Cancer Treatment and Weight Gain: Understanding the Causes and Finding Support

Healthy weight after and during cancer treatment depends on how much you weighed before treatment started, the stage you are in your recovery journey and the treatment you are receiving.

What is BMI?

The body mass index (BMI) is a tool for determining if your weight is healthy. While it is a helpful tool, it isn't perfect at telling how healthy your weight is, especially when you have cancer or are receiving cancer treatment. (2,3) 

Your BMI is your weight in kilograms divided by your height in metres squared. BMI  doesn’t consider your muscle mass, bone density, or body composition. So it’s not a good standalone measure of how healthy your weight is. Your healthcare provider will determine if your BMI is too high or too low.

What is the Healthy BMI Range in the UK?

The healthy BMI range for adults in the UK falls between 18.5 kg/m2 and 24.9 kg/m2.

Standard BMI ranges are: (3)

• Underweight: below 18.5 kg/m²
• Healthy weight: 18.5 to 24.9 kg/m²
• Overweight: 25 to 29.9 kg/m²
• Obese: 30 kg/m² and above

Why BMI May Not Tell the Full Story After Cancer Treatment

BMI may not accurately estimate how much fat you have because weight loss or weight gain during and after cancer treatment can be traced to things other than fat. Cancer treatments can:

• Reduce muscle mass
• Cause fluid retention
• Change how you store and distribute fat
• Affect digestion (e.g., cause bloating)

All these changes can make you weigh more without changing your body’s fat composition.

When Weight Gain Should be Checked by a Doctor

You should reach out to your doctor about your weight gain during cancer treatment if: (5)

• You are severely bloated
• Your feet, hands, or abdomen are swollen
• You feel breathless
• Your chest hurts, feels tight, or you are wheezing
• Your abdomen hurts persistently

Yes, cancer treatment can cause weight gain. But it varies from person to person and the type of cancer treatment you are receiving.  Weight gain could be due to more fat storage, fluid retention and reduced muscle mass. Some people gain weight while losing muscle mass, a situation sometimes called sarcopenic obesity. This can affect strength, mobility, and overall health even when body weight increases. Cancer treatments could increase your appetite, making you eat more and eventually gain weight. Or they could disrupt your metabolism, hormones or body function in ways that retain fluid, burn fewer calories or reduce your muscle mass.

Chemotherapy and Weight Gain

Chemotherapy-related weight gain can happen because the drugs may alter your gut function and make you too fatigued to sustain physical activity. Steroids used alongside chemotherapy can also contribute to weight gain. (6)

While some people may gain weight during chemotherapy, a large number lose weight instead. (7) Also, weight gain during chemotherapy and after chemotherapy are not the same thing. 

Weight gain after chemotherapy can continue for weeks and months or even years after cancer treatment has stopped. (8)

ABVD Chemotherapy and Weight Gain

Steroid use is important in ABVD chemotherapy to manage symptoms. (9) ABVD chemotherapy is a combination of four drugs used to treat Hodgkin’s lymphoma. However, steroid use has been associated with weight gain and may be the reason why weight gain happens during ABVD chemotherapy. (10)

Steroids used During Cancer Treatment and Weight Gain

Steroids like dexamethasone and prednisolone can increase your appetite, cause fluid retention and change the way your body metabolises and deposits fat around the body. The weight gain may go away when you stop taking the medication. (10)

Radiotherapy and Weight Gain

Radiotherapy doesn’t cause weight gain directly. However, it can make you feel fatigued, causing a reduction in physical activity leading to weight gain. Changes in appetite may occur during or after radiotherapy, although experiences vary between individuals depending on the treatment area and side effects. (11)

Immunotherapy and Weight Gain

Weight gain whilst undergoing immunotherapy treatment usually arises from its side effects. Side effects like fatigue, changes in hormone levels, and increased fluid can all contribute to weight gain. (12)  Speak to your healthcare team if you notice sudden or extreme weight gain.

