A few weeks later, the scales are down, your legs look slightly less full and someone helpfully informs you that cardio is “killing your gains”.

It is an alarmingly dramatic conclusion to draw from a pair of tired quadriceps.

The relationship between running and muscle is more complicated than the familiar comparison between a marathon runner and a bodybuilder suggests. Running creates a training stimulus, increases your energy needs and places repeated demands on the muscles of your legs. Whether you maintain, gain or lose muscle depends on how that running fits around your strength training, nutrition and recovery.

So, does running make you lose muscle? Usually, no. However, running can contribute to muscle loss when it creates an energy deficit you do not replace, leaves you too fatigued to lift effectively or becomes the only training stimulus your muscles receive.

Here are seven truths worth understanding before you blame your next easy run for stealing your glutes.

QUICK ANSWER

Running does not directly or inevitably cause muscle loss. Research suggests that combining aerobic and strength training generally does not prevent muscle growth or maximal-strength gains. The risk rises when a runner persistently underfuels, stops resistance training, increases mileage too quickly or cannot recover from the total workload. To protect muscle, continue strength training, eat enough overall, consume adequate protein and carbohydrate, and organise hard sessions so you can perform them well.

WHAT DOES “LOSING MUSCLE” ACTUALLY MEAN?

Before deciding that running has reduced your muscle mass, it helps to separate four things that are often confused:

What changed?	What it may mean
Your body weight dropped	You may have lost fat, water, glycogen, muscle or a mixture
Your muscles look flatter after a long run	Glycogen and associated water may be temporarily lower
Your legs feel weaker for a day or two	This may be ordinary training fatigue rather than tissue loss
Your strength and measurements decline for several weeks	This is more consistent with inadequate recovery or possible muscle loss

Muscle loss is a reduction in muscle tissue over time. It cannot be diagnosed from one weigh-in, one difficult gym session or the way your legs look after a dehydrating run.

Muscles store carbohydrate as glycogen, and glycogen is stored with water. A long or demanding run can reduce those stores, making muscles appear temporarily less full. Rehydrating, eating carbohydrate and recovering can reverse that appearance without a gram of new muscle tissue being built.

Consumer smart scales are also poor at detecting small, short-term changes in muscle. Hydration, food intake and the timing of your last workout can all shift their estimates. Look for trends across several weeks rather than allowing Tuesday morning’s bathroom scale to hold an emergency meeting about your quadriceps.

1. RUNNING DOES NOT AUTOMATICALLY “BURN” MUSCLE

Your body does not reach a particular mile, run out of patience and begin dismantling your biceps for fuel.

During running, energy comes mainly from carbohydrate and fat. Amino acids can contribute a small amount, particularly during prolonged exercise or when carbohydrate availability is low, but using some amino acids during a session is not the same as losing measurable muscle mass over time.

Muscle is constantly being broken down and rebuilt. What matters is the longer-term balance between muscle protein breakdown and muscle protein synthesis, alongside the training stimulus telling your body which tissue it needs to retain.

The strongest reassurance comes from research on concurrent training – the scientific term for combining aerobic and resistance exercise. An updated meta-analysis of 43 studies found that concurrent training did not compromise muscle hypertrophy or maximal-strength development compared with strength training alone.

That does not mean programming never matters. The same analysis found that gains in explosive strength could be reduced, particularly when aerobic and resistance work were performed in the same session. But “slightly less improvement in explosive power under certain conditions” is very different from “running eats muscle”.

For most recreational runners, there is no need to choose between having an aerobic system and owning a functioning set of hamstrings.

2. THE BIGGER RISK IS OFTEN UNDERFUELLING

Running increases how much energy you use. If your food intake stays exactly the same while your weekly mileage climbs, you may unintentionally create a substantial energy deficit.

When that deficit is small and temporary, the body can generally cope. When it becomes large or persistent, maintaining muscle becomes more difficult. A meta-analysis examining resistance training during an energy deficit found that insufficient energy impaired gains in lean mass, even when strength could still improve.

