Why Am I Gaining Weight Working Out? What's Really Going On

By Paul Butler · Online Coach & Personal Trainer

You finally start going to the gym, you step on the scales a fortnight later, and the number has gone up. The bathroom scales have a sense of humour. It just isn't a very good one.

So why are you gaining weight working out? In the first few weeks it is almost always water, not fat. Muscles hold extra fluid while they repair and as they get fitter. The other common reason is quieter: exercise makes some people hungrier, and they eat back more than they burned. Neither means the training isn't working.

Why am I gaining weight when I've started working out?

Mostly because new exercise makes your muscles hold on to more water while they repair. That can show up on the scales within days, and it hides any fat you are losing. If the gain continues for weeks, the more likely cause is eating more without realising, because exercise can turn your appetite up. Fat gain from the training itself is very unlikely.

The gym doesn't care that you had a bad week. Neither do your bathroom scales, the miserable so-and-so.

Reason one: your muscles are holding more water

When you train a muscle harder than it is used to, it gets tiny bits of damage, and the repair process pulls fluid into the area. That is part of why you feel stiff for a couple of days, and it shows up on the scales.

It doesn't stop there. As muscles get fitter they keep more water in them for good. In one study, 18 out-of-shape men took up cycling for four months. The water content of their muscles went up by 2%, while their fat mass dropped by 5.4% (Mora-Rodríguez et al., 2016). Their weight did come down, but the scales alone would have told them a lot less than was really going on.

Reason two: you might be eating it back

This is the one nobody likes hearing, so I'll be gentle. In a study of 35 overweight adults who did supervised exercise five times a week for twelve weeks, the average weight loss was 3.7kg. But that average hid a huge spread: some people lost nearly 15kg, and some gained almost 2kg.

The ones who lost much less than expected weren't lazy. Their appetite went up and they were eating around 270 calories a day more, which is about a chocolate bar's worth, every day.

"...expressing the exercise-induced change in body weight as a group mean conceals the large inter-individual variability in body weight and compensatory responses."

In plain English: exercise works differently for different people, and for some of them the biggest change is to their appetite. If that sounds like you, the post on always being hungry covers how to deal with it, and protein is a big part of the answer.

A few things help straight away. Plan what you eat after a session, rather than raiding the cupboard the moment you get home. Make sure it has some protein in it. And be wary of the "I've earned this" treat. A gym session usually burns less than it feels like it did, and a pastry can cancel it out in a few bites.

Reason three: a bit of muscle, though less than you'd hope

If you are new to lifting weights, you will build some muscle, and muscle does weigh something. But it goes on slowly. If the scales have jumped a couple of kilos in two weeks, that isn't new muscle, however nice it would be to think so. It is water and food.

What to measure instead of just your weight

The scales add up water, food, muscle and fat into one number, so on their own they can't tell you what is changing. Use a few measures together.

• Your waist. Once a month, same time of day. The belly fat post explains why it often shows progress the scales miss.
• Your strength. Write down what you lift. If the weights are going up, the training is working.
• The weekly trend. Weigh on the same day each week, or take a weekly average, and look at the direction over a month rather than any single day.
• Your clothes. Unscientific, but surprisingly honest.

When to get it checked

If your weight keeps climbing for several weeks and you are sure your eating hasn't changed, or if the gain comes with swollen ankles, breathlessness or feeling unwell, see your GP. Some medications and conditions, like an underactive thyroid, can cause weight gain, and none of those are things to guess at from a blog. This is general information, not medical advice.

Frequently asked questions

How long does water retention from exercise last?

For most people the extra fluid from a new routine settles down once the body gets used to the work. It can come back for a few days whenever you add something new or train much harder. Some extra water in fitter muscles stays, and that is a good thing.

Does muscle weigh more than fat?

A kilo is a kilo. But muscle is denser than fat, so a kilo of it takes up less room. That is why someone can stay the same weight while their clothes get looser and their waist gets smaller.

Should I stop weighing myself?

Not necessarily. Weighing can be useful as long as you look at the trend over weeks and pair it with your waist measurement. If one bad morning on the scales ruins your whole day, switch to weekly weigh-ins rather than giving up on the gym, which is the one move guaranteed not to help.

Will I put on weight when I start lifting weights?

You might see a small jump in the first couple of weeks, mostly water. It is nothing to worry about. Keep going, keep your eating steady, and check your waist after a month. The beginner gym programme is a sensible place to start if you want a plan.

This is general information, not medical advice. If your weight keeps rising for weeks without any change in eating, or comes with swelling or breathlessness, see your GP.

More honest guides on fat loss, nutrition and training

The 2008 King et al. study of individual responses to supervised exercise and the 2016 Mora-Rodríguez et al. muscle water study are linked in the text. Images from Pexels (free to use): Annushka Ahuja, Ketut Subiyanto, SHVETS production.