Every day, our body weight fluctuates. In fact, it can go up or down by as much as five or six pounds in a single day depending on a variety of factors, including what we ate, how much we moved, our stress levels and more.
But if you’re regularly stepping on the scale and are surprised to see your weight increasing, you might be wondering: Why am I gaining weight so fast? Why am I gaining weight for no reason?
Unexpected or unintentional weight gain can be frustrating and concerning. Often lifestyle factors can play a big role, but there are also times that added weight can be a sign of a medical condition.
Below, we highlight possible causes for unexplained weight gain, when to worry and what to do next. Keep reading or select one of the links below to go to a specific section:

What’s considered unintentional or unexplained weight gain?

Unintentional or unexplained weight gain is when you gain weight without increasing the number of calories you consume, and without reducing your activity levels.

Unexplained weight gain can often happen when your individual caloric needs – or the number of calories you body needs to function well each day – goes down, but your eating habits haven’t changed. Reasons why you may need fewer calories include aging, menopause and lifestyle choices.

But unexplained weight gain can also happen for reasons that have nothing to do with caloric intake. For example, you may have a known or unknown medical condition that’s causing water retention or abnormal tissue growth.

If you have any concerns, a great first step is making an appointment with a primary care doctor. They are able to diagnose and treat hundreds of conditions, as well as refer you to a specialist if you need more advanced care.

How aging can lead to unintentional weight gain

Does it feel like it’s harder to manage your weight as you get older? It’s not all in your head. While some of this weight gain is likely caused by behaviors, some is because your body is changing and needs fewer calories to work.

Slower metabolism

Your metabolism are the processes that your body uses to convert what you eat and drink into energy that’s used throughout your body. It never stops, even when you sleep.

A slower metabolism means that your body burns through food more slowly, so you need to eat less. Different factors affect how fast your metabolism works, including age, gender, genetics, hormonal factors, diet, muscle mass and activity levels. As you get older, your metabolism slows down, but physical activity and healthy eating can give your metabolism a boost.

Lower basal metabolic rate

Your basal metabolic rate (BMR) is related to your metabolism, but it’s not the same. The BMR is the minimal number of calories your body needs at rest to fuel essential bodily functions, like breathing. Everybody’s metabolic rate is different.

For most people, their BMR should account for 60-70% of their daily calories. So if you eat 2,000 calories, about 1,200-1,400 calories should go to basic body functions. The other calories are used to turn food into energy and to support physical activity.

Your BMR starts to decrease when you reach about 60 years old. So, it could be that your body only needs 1,000 calories – and not 1,200 – to support essential body functions. If you keep eating 2,000 calories, you could gain weight.

Changes in sex hormones

As you get older, your sex hormones decrease. A women’s estrogen level changes quickly during menopause or perimenopause – usually between the ages of 45-55. Men’s testosterone levels gradually decrease over time.

Changes in sex hormones can lead to weight gain. But hormones usually aren’t the primary cause of weight gain during menopause or later in life. More likely causes are a slower metabolism and less muscle mass.

Lifestyle reasons for weight fluctuations

It’s often normal to gain weight in adulthood – up to two pounds a year, according to a 2022 study. Aside from the natural aging process, these extra pounds can often be traced back to changes in behavior or lifestyle (such as the timing and size of your meals throughout the day). So changes in your weight may be easily explained actually, even if you don’t necessarily think anything major has changed.

Poor sleep

There are so many reasons why you should try to clock in 7-9 hours of sleep a night – and your weight is one of them. Numerous studies show that when people get less sleep than they need, their weight tends to go up.

The reason? When you don’t sleep enough it can affect your hunger hormones, metabolism and food choices. One analysis showed that sleep-deprived people ate, on average, almost 400 more calories a day. Not only that, but it’s also harder to get the exercise you need when you’re tired.

Yo-yo dieting

Repeat dieting, also called yo-yo dieting, can lead to unintentional weight gain over time. That’s because starting and stopping diets affects your appetite and may cause you to eat more. It can also affect cortisol, a stress hormone that holds on to fat in your body. Making sustainable behavioral changes, possibly with the help of a medical professional, can help people lose weight and keep it off.

Not drinking enough fluids

Not drinking enough can be a reason for unexpected weight gain. That may seem odd since water weighs a lot, but it’s true.

One reason dehydration can lead to weight gain is how you respond when you’re thirsty. While you may just need a tall glass of water, a thirst sensation can make you want to eat, even if you’re not hungry. Another reason is that your body doesn’t work as well when dehydrated – it may burn calories slower and be less effective at breaking down fat.

Quitting smoking or other tobacco products

Giving up smoking is one of the best things you can do for your health. But quitting tobacco can cause weight gain – most people gain 5-10 pounds in the months after they quit.

That’s because the nicotine in cigarettes and other tobacco products speeds up your metabolism, while also reducing your appetite. After you quit, you’ll be hungrier and may look to food as a way to replace smoking – but your body will be using up calories more slowly.

Gaining a few pounds is a small tradeoff for the increased health and energy that you’ll have when you give up smoking. But if you’re worried about weight gain after quitting smoking, talk with your primary care doctor.

Taking certain medications

If you recently began taking a new medication, there’s a chance that it could be the reason for your sudden weight gain.

The side effects of different medications can slow down your metabolism, making your body burn calories at a slower rate; increase your appetite so you eat more, cause you to retain water, or change how your body stores and processes food. Some medications may also have side effects that make it harder to exercise.