This is why the question is not simply, “How much am I running?” It is also, “Am I eating enough to support how much I am running?”

Low energy availability occurs when too little energy remains for normal physiological function after exercise demands have been accounted for. It can affect runners of every sex and body size. The International Olympic Committee’s consensus statement on Relative Energy Deficiency in Sport links problematic low energy availability with consequences affecting health, recovery and performance.

Possible warning signs include:

unintended weight loss;
persistent fatigue or irritability;
declining performance despite continued training;
poor recovery and repeated injuries;
disrupted or absent periods;
reduced libido;
feeling unusually cold;
disturbed sleep;
frequent illness.

None of these proves that you are losing muscle, and each can have several causes. Together, however, they are a reason to review your training and food intake. Our guide to the signs of underfuelling in runners explains the wider pattern.

Trying to increase mileage, lose weight rapidly and build muscle at the same time asks your body to pursue three demanding goals with one recovery budget. It may manage for a while. It is unlikely to send a thank-you card.

3. RUNNING CANNOT REPLACE PROGRESSIVE STRENGTH TRAINING

Running uses your calves, quadriceps, hamstrings, glutes and core. That does not make every run an effective muscle-building session.

Steady running involves thousands of relatively low-force contractions. Your muscles become better at resisting fatigue, using oxygen and producing repeated movement economically. Those are valuable endurance adaptations, but they are not identical to the progressive overload created by squats, deadlifts, lunges, presses, rows and calf raises.

If you previously lifted three times per week, begin marathon training and quietly remove every strength session, some loss of muscle or strength becomes more plausible. Running may be blamed, but the missing resistance stimulus is part of the explanation.

To maintain muscle, you do not necessarily need a bodybuilding programme. Two well-planned full-body sessions each week can provide a useful signal, particularly when they include:

a squat or split-squat pattern;
a hip hinge, such as a Romanian deadlift;
a calf exercise using a straight and/or bent knee;
an upper-body push;
an upper-body pull;
trunk and hip stability work.

The goal is not to leave the gym unable to use stairs. It is to expose the major muscle groups to meaningful resistance and retain enough consistency to progress or maintain the load. See our complete guide to combining running and strength training for weekly scheduling and exercise selection.

4. HIGH MILEAGE CAN MAKE MUSCLE GAIN HARDER – WITHOUT MAKING IT IMPOSSIBLE

Running and lifting demand different adaptations, but the body is capable of developing both. The difficulty usually comes from the total dose.

Someone running three times per week while completing two strength sessions has a very different recovery challenge from someone running 100 kilometres per week, lifting four times and trying to add muscle as quickly as possible.

As running volume rises, several things can happen:

more energy is required simply to maintain body weight;
leg fatigue can reduce the quality of strength sessions;
soreness and tissue stress can accumulate;
sleep and recovery become more important;
time available for resistance training shrinks;
maximising muscle growth becomes less compatible with maximising endurance performance.

This is sometimes called the interference effect. It is real, but frequently exaggerated. The updated concurrent-training research found no overall reduction in muscle hypertrophy or maximal strength. Explosive strength was the outcome most likely to be affected, particularly when both training modes were crammed into the same session.

The practical lesson is not to avoid running. It is to decide which quality currently has priority.

If your primary goal is a marathon PB, your strength training may aim to maintain muscle, force production and resilience rather than maximise leg size. If your main goal is hypertrophy, you might keep running but limit the amount of demanding endurance work that compromises lifting performance and recovery.

You can pursue both. You simply cannot always maximise both at exactly the same time.

5. PROTEIN HELPS – BUT IT CANNOT RESCUE AN EMPTY PLATE

Protein provides the amino acids used to repair and remodel tissue after running and resistance training. Endurance athletes generally need more than sedentary adults because training increases protein turnover and recovery demands.

