Nutrition during cancer treatment: when to see a dietitian

Eating well during cancer treatment helps you keep your weight and muscle, cope with side effects and recover. Cancer and its treatment often make eating harder, and unplanned weight loss is common. If you are losing weight or struggling to eat, ask your hospital doctor or specialist nurse to refer you to a dietitian [4][13]. Keeping active alongside eating well helps protect your muscle [5][13].

Why does nutrition matter during cancer treatment?

People with cancer are at high risk of malnutrition, because both the cancer and its treatment can affect how well you eat and how your body uses food [1]. Malnutrition and loss of muscle are common in cancer and are linked with worse outcomes from treatment [3].

UK prehabilitation guidance published in 2019 by Macmillan, the Royal College of Anaesthetists and the NIHR Cancer and Nutrition Collaboration said people who are underweight and losing weight have "an increased risk of poorer outcomes", including longer hospital stays, wound infections after surgery and lower quality of life [1].

Most of that evidence comes from surgery. Weight loss matters even if you started out heavier: "people who are overweight or obese who then lose weight have poorer outcomes" [1].

Day to day, Cancer Research UK says a balanced diet "can help you feel better" and may give you more energy to cope with treatment [12]. Nutrition is also one of the three parts of prehabilitation, with exercise and psychological support [2]. See our prehabilitation guide.

What eating problems can cancer treatment cause?

Common problems include weight loss, a poor appetite, feeling or being sick, pain, taste changes, swallowing problems, and diarrhoea or constipation [4]. Some come from the cancer itself and some are side effects of treatment [4].

Some types of cancer make your body use up more energy, and some people feel full quickly or find that food tastes different [6].

Weight gain happens too. Chemotherapy, steroids and hormonal therapies, such as tamoxifen, anastrozole or hormone therapy for prostate cancer, can cause weight gain, for example by increasing your appetite [7]. Macmillan says: "It is important to keep taking your medication even if you think it might be causing you to put on weight." Speak to your GP, cancer doctor or nurse before trying to lose weight [7].

Macmillan's 2025 prehabilitation guidelines do not recommend routine weight-loss plans for people preparing for or going through cancer treatment [18]. Our guides on breast cancer and exercise and prostate cancer and exercise cover activity on hormone therapy. If you have a stoma, see bowel cancer and exercise and ask your stoma nurse about eating and drinking.

What should you eat during chemotherapy?

There is no single right diet for chemotherapy. Eat a balanced diet when you can, but if you are losing weight, getting enough energy and protein comes first [13].

World Cancer Research Fund's guide, written with support from the British Dietetic Association's Oncology Specialist Group, says the best way to slow or stop weight loss is to "eat and drink as many calories (energy) as possible", choosing protein-rich foods "to help your body maintain muscle and repair tissue". Smaller meals or snacks through the day can help [13]. This general advice may not suit you if you are eating very little, losing a lot of weight, receiving palliative care, or follow a special diet for diabetes, heart or kidney disease or a stoma; ask your team or dietitian first [13].

Treatment can also lower your immunity. For most people this does not last long, so Macmillan says there is no need to follow a special diet [19]. But if your immunity is very low, for example during high-dose chemotherapy or after a stem cell or bone marrow transplant, your healthcare team may ask you to avoid some foods for a time. Follow their advice, and ask your team if you are not sure whether it applies to you [19].

Macmillan calls this a "building-up diet" and says "if you lose a lot of weight, it is important to talk to a dietitian" [5]. This article is not a diet plan. A dietitian can tailor advice to your treatment, side effects and other conditions.

When should you ask for a dietitian?

Ask for a dietitian if you are losing weight without trying [5], or if eating problems such as a poor appetite, taste changes or difficulty swallowing [4] mean you are struggling with your diet. You do not have to wait to be offered one [13].

How to get a referral. "You can ask your doctor or nurse at the hospital to refer you to a dietitian," says Macmillan, and "in some hospitals, you can refer yourself" [4]. Outside hospital, your GP or district nurse can tell you whether to refer yourself or refer you [4]. Your clinical nurse specialist (CNS) is a good person to ask: a CNS can "refer you to other services that can help" [17].

Screening. Macmillan's 2025 prehabilitation guidelines recommend that everyone is screened for nutrition problems "using a validated tool at the earliest opportunity and before initiating cancer treatment" [18]. If screening shows a raised risk, you should have a full assessment by a registered dietitian [18]. If nobody has weighed you or asked about your eating, raise it.

