13 Common Causes Of Rapid Weight Gain

Though it may be a bitter pill to swallow for some, it’s difficult to deny that most people also grow in weight as they steadily grow in age. However, suppose you’re experiencing rapid weight gain that defies logic and leaves you confused and concerned. It could be a sign that something is happening underneath the surface that needs investigating.

Contrary to popular belief, the culprit for rapid weight gain is often a sudden spike in caloric intake, with a simultaneous drop in output.

In short, people’s overall activity has decreased, but their food intake has increased, thus causing a gain in weight.

However, it could be many reasons you are experiencing unintentional weight gain.

My advice is to keep a food log of everything you eat daily, plus your exercise routine (if you have one), to review the findings for at least two weeks.

Still, before I get into some of the common causes, I’d like to clarify that it’s always worth consulting with a doctor who can give you a definitive explanation of what is happening and why the sudden weight gain has occurred.

1. Fluid Retention

In laypeople’s terms, edema is the medical word for swelling.

It causes body parts to swell, usually due to injury or inflammation.

At the risk of sounding too technical, edema happens when the small blood vessels leak fluid into tissues nearby.

The additional fluid builds, which causes the tissue to swell.

Edema can happen in any body part, though it’s more common in the legs, ankles, and arms.

But don’t be gravely concerned as edema can be caused by standing for long periods or if you’re pre-menstrual.

If the swelling is severe, it is, without a doubt, a good idea to seek professional advice from your doctor, as excess swelling can cause breathlessness, which in turn could lead to serious health issues.

2. Bloating

Bloating is usually when the GI tract becomes filled with gas or air.

You can experience pressure, tightness, or fullness in the abdomen, which can cause a swollen appearance.

So if the scale is up in weight and you’re feeling bloated, and your pants are tighter than ever, then it’s likely not legitimate fat gain, so it’s probably nothing that should overly concern you.

3. You Stopped Smoking

It’s common knowledge that smoking acts as an appetite suppressant, so cravings can go through the roof once you stop smoking.

Smoking can cause a rise in dopamine, the neurotransmitter that plays a role in feeling pleasure.

Think of the same kind of pleasure you experience from eating candy or ice cream.

What’s more, there’s a dip in dopamine once smoking has ceased, but the issue is that your cravings for it remain, and this craving for a hit of dopamine can potentially lead to eating something that satisfies you more often than usual.

If this relates to you, I will encourage you to exercise frequently.

Exercise offers a nice distraction away from smoking, but it also releases those feel-good endorphins you’re missing since stopping smoking.


4. You’re Eating Too Many Calories

Controlling your weight comes down to one thing:

Calories in vs. calories out.

If you eat less than your body burns each day, you will lose weight.

If you consume more calories than your body burns each day, you will gain weight.

Frequent snacking and underestimating how much you are eating each day leads to the overconsumption of calories.

However, knowing your daily caloric needs can be challenging.

I suggest using a trusted online calorie calculator, which will give you a starting point as to how many calories you need each day to start a successful weight loss diet to achieve a healthy weight.

5. Underactive Thyroid

Hypothyroidism is a thyroid disorder that can cause weight gain as one of its side effects is the metabolism slowing down.

Hypothyroidism has a direct impact on the kidneys, which causes the body to hold additional fluid.

Some other symptoms of hypothyroidism include:

  • Constipation
  • Constant fatigue
  • Aching muscles and stiff joints
  • Feeling cold more often
  • Brittle nails
  • Dry skin and hair

6. Poor Sleep Hygiene

Studies show that people who have poor sleep hygiene and get less than 7 hours of sleep each night are more likely to be overweight than those who get 9 hours of sleep.

If you’re someone who the chance would be a fine thing to get nine or more hours of sleep each night, then I’m with you.

It’s not always possible, but here’s the thing.

Sleep-deprived people show to have lower levels of leptin, the chemical that helps you feel full so the body doesn’t trigger hunger signals when it doesn’t require the energy.