For many runners, roughly 1.2–1.6g of protein per kilogram of body weight per day is a useful general range. Runners completing heavy training, combining running with hypertrophy-focused lifting or deliberately losing weight may benefit from working towards the upper end – or seeking individual guidance from a registered sports dietitian.

For context:

Body weight	1.2g/kg	1.6g/kg
55kg	66g	88g
65kg	78g	104g
75kg	90g	120g
85kg	102g	136g

A small controlled study involving endurance-trained men estimated higher needs after a 20km run, although its six-person sample and demanding protocol mean it should not become a universal target for every runner. The more useful conclusion is that protein requirements can rise with training load – not that everyone needs an industrial-sized shaker.

Spread protein across the day rather than placing the entire responsibility on dinner. Meals and snacks containing approximately 20–40g can make this easier, depending on body size and total needs. Useful options include Greek yoghurt, eggs, milk, fish, lean meat, tofu, tempeh, lentils, beans and soy products.

Protein powder can be convenient, but it is not compulsory. The relevant question is whether it helps you meet your overall intake. Our guide to the best supplements for runners explains where protein powder and creatine may – and may not – be useful.

Most importantly, protein does not cancel out chronic underfuelling. A high-protein yoghurt cannot single-handedly compensate for a day that is hundreds of calories short of supporting your training.

6. CARBOHYDRATE PROTECTS THE QUALITY OF YOUR TRAINING

Muscle-preservation advice often focuses entirely on protein. Runners also need carbohydrate.

Carbohydrate is stored as glycogen in the muscles and liver and provides important fuel for moderate- and high-intensity running. If glycogen is repeatedly low, pace can fall, perceived effort can rise and the quality of both running and strength sessions may suffer.

Adequate carbohydrate supports muscle indirectly by helping you:

complete effective strength sessions;
maintain intensity during harder runs;
recover glycogen between sessions;
reduce the need to create energy from other substrates;
meet your overall energy requirements.

This does not mean every 30-minute easy run requires a ceremonial bowl of pasta. It means matching intake to workload. A rest day, an easy 5K and a 30km long run do not create identical fuelling demands.

If you regularly finish long runs depleted, struggle through evening gym sessions or feel as though every workout starts with an empty battery, review your carbohydrate intake as well as your protein. Our carbohydrate guide for runners covers everyday intake and before-, during- and after-run fuelling.

7. SOME RUNNING CAN BUILD MUSCLE

Running is not the most reliable route to maximum hypertrophy, but it is not physically incapable of stimulating muscle growth.

New runners may gain some lower-body muscle because running provides a novel load. Hills, short sprints and faster intervals recruit more high-threshold muscle fibres and require greater force than relaxed, level jogging. People returning from inactivity may also experience early strength and muscle changes simply because the stimulus is new.

However, adaptation is specific. Your body becomes efficient at meeting the demand you repeatedly impose. Long-distance running prioritises economical force production and fatigue resistance; bodybuilding prioritises progressively increasing muscular tension and volume.

That is why sprinting and hills can complement strength, but they do not make resistance training redundant. Running might build some muscle. A progressive strength programme is far more controllable if muscle growth is the actual goal.

HOW TO RUN WITHOUT LOSING MUSCLE

You do not need a complicated “hybrid athlete” protocol. Start with the foundations.

1. Keep at least two strength sessions

Train the major muscle groups consistently. Use loads that feel challenging while keeping good technique, and record enough information to notice whether strength is stable or declining.

If you are in a high-mileage phase, reduce unnecessary gym volume before removing strength completely. A shorter, focused session is more useful than a heroic workout that ruins the next three days of running.

2. Replace the energy your running requires

You do not have to match a watch’s calorie estimate with mathematical precision. You should, however, notice whether mileage has risen while meals have stayed small and hunger, fatigue or weight loss have increased.

Add food around demanding sessions, increase portions on high-volume days and avoid treating rest days as punishment days with dramatically reduced intake. Recovery still uses energy.

