Started the gym and gained weight? Here are some possible reasons why

If you’ve started the gym and gained weight, don’t panic! Our personal trainer explains why this might happen.

Joining a gym should be a happy experience. Most people’s primary reason for joining is to improve their health and wellbeing – a positive step. There are many benefits of exercise, including an improvement in mental wellbeing and the prevention of injuries. But let’s be realistic – a lot of us (me included) join a gym to lose some weight.  

Body positivity is an important thing – we should love ourselves no matter what size we are – but if you want to lose weight, and are taking steps to do just that, it can be incredibly frustrating if the scales start moving in the opposite direction. 

However, you don’t need to despair if you’ve joined a gym and gained weight. Your efforts will be rewarded. There can be many reasons for the weight gain and there are other ways to measure your progress that don’t involve worrying over a number on the scales.

Women in the gymCredit: Shutterstock / YAKOBCHUK VIACHESLAV

Does muscle weigh more than fat?

It’s worth starting by debunking one of the biggest fitness-related myths out there: muscle does not weigh more than fat.  

A pound of muscle weighs the same as a pound of fat, a pound of feathers or a pound of bricks. The difference between muscle and fat (and indeed bricks and feathers), is the amount of space they take up. A pound of muscle takes up less space in the body than a pound of fat so, when you’re exercising, you can end up weighing more because you reduce body fat but gain muscle. 

I am an excellent example of the above point. I weigh nearly a stone more than when I first started exercising. However, I am two dress sizes smaller. The reason is I have less body fat but more muscle because I lift weights regularly.


Why am I gaining weight after starting the gym?

Weight loss is rarely linear and there are many reasons for fluctuations, as we’ve outlined below.

I recommend weighing yourself first thing in the morning, after using the toilet, so that you get a more reliable and consistent picture.

Some tracking apps, such as My Fitness Pal, will record your weight loss on a graph. This can be a useful motivation to keep going even if you see a spike on the scales, but the overall picture is still one of loss.


This is the most obvious reason for weight gain after joining a gym.

Working out, and particularly strength training, causes the muscle fibres to break down and grow back stronger. This can cause inflammation in the muscles while this repair and growth takes place. In turn, that inflammation can be reflected as weight gain.

This can be frustrating, as you might weigh yourself the day after a workout and see a spike, but when you weigh yourself after a rest day, you see a drop.  

There’s also the possibility that you’re rewarding yourself with food for exercising and so not losing weight. The basic principle of weight loss is that you must be in a calorie deficit. If you’re working out on a regular basis but still eating an excess of calories, you won’t lose weight. 

Water retention

There are many variables that can lead to weight gain due to water-retention.

Sodium levels are one example. Eating processed foods that contain lots of sodium or adding salt to your food can cause a temporary weight gain the next day, as sodium makes your body hold onto water. Drinking electrolytes can also have the same effect because of the sodium in them.  

Fluctuating hormone levels can lead to water retention for many women during their cycle. This is especially true during perimenopause, when hormone levels change.  

Glycogen is a form of energy that binds itself to water within our muscles. Again, it can be reflected as weight gain.  


Cortisol is also known as the stress hormone and it can affect body weight if it’s too high. You might see a temporary weight spike after drinking alcohol because it causes your cortisol levels to increase. Long term, too much alcohol can cause the liver to prioritise breaking down alcohol over fat, leading to weight gain.  

A poor night’s sleep can also raise cortisol levels, inhibiting the body’s ability to burn fat. And, of course, stress can also raise cortisol. This can make you feel hungrier and crave unhealthy foods.  



Broadly speaking, a stool can weigh between a quarter and half a pound (113-227g), and more if you’re very constipated. While it’s good to eat a lot of fibre-rich foods, this does increase water retention in the colon.

Should I worry about weight gain after joining the gym?

If you’ve joined a gym or started exercising and have gained some weight, my advice is: don’t worry about it.  

If weight loss is your goal, it can be hard to hear someone telling you not to worry, but bear with me.

It’s likely this is a temporary gain and will settle down in time, provided you are generally eating healthily and continue to exercise. To gain a pound (454g) of fat in a week, you’d need to consume about 3,500 extra calories across the week in addition to your usual amount. While that is possible, it’s unusual for most people to do it on a regular basis, and if you’re exercising and burning more calories than you used to, that amount would need to be even higher.  

