What Changes In Your Body When You Start Running?


If you’ve just recently started running, you may be noticing some changes to your body shape, metabolism, muscle mass, and energy levels.

Runners who have recently begun but haven’t yet noticed any difference, keep going! It may take a little while, but you may be surprised at what changes in your body when you start running.

Although significant changes to the body are often most noticeable when you’re a beginner, even experienced runners can see benefits. Switching up your routine, trying new training runs, or tweaking your nutrition slightly can lead to changes that you may not have expected.

Keen to start but not sure how? Download our 30-Day Run Challenge to start you off on the right foot!

Let’s explore what happens to your body when you begin to take running seriously.

How Does Running Change Your Body?

There are plenty of small changes you’ll notice in your body when you start running consistently.

We’ll get into some of the most common ones down below, but all of them result from the main three changes that happen:

  • Weight loss or fat burning
  • Muscle gain
  • A speedier metabolism

Of course, you’ll have to stick to a healthy diet and make sure you’re eating fewer calories than you’re burning in order to actually lose weight.

So, running in itself has many benefits, but ultimately it’s meant to be just one part of a healthy lifestyle. This way, your changes will be constant, meaningful, and sustainable, rather than being just yo-yo changes!

How Long Does It Take For Running To Change Your Body?

The answer to this question depends on three things:

  • How regularly you run
  • How intensely you run
  • Your nutrition

You can’t run once a week at low intensity and expect to see changes. You need to be on the road for at least 30 minutes, 3 times a week, at a moderate to high intensity. That’s when real change will happen!

We strongly recommend doing some form of exercise on at least 2 other days of the week. If you have no other exercise equipment, then walking is a good option. It’s less intense than running, but it will keep you active and help you form a habit.

If you have access to a bicycle or exercise bike, cycling is a super form of cross-training. It strengthens all the muscles that are important for running and burns a good lot of calories when done at a decent intensity.

It’s also incredibly important to eat a healthy diet. You can run all you want, but if you’re eating too much or choosing the wrong foods, you just won’t see changes.

If you can manage this level of dedication, you should be able to see small changes within 3 or 4 weeks of starting. Keep going for 2 to 3 months and you’ll notice larger changes – and others will start noticing too!

The Benefits You Gain From Running

Once you’ve been running for 3 or 4 weeks, take some time to assess your body and compare it to your pre-running body.

If you’ve been running regularly, eating healthy, sticking to a calorie deficit (here’s how to calculate yours!), and doing some form of cross-training on your off-running days, here are some of the things you should notice.

Improved Endurance

When you start running, you may find that you can hardly make it through a couple of miles without wanting to pass out. Give it a few weeks, though, and you’ll slowly but surely improve.

By the end of week 3 or 4, you should be able to run a much farther distance than you could when you first began. Your lungs and legs will have adapted to the level of exercise you’re doing, and you can run further (and faster) before your legs become fatigued or your lungs start to burn.

Boosted Metabolism

When your body becomes more active, it’s not only the outside that starts to change. Your immune system, cardiovascular system, and digestive system also begin to work more efficiently.

You can expect your metabolism to work faster and more effectively. This means that your body will begin to use fuel (ie. what you eat and drink) more efficiently to produce energy.

Even if you are on a calorie-restricted diet, you can expect to feel more energetic due to the speed at which your body is processing the food you’re taking in.

Burns Fat

When you exercise, you burn calories. This isn’t just some abstract concept, though. When we talk about burning calories, we mean using energy and burning fat.

When you run, your body first and foremost utilizes the glycogen stores in your muscles. These stores are filled by the food you eat.

But once it’s burned through that, it begins to use your own body fat to fuel itself through the rest of the exercise. This is where the real fat burning begins!

You can run on an empty stomach to increase the amount of body fat you burn. By “empty stomach”, we mean not having eaten or drank anything for around 8 hours or more. In other words, running fasted. This is usually easiest first thing in the morning.

After 3 or 4 weeks of consistent exercise (fasted or not). you should notice that you’ve lost fat in places like your arms, thighs, back, and around your face. Of course, this also depends on you being in a calorie deficit.

Builds Muscle

Running might not seem like the most muscle-building exercise out there, but it’s a fantastic way to gain strength and muscle in the lower body. Here’s what you should start to notice:

Strong Butt

The glutes work constantly when you run, especially if you’re running with good form. Activating these muscles consistently will lead to them gaining strength.

If a strong butt is one of your main goals, then we suggest pairing your running with strength training. This should include squats, lunges, hip thrusts, and glute bridges.

Toned Thighs

Your entire lower body gets a super workout when you run. Your quads especially carry almost your whole body weight, and they also absorb a lot of the impact of your foot strike. They really work hard when you’re running downhill, especially.

It’s important to note that many runners (especially new ones) develop a quad-dominant style. This may not be an issue in the beginning (in fact, it can contribute to toned thighs), but in the long run it can cause knee instability and increase the chances of injury.

It’s a good idea to develop correct form from the beginning so you can avoid this!

Stronger Calf Muscles

Pushing off on each step activates the calves. If you haven’t really had muscular calves before, prepare for muscles to develop now!

Making sure that you’re wearing the right shoes for your feet is essential if you really want your calf muscles to grow. If you’re wearing incorrect shoes, your feet and legs won’t be in the right position and your calves won’t activate properly.

Improves Posture

If you’re running with correct form, your body will begin to develop better posture. You can’t run properly while you’re hunched over!

This is especially helpful for those who spend their days in front of the computer or sitting over a desk.