3. Eat enough protein across the day

Aim for a consistent daily total rather than obsessing over a 30-minute “anabolic window”. Include a meaningful protein source in several meals and snacks. A post-training meal is useful; missing a shake by 17 minutes is not a physiological emergency.

4. Fuel hard and long runs with carbohydrate

Begin important sessions adequately fuelled and practise taking carbohydrate during longer runs when appropriate. This supports training quality and makes it easier to meet overall energy needs.

5. Separate demanding sessions when possible

If muscle, maximal strength or explosive power is a high priority, separate hard running and heavy lower-body lifting by several hours or place them on different days. The concurrent-training meta-analysis found less concern for explosive-strength development when sessions were separated by at least three hours.

That is not a magic cut-off. It simply gives fatigue time to fall and creates an opportunity to eat, rehydrate and approach the second session with more quality.

If you must combine them, complete the priority session first. Someone focused on muscle gain may lift before running. Someone preparing for an important race may complete the key run first. An easy jog alongside upper-body training is a much smaller conflict than intervals followed immediately by heavy squats.

6. Keep easy running genuinely easy

Turning every run into moderate-intensity grey-zone work adds fatigue without necessarily producing the best endurance stimulus. Easy running should leave room for your key sessions and strength work.

7. Monitor trends, not single bad days

Useful signs include:

body weight trending down unintentionally;
gym performance declining across several weeks;
persistent soreness or fatigue;
difficulty completing usual running paces;
disrupted periods or other signs of low energy availability;
measurements or professionally assessed body composition decreasing consistently.

One poor session means you had one poor session. A pattern deserves attention.

A SIMPLE WEEK FOR RUNNING AND MUSCLE MAINTENANCE

There is no perfect schedule, but a recreational runner completing four runs and two strength sessions might use:

Day	Training
Monday	Full-body strength
Tuesday	Easy run
Wednesday	Intervals or threshold session
Thursday	Rest or gentle mobility
Friday	Easy run plus short full-body strength session
Saturday	Long run
Sunday	Rest

This arrangement is only an example. The best week depends on training history, race distance, gym goals, injury history and the days you can realistically train.

If Friday’s strength session consistently damages Saturday’s long run, move it, reduce its lower-body volume or place strength after a harder running day. A timetable is supposed to organise fatigue – not merely colour-code it.

RUNNING AND MUSCLE LOSS: RISK CHECK

Lower risk	Higher risk
Mileage increases gradually	Mileage rises suddenly
Energy intake broadly matches training	Persistent calorie deficit or unintended weight loss
Protein is distributed across the day	Protein intake is consistently low
Strength training continues	Resistance training is removed completely
Hard sessions are organised and fuelled	Hard running and heavy lifting are repeatedly stacked while depleted
Sleep and rest are protected	Fatigue accumulates for weeks
Goals have a clear priority	Trying to maximise weight loss, mileage and hypertrophy simultaneously

FREQUENTLY ASKED QUESTIONS

Does running burn fat or muscle first?

The body does not use one fuel in a strict first-then-second order. Running uses a changing mixture of carbohydrate and fat, with a smaller possible contribution from amino acids. Intensity, duration, recent food intake, glycogen availability and training status all affect that mixture. Measurable muscle loss is a longer-term outcome, not an automatic event during one run.

Will running 5K make me lose muscle?

For most people, a few 5K runs each week will not cause muscle loss when they eat adequately and continue resistance training. The total workload and recovery context matter more than the distance printed on one activity.

Can I train for a marathon without losing muscle?

Yes, many runners maintain muscle while preparing for a marathon. The challenge increases as mileage rises because energy needs and fatigue increase. Keep strength work in the plan, fuel the training and accept that maximising hypertrophy may not be realistic during the highest-volume marathon weeks.

Does running after lifting ruin muscle growth?

