Why Does Weight Loss Happen During Cancer Treatment?

Perhaps you are leaving more food on your plate, or your clothes feel loose even though you have not been trying to lose weight. You may have noticed the change yourself, or someone close to you may have mentioned it.

During cancer treatment, eating can become difficult for everyday reasons. Food tastes different. Your mouth feels sore. Nausea puts you off a meal, or you are too tired to prepare one. But eating less is only part of the picture. Cancer can also affect how your body uses nutrients and energy, so weight and muscle loss sometimes continue even when you are eating.

Is weight loss common during cancer treatment?

Weight loss happens to many people with cancer, but everyone’s experience is different. Some people maintain their weight, while others gain weight during treatment.

The changes depend on your cancer, treatment and overall health. There is no single amount of weight loss that should be considered “normal” for everyone.

Along with your weight, notice how you feel. Are you eating less, feeling weaker or finding everyday activities more tiring? These details can help your care team understand what is happening.

What causes weight loss during cancer treatment?

Several things may contribute at the same time. You might have less appetite, find food unpleasant or feel too tired to prepare a meal.

A smaller appetite

Mealtimes may come around without you feeling hungry. You might sit down to eat, manage a few mouthfuls and feel you have had enough. Over several days, these smaller portions can leave you short of the energy and protein your body needs.

Nausea and vomiting

It is hard to eat when you feel sick. Even the smell of food cooking can be enough to put you off. If you are also vomiting, keeping down drinks may become difficult.

Let your care team know, especially if you cannot keep fluids down. They can review your symptoms and treatment to help you manage the nausea.

Changes in taste and smell

A favourite dish may no longer taste the way you remember. Food can seem bland or leave a metallic taste, and smells that never bothered you before may become unpleasant.

You may find yourself avoiding certain foods or stopping halfway through a meal because the taste is difficult to tolerate.

Mouth or swallowing problems

A sore or dry mouth can make each mouthful uncomfortable. Swallowing may hurt, or food and drinks may become difficult to get down.

Contact your care team promptly if swallowing is painful or difficult. They can assess what is causing the problem and advise whether changes to food texture, fluids or other support are appropriate for you. 

Digestive problems

When you are constipated, bloating and discomfort can leave little desire to eat. Diarrhoea can cause fluid loss and, in some cases, affect nutrient absorption.

Cancer and its treatment can also change how you digest food. If bowel problems continue, describe them to your team so they can work out what help you need.

Pain, tiredness and emotional strain

Sometimes, the difficulty starts before the meal. Shopping and cooking may take more energy than you have, particularly when you are tired or in pain.

Worry or low mood can make eating harder, too. Mention these difficulties to your care team, even if they seem less pressing than other treatment concerns. They are part of your care.

What is cancer cachexia?

Sometimes, weight loss continues even when a person is trying to eat enough.

Cancer cachexia is a condition involving ongoing muscle loss, often with loss of body fat. It can also cause weakness, tiredness and reduced appetite.

Inflammation and changes in how the body uses energy and nutrients contribute to this process. Eating more may therefore not fully reverse the weight and muscle loss.

Cachexia is not a matter of willpower. It does not mean someone is failing to try hard enough to eat.

Nutrition remains important, but care may also need to address symptoms and other changes happening in the body. Your treating team can assess whether cachexia is contributing to your weight loss.

Is cachexia the same as malnutrition?

They are related, but they are not the same.

Malnutrition develops when the body does not get or use enough nutrients to meet its needs. Eating less, swallowing difficulties and problems with digestion or absorption can all contribute.

Cachexia involves muscle loss alongside changes in inflammation and metabolism, meaning how the body uses energy. Nutrition alone may not fully reverse it.

A person can have both. This is why an assessment looks beyond how much you are eating.

Why does muscle loss matter?

Muscle helps you get out of a chair, climb stairs, carry shopping and move around. Losing it can make ordinary activities feel harder.

Muscle loss is not always obvious from appearance or body weight. It can happen in someone who does not look underweight.

Your team may ask about changes in strength and daily activities, as well as your weight and appetite. Mentioning that you now struggle with something you previously managed easily can be useful.

What can your care team do?

The first step is to understand why your weight is falling.

Your team may ask about:

• How much weight you have lost and over what period.

• What you can eat and drink.

• Changes in appetite, taste or swallowing.

• Nausea, bowel symptoms, pain or tiredness.

• Changes in strength and everyday activities.

• Your treatment, medicines and emotional wellbeing.

They may also assess you for malnutrition or cachexia. A dietitian experienced in cancer care can help you work out how to meet your nutritional needs.

The plan might include help with symptoms, changes to meals or nutrition support. What is appropriate will depend on your health and the cause of the weight loss.

Start by explaining what is getting in the way. “I feel full after three mouthfuls” or “the smell of cooking makes me feel sick” gives your team something specific to work with.

Depending on your needs, they may suggest:

• Smaller meals or snacks more often.

• Foods that provide more energy and protein in smaller portions.

• Ready-to-eat meals or help with shopping and cooking.

• Nutrition drinks if ordinary meals are not meeting your needs.

You do not have to work out the right diet by yourself. Swallowing problems, diabetes, kidney disease or bowel problems may mean you need different advice.

If you are struggling to maintain your weight, avoid starting fasting plans or restrictive “anti-cancer” diets without discussing them with your treating team.

If supplements or complementary approaches are part of your care, include them in your overall care plan and keep your healthcare team informed. These questions to ask before starting supportive cancer therapy can help you prepare for that conversation.

When should you contact your cancer care team?

Contact your team when you notice weight loss you were not trying to achieve. You do not need to wait until you have lost a large amount.

Ask for prompt advice if you:

• Cannot eat or drink enough.

• Have continuing vomiting or diarrhoea.

• Develop painful or difficult swallowing.

• Have mouth sores that interfere with eating or drinking.

• Feel increasingly weak or dizzy.

Follow the contact instructions given by your treatment team. Seek urgent medical help for new confusion, fainting, signs of severe dehydration or a sudden significant deterioration.

What should you ask at your appointment?

A few notes can help you explain what has changed. Write down your weight changes, appetite and any symptoms making eating difficult.

You could ask:

• What might be causing my weight loss?

• Do I need a nutrition assessment?

• Can we do more to manage symptoms affecting my appetite?

• Would advice from a dietitian help?

• Which food changes are suitable for me?

Supporting your health alongside cancer treatment

Eating difficulties can affect more than your weight. They may change your routine, make family meals stressful or leave you worried about maintaining your strength.

Supportive care during cancer treatment gives you space to discuss everyday wellbeing concerns while continuing your prescribed care. Your treating team should assess unexplained weight loss and guide any nutrition or cachexia management.

If you would like to discuss additional support, contact Dr Tarang’s care team to arrange a consultation. You can discuss what is affecting you most during your clinical consultation.