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6 common causes of rapid weight gain and how to tackle it

Weight fluctuations are completely normal but can be perplexing as to why it happens.

Written by
Emma Norris
Medically reviewed by
Dr Claudia Xiao
Last updated
May 15, 2024
min read
6 common causes of rapid weight gain and how to tackle it
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So, you've been cracking on with life as usual, doing all the same things you normally do. Then, one day, you catch a glimpse of yourself in the mirror and are taken aback (and not just because you're wondering 'who's that handsome devil?')

You seem to be carrying a few extra spare tyres around the middle that definitely weren't there before. Or maybe you stepped on the scales and the number has inexplicably jumped up a few digits. What gives?

Rapid weight gain can be bloody frustrating, especially when you can't quite pinpoint the culprit. After all, how are you supposed to know what to stop (or start!) doing, if you don't know what's causing it?

If you've ruled out all the usual suspects, like a change in eating habits or exercise routine, you're probably feeling a little perplexed. Thankfully, there are a few other simple explanations for sudden weight gain — and by getting to the bottom of it, you can start to feel in control of your health again.

What is considered rapid weight gain?

You might be wondering what's considered normal when it comes to weight gain. Firstly, it's important to note that weight fluctuations are completely normal.

The average adult's weight fluctuates by as much as 1-2 kg in a day [1]. Some factors that can impact the number on the scales include fluid retention, sodium intake, and exercise. So, if your weight creeps up after a big meal, don't panic — it's unlikely to be actual body fat gain.

However, if you've noticed that you've gained a few kilos over the course of a few weeks — and that number doesn't seem to be going down — it could be a sign that something is awry. The good news is, becoming aware of what's causing weight gain is the first step to doing something about it.

Causes of rapid weight gain

Some of the most common causes of a sudden weight increase include:

Sodium intake

Whether it's grabbing fast food after a stressful day at work or indulging on holidays, frequently eating out can contribute to weight gain. This isn't only because the meals tend to be more decadent than home-cooked food. Restaurants and cafes tend to add lots of extra salt to their food for maximum flavour.

Unfortunately, consuming too much sodium causes the body to cling to water for dear life. Research shows that eating high-sodium meals can lead to a weight gain of about 1-1.5 kg [2]. So, if you're eating a bunch of takeaway meals consecutively, you can imagine how that would add up to noticeable weight gain.

Alcohol consumption

There's nothing wrong with enjoying a cold frothy or a glass of red on occasion. But unfortunately, regular alcohol consumption and weight gain go hand in hand. It's called a beer belly for a reason!

It’s important to note that the Australian government's safe drinking guidelines for healthy adults are as follows [17]: 

“To reduce the risk of harm from alcohol-related disease or injury, healthy men and women should drink no more than 10 standard drinks a week and no more than 4 standard drinks on any one day.”

Alcohol leads to unintentional weight gain in a few different ways. Firstly, alcohol tends to be high in calories but has very little nutritional value (in other words, it's empty calories). This is especially true of beers and any drinks that use soft drinks as mixers, such as rum and Coke.

Booze also tends to increase hunger levels [4], and make us crave greasy, unhealthy foods (we're looking at you, 3am kebab). Not only that, but the body is unable to effectively burn fat when we have alcohol in the system. In other words, alcohol creates the perfect conditions for weight gain!

So, if the number on the scales has been creeping up recently, it might be time to cut back or switch to non-alcoholic versions.


Much like alcohol, excess stress creates a perfect storm for sudden weight gain. When we're chronically stressed, our adrenal glands release a hormone called cortisol. This can trigger the 'flight' or 'fight' response and signals to the body that we're in danger — which, in turn, causes the body to hold onto excess fat, especially around the abdomen [5].

Not only that, but multiple studies have found that stress increases the intake of foods high in fat, sugar, or both [6]. This is likely due to the increased appetite from the stress hormone. Plus, many of us reach for high-fat or sugary foods in difficult times because it gives us a boost of the feel-good hormone, dopamine.

Combine too much cortisol with the fact we tend to exercise less when we're stressed, and it's not hard to see how rapid weight gain could occur. It might be a good opportunity to consider how you can regain some balance in your life.

Thyroid issues

If your weight seems to be on a mysterious upward spiral, it could be a sign of underlying thyroid issues. The thyroid is the small, butterfly-shaped gland located in the front of the neck, just underneath the Adam's apple. It's responsible for many important processes in the body, including controlling energy levels and how fast you can burn calories.

Sudden and unexplained weight gain is one of the most common signs of hypothyroidism: a condition where your thyroid hormone is underactive.

The initial weight gain you experience with an underactive thyroid is usually due to holding excess water and salt [7]. However, if left untreated, it can lead to excessive weight gain that's hard to shift, as well as other health risks like heart disease.