Not automatically. However, a hard or long run performed immediately after heavy lifting may reduce the quality of one or both sessions. Research suggests concurrent training generally preserves hypertrophy and maximal-strength gains, although explosive-strength development may be more sensitive when both are completed in the same session.

Is fasted running bad for muscle?

An occasional short, easy run before breakfast is unlikely to strip away muscle. Repeated long or intense fasted sessions become less sensible when they contribute to low energy availability, poor training quality or inadequate recovery. Fuel according to the demands of the session rather than treating fasted running as inherently superior.

How much running is too much if I want to build muscle?

There is no universal mileage threshold. Running becomes “too much” for muscle gain when it repeatedly prevents you from completing productive strength training, maintaining energy intake or recovering between sessions. Your current experience, pace, terrain, sleep and gym volume all change the answer.

Can I gain muscle and improve my running at the same time?

Yes – particularly if you are relatively new to one or both forms of training and your goals are moderate. As you become more advanced, maximising both simultaneously becomes harder. Use training phases: maintain one quality while placing greater emphasis on the other.

Does creatine help runners maintain muscle?

Creatine can support strength-training performance and lean-mass gains when paired with appropriate resistance exercise. It does not replace adequate energy, protein or training, and it may increase body weight through water stored in muscle. Read our evidence-led guide to creatine for runners before deciding whether the trade-off suits your goals.

THE BOTTOM LINE

Running does not automatically make you lose muscle.

The clearer threats are the conditions that sometimes accompany running: a large energy deficit, insufficient protein and carbohydrate, rapidly increasing mileage, abandoning strength training and accumulating more fatigue than you can recover from.

Combine running with consistent resistance training, fuel the work you are asking your body to perform and organise your hardest sessions with some thought. You may not maximise marathon performance and bodybuilding progress in the same training block, but you can become a strong runner without watching your muscle disappear at the roadside.

Run enough to build the engine. Lift enough to keep the chassis. Eat enough to support both.

REFERENCES

Schumann, M., Feuerbacher, J. F., Sünkeler, M., et al. (2022). Compatibility of Concurrent Aerobic and Strength Training for Skeletal Muscle Size and Function: An Updated Systematic Review and Meta-Analysis. Sports Medicine, 52(3), 601–612.
Murphy, C., & Koehler, K. (2022). Energy Deficiency Impairs Resistance Training Gains in Lean Mass but Not Strength: A Meta-analysis and Meta-regression. Scandinavian Journal of Medicine & Science in Sports, 32(1), 125–137.
Morton, R. W., Murphy, K. T., McKellar, S. R., et al. (2018). A Systematic Review, Meta-analysis and Meta-regression of the Effect of Protein Supplementation on Resistance Training-Induced Gains in Muscle Mass and Strength in Healthy Adults. British Journal of Sports Medicine, 52(6), 376–384.
Kato, H., Suzuki, K., Bannai, M., & Moore, D. R. (2016). Protein Requirements Are Elevated in Endurance Athletes after Exercise as Determined by the Indicator Amino Acid Oxidation Method. PLOS ONE, 11(6), e0157406.
Mountjoy, M., Ackerman, K. E., Bailey, D. M., et al. (2023). 2023 International Olympic Committee’s Consensus Statement on Relative Energy Deficiency in Sport. British Journal of Sports Medicine, 57(17), 1073–1097.
Jäger, R., Kerksick, C. M., Campbell, B. I., et al. (2017). International Society of Sports Nutrition Position Stand: Protein and Exercise. Journal of the International Society of Sports Nutrition, 14, 20.

Disclaimer: This article is for general informational and educational purposes only. It is not a substitute for personalised medical, nutritional or dietetic advice, diagnosis or treatment. Speak to a GP or registered sports dietitian if you experience unintended weight loss, persistent fatigue, disrupted periods, repeated injuries, declining performance or other possible signs of low energy availability. Individual energy and protein needs vary with body size, health, training load and goals.