Strengthens Bones

Running is a weight-bearing activity. Although that means there’s a slight risk of impact-related injury to joints, it also means that your bones gain strength. Regular running can have a noticeable positive effect on bone density or bone mass.

This is particularly important for older runners. Running can help prevent age-related bone loss or weakening, keeping you stronger for longer.

Better Cholesterol Levels

Regular exercise, even at a moderate intensity, can decrease bad cholesterol and increase the good stuff. This, along with an increase in cardiovascular endurance, reduce your chance of heart disease.

More Relaxed

When you’re recovering from your run, your muscles are likely to be pretty relaxed. You can always use a foam roller or compression gear to help increase blood flow to muscles if they do feel a little tense.

But, even more importantly, regularly running has a positive mental effect. Out on the road or trail with the hypnotic pounding of your feet on the ground helps your mind switch off a little and stop worrying about its usual things.

Also, runner’s high is a real thing! Exercising, but running in particular, releases endorphins, which make you feel great, mentally and physically. It’s not an extremely long-lasting effect, but it does help you become more calm and relaxed.

Get Outside More Often

There are many well-documented benefits of being outdoors. Combine those with the benefits of running, and you’re getting twice as much goodness!

You’ll be getting more fresh air, possibly exploring new places in your town, and getting out of the house for a bit.

The Less Good

Like everything, there are a few downsides to running regularly. Thankfully, these are easy to overcome with just a bit of planning and preparation!


This is probably the one disadvantage that can’t be overcome with planning, and it affects women more than men. When you run regularly and you’re burning fat, one of the first places it will come off is the chest.

For men, especially those who are doing weight lifting and developing chest muscle, this won’t be a problem. You’ll be developing a well-defined chest with less fat and more muscle.

Ladies, you’ll most likely go down a cup size! For some, this may be a benefit. But for most, this may be an unwelcome change. The chest is a fatty area, and when your body burns fat, it looks for the easiest spot to take it off. For us ladies, the chest is that first target!


This is something most new running will experience. In fact, even experienced runners still have to deal with it!

Chafing is caused by friction when your socks or clothing rub against your skin. In some cases, skin against skin can be a problem too. It leads to inflammation, pain, and general discomfort.

Making sure all your gear fits you properly is the first step to preventing chafing. You can also use an anti-chafing cream or lotion.


Blisters are also caused by chafing, or by pressure points caused by ill-fitting shoes. On occasion, running long distances or at a faster pace than normal can contribute to blisters, even if your shoes fit well. It’s advisable to increase your distance and pace slowly to prevent this.

The wrong size socks, or socks made from low-quality material, can also form blisters. Your socks should fit snugly, without bunching up or slipping around on your feet.

If you’re wondering how to deal with blisters, we’ve written about it in detail here!

Black Toenails

Also known as Runner’s Toe or Runner’s Toenail, black toenails can be alarming to see! But it’s a very common occurrence in runners of all levels.

The constant bumping of your toes against the front of your shoe can cause bruising, blood blisters, or even cause the toenail to lift. Most often, the black color is simply bruising underneath the toenail.

Arms May Shrink

Running does technically involve the arms, but not in a muscle-building kind of way. If you aren’t doing some kind of cross-training that builds muscle in your arms, it’s likely that they will get smaller.

For some, this may be a good thing. But if you’re after a well-balanced physique, just running isn’t going to help you develop great arms.

Runny Nose

This is a common side effect of running, especially in colder weather. When the nasal passages become irritated, they increase the amount of mucus they produce, which can result in an annoying runny nose.

You may need to take extra tissues or a handkerchief with you when running. If it begins to interfere with your running, ask your doctor about medication for exercise-induced rhinitis.

Toilet Breaks

You may find yourself suddenly needing to go in the middle of your run. This is due to the jarring nature of running, although it can be made worse if you’ve had a lot of caffeine or a lot to drink before your run.

Avoid drinking a large amount before heading out. Limit your caffeine intake and try to avoid sugary foods too. This should improve over time!

Side Stitches/Cramps

This is a common occurrence in runners of all fitness levels. In fact, up to 70% of runners experience it! It’s caused by a spasm in the diaphragm and can show up on either side of your abdomen.

It’s not typically a severe issue. But it could be enough to slow your pace or even force you to walk for a few minutes. This isn’t usually a problem if you’re new to running, because you’re not likely to be training hard for pace or time.

Itching Thighs

Itching thighs can be caused by dry skin, chafing, or the blood vessels rapidly expanding and bringing blood closer to the surface, stimulating nerves nearby.

Cracking Knees

Your knees may begin to crack more often than they used to. This is most likely due to them being used more often and more intensely than usual. It’s usually nothing to worry about, though.

The cracking sound can be caused by one of two things: either, tendons and ligaments are moving over the bone as the joint moves, or nitrogen bubbles in the joints pop and create the sound.

Either way, it’s nothing to worry about! If you can feel a grinding or scraping feeling when it cracks, then it may be time to get it checked out.


Headaches are a common side effect of running. There’s a wide variety of causes, and you can find out all you need to know about it in this article!

The two most common reasons, though, are tight muscles in the neck and bad hydration. Both of these can be easily remedied!

If you’re prone to headaches after running, make sure to stretch your neck and shoulder muscles well both before and after running. A foam roller may be helpful to ease tension and get rid of knots.

Of course, make sure to stay well-hydrated when running and after your run! If necessary, use an electrolyte supplement or sports drink to keep you feeling good while you’re on the road.

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Shanna is a writer who runs... And cycles, jumps rope, and lifts weights. She lives in beautiful South Africa and enjoys sharing her knowledge and experience with other avid athletes.