However, the same people show higher ghrelin levels, the “hunger hormone,” which can lead you to believe you’re hungry when your body doesn’t necessarily need food.

Constant tiredness usually results in less activity throughout the day but more snacking to keep the body’s energy levels up, leading to weight gain.

7. Possible Medical Conditions

If you’ve looked at all the eventualities that could have caused you to gain weight and you’re still unsure what’s going on, it would be good to get your doctor to test you for possible endocrine disorders.

Though rarely an explanation for weight gain, the endocrine system includes the thyroid, ovaries, and adrenal glands and helps regulate hormone levels that make the body function.

So although the unexplained weight gain may be linked to an endocrine disorder, the chances are slim, but it is still very much worth checking.

8. Dehydration

The truth is that most people aren’t drinking anywhere near enough water as they should be, and this can be mistaking the feeling of thirst for hunger as both signals are similar and are easily confused.

If you’re tired, lightheaded, or feeling confused, these are all mild dehydration signs.

However, the important note to take away here is that these signs are similar to how we feel when hungry.

Without drinking enough water, the body cannot convert your food into energy at a respectable rate as it otherwise would if you were adequately hydrated.

9. Pregnancy

Pregnancy is a prevalent cause of unintentional weight gain.

Many pregnant women increase caloric intake to support the baby’s growth, so a large percentage of women inevitably put on weight as the baby grows.

If you’re a postpartum mom who wants to lose weight, I highly recommend you read “Postpartum Exercise – The Ultimate Plan for Weight Loss After Pregnancy.”

10. Medication

According to an article written by the University of Rochester Medical Center, some medicines cause people to gain between 10-and 20 pounds in a few months as a side effect.

The treatment of the following can cause people to experience a rapid weight gain from taking certain medications:

  • Psychiatric and depression disorders
  • Seizures
  • High blood pressure
  • Diabetes

Please do not stop taking medication and seek medical advice first.

11. Stress

Stress and anxiety affect us all differently.

While some people experience losing weight, others gain weight.

The reason weight gain happens is that people develop poor eating habits and turn to food for comfort, which is a coping mechanism.

However, a never-ending “stress cycle” can occur as gaining weight from feeling stressed can make you more stressed, resulting in someone gaining even more weight.

My tip would be to find a healthy distraction like exercise or an exciting hobby, getting out in nature, or speaking with a friend if you know you are prone to emotional eating.

12. Menstruation

Rapid weight gain can often be down to a woman’s menstrual cycle.

Many women experience bloating and an increase in water retention when their cycle begins as their progesterone and estrogen levels change, which may cause an increase in weight.

The amount of weight gained is down to the individual; however, it can be substantial in some women.

Although it is a rapid weight gain initially, it subsides once the menstrual period ends.

13. Ageing

As we age, we lose muscle mass because we are less active.

However, muscle is efficient at burning calories, so the less we have, the fewer calories we burn.

The issue many people come up against is that not only do they eat and drink the same amount they always have for years, if not decades, but they are also less active than they ever were.

It is a logical explanation for why we gain weight as we age.

In my opinion, our physical activity should never dramatically decrease due to age.

We should incorporate an exercise program as part of our routine irrespective of age, as long as we are physically able to.

If we continue to stay active, we lower the risk of losing muscle, which means we are less likely to gain as much weight as we would if we didn’t remain active.


When it comes to unintentional weight gain, there is no one-size-fits-all.

It can happen for several reasons.

Changes to lifestyle and age are the most common factors people experience weight gain; however, rapid weight gain can be a sign of undiagnosed health issues.

If you are experiencing unexplained, fast weight gain, you should visit your doctor to get medically reviewed and treated as necessary.

About the author

Chris Jones

I'm Chris. I have a vast interest in all things relating to health, wellness, exercise, and nutrition. I also love to improve my mindset and learn how to increase my productivity. If you'd like to say hello or ask me a question, please visit my contact page, and I'd be glad to hear from you. Alternatively, you can find me on Twitter (@liveliftlife).