Up to 80% of hypothyroidism is caused by autoimmune diseases like Hashimoto's disease [8]. And, while it's much more common in women (as they are more susceptible to autoimmune disease), it can still affect men, particularly those over 60 [9].

Other symptoms of thyroid disorder include tiredness, increased sensitivity to the cold, muscle soreness, and coarse hair and skin [10]. If you notice these symptoms, it's important to seek medical attention. Your doctor will be able to run blood tests and other assessments to rule out this underlying cause.

Quitting smoking

If you've recently made the decision to quit smoking or vaping, this is cause for celebration. However, it can be disheartening when all your hard work is 'rewarded' with rapid weight gain!

It's not uncommon to put on weight when you give up smoking. On average, people gain between 2.25-4.5 kg in the months after quitting [11]. There are a couple of reasons behind this.

Firstly, nicotine speeds up the body's food processing system. So, when you give up smoking, it can slightly slow down your metabolism and lead to weight gain. Further, it's well-known that nicotine suppresses appetite. Many people find that when they quit, they end up eating more (even if it's just due to the body restoring its natural equilibrium).

Some people also rely on smoking as a band-aid solution for stress. As a result, they experience increased food cravings — because the part of the brain that was once reliant on nicotine seeks a different way to get a dopamine boost [12].

Of course, quitting smoking is an important decision to make for your health. In most cases, the benefits of kicking the habit will far outweigh the potential weight gain. However, it's a good idea to talk to your doctor if you notice you're gaining weight, as they will be able to recommend an action plan.

Certain medications

If you've recently started on a new medication, this could be the cause of your sudden weight gain — many come with the potential side effect of a few extra kilos.

This includes medications for mental health conditions such as depression and anxiety. Other medications that can cause weight gain include beta-blockers and epilepsy medication [13].

It's important to note that these medications can be life-saving for the people who need them. So, rather than suddenly going off them, it's crucial to visit your GP. They may be able to suggest an alternative without the side effect of weight gain.

What causes rapid weight gain in your late 20s?

It's no secret that our metabolism slows as we age. You may have noticed that you can no longer get away with a pub feed and multiple pints like you could in your uni days.

However, the drop in metabolism that happens in our late 20s and early 30s isn't quite as steep as was once initially thought. Experts believe that our basal metabolic rate (the amount that we burn at rest) drops roughly 1-2% per decade [14].

So, while you can expect to see a slowing in metabolism as you leave early adulthood, this effect should be fairly minimal. If you do find you gain excess weight in your late 20s, it may be a result of changes to your lifestyle, diet or activity levels, or some of the other aforementioned factors.

What are the risks of rapid weight gain?

Let's be honest, most people aren't thrilled about gaining weight unless it also happens to be accompanied by bigger biceps. However, the concerns of rapid weight gain aren't just superficial. Other health concerns of rapid weight gain include:

Sleep apnoea

Snoring can be a bit of a 'chicken and the egg situation.' Research shows that people who are overweight are far more likely to experience sleep apnoea [15].

But on the flip side, sleep apnoea is actually a surprising cause of weight gain, as sleep deprivation can lead to overeating. Either way, it's best to get this one checked out — for the sake of your health, and for your relationship!

Heart issues

Being overweight carries a heightened risk of high blood pressure and heart disease. Excess weight can lead to a build-up of fatty material in the arteries.

This can put a lot of pressure on the arteries that carry blood to the heart and if untreated, can lead to heart failure or other serious issues.

Kidney disease

If you have underlying kidney issues, rapid weight gain can (in extreme cases) lead to kidney diseases.

This is because holding excess weight can cause inflammation and negative metabolic effects, which can lead to structural damage to the kidneys and a decline in kidney function [16].

Erectile dysfunction

It's not what anybody wants to hear, but research shows that excess weight can reduce your risk of ED. One study found men with a high body mass index (more than 28.7) were at a 30% greater risk of erectile dysfunction than those with a normal body mass index (less than 25).

Plus, carrying excess weight can affect your confidence, which can have a knock-on effect in the bedroom.

Treating rapid weight gain — and keeping it off

The most common strategies for maintaining a healthy weight are diet, exercise, and lifestyle change.

That said, if you're doing all the right things and still can't seem to get a handle on your weight, don't beat yourself up. There are other factors that can affect body weight, and not all of them are in our control.

Pilot's Metabolic Reset Program may be the extra piece of the weight loss puzzle you've been looking for. Backed by science, our treatment plan is an effective strategy for long-term weight loss, when accompanied by lifestyle changes.

Our program combines breakthrough modern medicines with community support from our medical team and health coaches, while also connecting you with a supportive community of like-minded men to help keep you motivated and accountable to your weight loss goals.

Take our short quiz today to find out how Pilot can help you lose weight and feel healthier.

Image credit: Getty Images

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