The amount of weight you may gain from a medication depends on your age, medical conditions, how long you need to take the medication and how it changes your lifestyle. Some people might gain a couple pounds in a year, but others might gain 20 pounds in a couple of months. It’s also possible that some people may lose weight when taking the same medications that cause others to gain weight.

Medications that may cause weight gain include diabetic medicines such as insulin, as well as antipsychotics, antidepressants, epilepsy medicines and some cancer treatments.

If your weight is being caused by your medications, there are ways to treat it. You may be able to switch to a different medication or take a smaller dose of your current medication. But don’t make any changes without talking to a doctor.

Medical conditions that can cause unexplained weight gain

While most weight gain is caused by behaviors or aging, there are some medical conditions and illnesses that make you gain weight. In some cases, medical conditions can cause unintentional weight gain that’s expected – for example, it’s not surprising to gain weight when pregnant. But often weight gain from medical conditions is both unexplained and unexpected.

Underactive thyroid

Your thyroid gland produces a hormone that helps control your metabolism, heartbeat, temperature, mood and more. If your body doesn’t make enough of this hormone, you may have side effects such as weight gain, tiredness and depression.

Both men and women can have underactive thyroids, but it’s more common in women. The only way to diagnose the problem is with a thyroid function test. The good news is than an underactive thyroid can usually be successfully treated with medications.

Mental health conditions

Mental health conditions affect people in different ways. Some people can gain weight while others may experience unexpected weight loss.

There are different reasons why a mental health condition can cause weight gain. People with anxiety, depression or bipolar disorder may use food as a way to cope and eat too much or make poor food choices. Plus, some mental health conditions, like depression, can make it difficult to find the motivation or energy to exercise or be active. Also, a side effect of common mental health medications, like antidepressants and antipsychotics, is weight gain.

Increased cortisol

Cortisol is known as the stress hormone because it’s released into your body when you’re under stress. It’s your body’s way of preparing you for potentially dangerous situations.

As part of this fight-or-flight response, your body gets a boost in energy and heart rate. You may also crave sweet and fatty foods since they provide quick energy. At the same time, bodily functions that aren’t needed to protect you from immediate danger are temporarily paused or slowed down – one of these functions is your metabolism.

About four hours after a stress response, your cortisol levels (and your bodily functions) should be back to normal – unless your body has released more cortisol in response to stress, you’re taking an oral corticosteroid medication or you have medical condition like Cushing’s syndrome.

If your cortisol levels remain high for a long time, it can cause unexpected weight gain since your body may crave high calorie foods and your metabolism may be slower than normal.

Diabetes

It’s possible to have unexpected weight gain with both type 1 and type 2 diabetes. Type 2 diabetes, in particular, is often linked with weight gain.

People with diabetes may be more likely to experience weight gain because of diet, medications and lower activity levels. There is also data showing that insulin resistance (the cause of type 2 diabetes) can decrease energy and increase appetite.

Fluid retention

The amount of fluid in your body changes throughout the day. It’s totally normal for your weight to change up to four pounds in a single day, based on your fluid levels.

Fluid retention, or edema, is when a lot of fluid builds up in the body. This can cause a sudden weight gain of 15 or more pounds. If you have edema, your limbs, hands, feet, face or abdomen will likely look swollen.

People with medical conditions, such as heart failure, kidney disease, liver disease, or those taking certain medications may experience this type of weight gain. But it’s also possible that your diet could be leading to fluid retention – for example, if you’re eating too many salty foods.

Polycystic ovary syndrome (PCOS)

Polycystic ovary syndrome (PCOS) is a condition that affects the ovaries, causing them to produce abnormally high amounts of androgens (male sex hormones). Most women have these androgens in small amounts, but higher levels of androgens can affect how a woman’s body works.

PCOS can cause weight gain, especially around the belly, and make it difficult to lose weight. PCOS is a common cause of irregular periods and missed periods. The condition can also cause acne or oily skin, and hair growth on the face and body.

Can cancer cause unexplained weight gain?

Yes, it’s possible for weight gain to be a sign of cancer, but it’s more likely related to changes in behavior or a different underlying medical condition. Those with cancer are more likely to lose pounds instead of gain them. That’s because your body needs to work very hard to try to fight off the cancer.

Weight gain for someone with cancer may happen in later stages of the disease if tumors grow larger or someone begins retaining fluid, usually in their legs or abdomen.

Which cancers can cause unexplained weight gain?

Unexplained weight gain can happen with any type of cancer, but fluid retention is more common with ovarian and colorectal cancer.

It’s also possible for someone to gain weight during cancer treatment because of the medications they’re taking. This is especially true for people who have breast, prostate cancer or ovarian cancer.

When should I be concerned about unexplained weight gain?

Unexplained weight gain can be an immediate or long-term health concern. If you think your weight changes are caused by a medical condition, make an appointment with your doctor.

Seek medical care right way if your unexplained weight gain is rapid or comes with any of the following symptoms:

  • Fever
  • Skin sensitivity
  • Shortness of breath or difficulty breathing
  • Heart palpitations
  • Sweating
  • Changes in vision

If it’s possible that your weight changes are caused by aging or lifestyle behaviors, it’s still a good idea to talk to a doctor. Being overweight is a risk factor for medical conditions such as type 2 diabetes, heart disease and certain cancers.

Your doctor can help no matter what is causing your unexpected weight gain. If it turns out that you should see a specialist, they’ll connect you with someone who can care for your needs.