Instead, your weight gain – or temporary spike – is probably attributable to one or more of the reasons listed above.  

Also bear in mind that there is a difference between weight loss and fat loss.  Weight gain or loss can be influenced by the factors already listed. However, when most people talk about wanting to lose weight, they don’t mean “I want to lose water, some muscle, and a bit of body fat” – they are really referring to fat loss. 

This is a key point to remember when you look at the scales. If it’s suddenly up or suddenly down, only a small part of that will be attributable to fat. The rest will be based around bodily functions and other physiological factors. Fat loss occurs gradually over time by creating a calorie deficit through diet and exercise (burning more than you consume). 

What can I do about my weight gain?

There is no quick fix when it comes to weight loss. It really comes down to time. If you’ve put on weight (as body fat) over a period of time, it stands to reason it will also take time to lose it again.

Patience is key. You’re much more likely to be successful if you make small changes rather than trying to follow a restrictive diet or punishing exercise routine.  

If it’s a sudden weight spike, you just need to ride it out and wait for it to settle. And it will! If it helps your mindset, weigh every morning at the same time so you can monitor peaks and troughs. However, the scales are only one tool used to measure progress. There are many other – and frankly better – options.  

Other ways to track progress


One of my favourite ways to measure progress is by taking photos. It can be daunting if you’re not feeling good about your body, but being able to see your progress is a great feeling and we don’t tend to notice it when we just look in the mirror and have nothing to compare it with. I put my phone on selfie mode, set a timer, and take full body front, back, and side pictures. I just wear my underwear for these (they’re only for my eyes) but you could wear leggings and a sports bra if you prefer. Men might like to wear just shorts. Try to wear the same clothes – nothing too baggy – for the photos and take them every four to six weeks to check progress.  


I like to combine progress photos with measurements, as the progress on the tape measure is undeniable. I measure in inches, but you can use centimetres if you prefer. Hold the tape measure securely but not too tightly. The following areas are good to measure:  

  • chest (under breasts for women) 
  • waist (above belly button) 
  • hips (at the widest part) 
  • thighs 
  • tops of arms (above biceps) 

Fit of clothes

A good gauge of fat loss is the fit of your clothes. If your waistband is getting looser, it’s a sure sign you’ve shed some fat around the middle. If you’re exercising, there’s the added possibility you’ve put on some muscle, so even if the scales are the same or higher, you could still be looking slimmer.  

Energy levels

Exercise increases energy levels, and that’s a big reason to keep going. Forget about the number on the scales and focus on how you feel. Is it easier to walk up the stairs? Do you feel brighter and more energised during the day? Can you complete your cardio exercise with ease? Can you get up and down from a chair or the floor easier? Then it’s working – keep going!  

Getting stronger

There are many benefits of strength training and, of course, one is that it makes you stronger. A temporary weight spike from muscle inflammation (that will settle, don’t forget) is a small price to pay for getting stronger. Building strength protects joints and bones as we age. Take note of signs you’re getting stronger, whether it’s in the gym or not. Maybe it’s easier to carry heavy shopping bags, or maybe you can increase the weights you’re using for exercises like squats.  

Feeling better in yourself

It’s easy to become almost obsessed with the scales and weight loss but try directing your focus towards your overall health and wellbeing to measure progress. If any of the above points are ringing true, then it’s far better to focus on these positives than a unit of measurement that might not be correct for many reasons. Do you sleep better? Are you happier? Have friends and family commented that you seem brighter? Exercise is a chance to move our body and improve our health. It’s not – and never should be – a chore or punishment. Instead, celebrate every day as a new opportunity to shine.  

Becky Fuller

Written by Becky Fuller she/her


Becky Fuller is a fully qualified Personal Trainer, specialising in strength and conditioning for over 50s. Becky’s focus is helping people to become stronger both in body and mind, and to move well without pain. Becky also has many years’ experience working as a freelance journalist, writing for a wide variety of publications such as Screen Rant, Geek Feed, and Daily Actor. She also regularly reviews theatre productions for UKTW.

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