Hormone Therapy and Weight Gain

Hormone therapy can slow your metabolism, reduce muscle mass and make your body store more fat. Cancers like breast and prostate cancer, which depend largely on hormone therapy, may result in weight gain during treatment. Androgen deprivation therapy (ADT) is particularly known to cause a decrease in muscle tissue and an increase in body fat. (13) 

It can be confusing when you notice that you are gaining weight during and after cancer treatment. That happens for several reasons, not limited to hormonal changes, burning fewer calories than usual, and changes in muscle mass.

Reduced Activity and Fatigue

Feeling tired all the time as a result of cancer treatment can make you move less. This means you are going to use less energy than usual; if your calorie intake stays the same or improves, you are likely to gain weight. (14)

Changes in Metabolism and Muscle Mass

You may lose muscle during cancer treatment, which reduces your calorie needs. If you keep eating the same quantity of food (or more), you are likely to gain weight. Your metabolism may also be slower, making your body retain more fat.

Hormone Changes (Including Early Menopause)

Menopause-related changes can contribute to weight gain, especially after breast cancer and your body may store more fat, often around your belly. (15)

Stress, Emotional Eating and Disrupted Routines

Your mental state affects your body. If you feel physically, emotionally, and mentally stressed, you may want to eat more. You may also change your routines and exercise less. This contributes to weight gain. (16)

Fluid Retention and Bloating

Your body can retain fluid during cancer treatment. This can show up as swelling in your limbs, feeling bloated, oedema, ascites, or just rapid weight gain. (17)

Although weight loss is the more common experience, it is possible to gain weight during cancer and cancer treatment. Certain cancers are more likely to cause weight gain, including:

Ovarian Cancer and Weight Gain

Ovarian cancer and its treatment can cause fluid retention around the abdomen. (18) Doctors call this “ascites”. Ovarian cancer can also cause hormonal changes and make you more sedentary. You should contact your doctor at once if your symptoms become severe or too sudden.

Lung Cancer and Weight Gain

Some people receiving treatment for lung cancer may experience weight gain due to reduced activity, steroid use, fluid retention, or treatment-related changes in body composition. (20)

Liver Cancer and Weight Gain

Liver cancer weight gain is mostly caused by your body storing up more fluid than usual. Reduced physical activity due to fatigue also contributes. (21)

Bladder Cancer and Weight Gain

Bladder cancer can cause swelling and pain, which makes you move around less. And the steroids used to treat it can cause weight gain by increasing appetite and causing your body to store more fluid than it needs.

If you are concerned about weight gain, changes in body composition or changes in eating habits after cancer treatment, ask your healthcare team whether a referral to an oncology dietitian would be helpful. It’s best to manage weight gain after cancer treatment gradually. Aim for slow, sustainable changes you can commit to. 

Safe Exercise after Cancer Treatment

After cancer treatment, safe exercise can help you regain energy and burn fat. Aerobic exercises (exercises that increase your heart rate) like brisk walking, some gentle strength training and guided physiotherapy are all good options. (22)

Nutrition Tips for Weight Management 

A healthy weight begins at the plate. (23) You should aim to:

• Follow a healthy, balanced diet rich in plant-based or lean proteins, whole grains, fruits, and vegetables.
• Stay well hydrated to support recovery and help manage weight changes.
• Avoid extreme dieting or restrictive eating (e.g., cutting out whole food groups such as carbohydrates or fats) because this can affect your energy levels and hinder recovery.

Oral nutritional supplements may be recommended for people who are unable to meet their nutritional needs through their normal diet, particularly if they are losing weight, eating poorly, or experiencing treatment-related side effects. Your healthcare team or dietitian can advise whether they are appropriate for you.

Sleep and Stress Management

Your body releases a hormone called cortisol when you become stressed; cortisol increases appetite and contributes to weight gain. Getting enough sleep helps reduce cortisol levels and support a healthy weight. (24)