Supplement drinks and tube feeding. The same guidelines say eating and drinking by mouth should come first, helped by personalised advice and oral nutritional supplements where needed [18]. If you are not managing to eat enough, or are losing weight, your dietitian, GP, cancer doctor or specialist nurse may recommend nutritional supplements that add extra energy or protein [8]. If you have diabetes, ask your GP, specialist nurse or dietitian before using them [8]. Some people need tube feeding for a time [11].

What is the difference between a dietitian and a nutritionist?

A dietitian is a statutorily regulated health professional: the title is protected by law, and anyone using it must be registered with the Health and Care Professions Council (HCPC) [14][15]. "Nutritionist" is not a protected title [14].

The British Dietetic Association says: "Dietitians are the only nutrition profession with statutory (legal) regulation, meaning the title dietitian is protected by law." Dietitians "assess, diagnose and treat dietary and nutritional problems" [14]. The HCPC lists dietitian and dietician among its titles "protected by law" [15].

Nutritionists can join a voluntary register: only people on the UK Voluntary Register of Nutritionists should call themselves a Registered Nutritionist (RNutr) [14]. The BDA describes dietitians as the professionals who assess, diagnose and treat dietary and nutritional problems [14], so for eating problems caused by cancer or its treatment, a dietitian is the right person.

What is an oncology dietitian? A dietitian who specialises in cancer. It is a specialism, not a separate protected title. The BDA's Oncology Specialist Group began in 1988 among dietitians working with people diagnosed with cancer, and has sub-groups including head and neck, haematology and prehabilitation [16]. If you see anyone privately, check they are HCPC registered as a dietitian.

How do diet and exercise work together during cancer treatment?

Eating enough and staying active both help protect your muscle. European nutrition guidelines say "nutritional care should always be accompanied by exercise training" [3], and Macmillan advises: "Keeping active will stop your muscles getting weak" [5].

This holds even when your weight is falling. Macmillan says "it is important to try to keep active, even if you are losing weight" [5]. World Cancer Research Fund says muscle-strengthening exercises "are especially important because they can keep you from losing too much muscle and strength during treatment", and suggests telling your cancer care team before becoming more active, as they may be able to put you in touch with a qualified exercise specialist [13].

The two work best when planned together. Your dietitian manages the eating side; a clinical exercise physiologist can test your strength and fitness and build a programme around your treatment and energy levels. See exercise and cancer, exercise during chemotherapy and the cancer rehabilitation team for who does what.

Should you take supplements or follow a special cancer diet?

Do not start supplements or a special diet without speaking to your cancer team first. Some supplements may make cancer treatment less effective, and no diet can cure cancer or replace treatment [9][10][13].

Macmillan says: "If you are thinking of taking any food supplements, it is important to talk to your cancer doctor, specialist nurse, pharmacist or dietitian first." It warns that high-dose antioxidant supplements during treatment may make treatment less effective [9], as does Cancer Research UK, which adds that "there is no reliable evidence that dietary or nutritional supplements can prevent, cure or control the growth of cancer" [10].

On diets that promise to fight cancer, Macmillan is direct: "there is no diet or food supplement that can be used instead of conventional cancer treatment" [9]. Very restrictive "fad" diets may not give your body the nutrients it needs during treatment. If a vitamin or mineral is low, your doctor or dietitian can prescribe a supplement that is safe alongside your treatment [13].

How should you eat after cancer treatment ends?

Once treatment ends and you are eating normally again, the focus usually moves back to healthy eating and keeping active [13]. If you are still losing weight, or gained weight during treatment, speak to your team before changing your diet [7].

European guidelines advise that cancer survivors "should engage in regular physical activity and adopt a prudent diet" [3]. Our guide to exercise after cancer treatment covers rebuilding fitness, and cancer rehabilitation in the UK explains NHS and private support, with a page on cancer rehabilitation in London.

Talk to your oncology team first

Your oncology team knows your cancer, your treatment and your other health conditions. Tell them about any change in your weight, appetite or ability to eat. Speaking to your doctor, dietitian or specialist nurse "can be the start of getting help" [11]. If you have diabetes, heart disease, kidney failure or a stoma, general eating advice may not suit you [13].

The Health Nav does not currently match people with dietitians; the route to a dietitian is through your NHS cancer team or GP. We can help with the exercise side, building and keeping strength alongside your eating plan. Request a match with a verified clinical exercise physiologist through The Health Nav. You can also email support@thehealthnav.co.uk.

This article is general information and does not replace advice from your own healthcare team.

References

1. Macmillan Cancer Support, Royal College of Anaesthetists, NIHR Cancer and Nutrition Collaboration. Prehabilitation for people with cancer: principles and guidance for prehabilitation within the management and support of people with cancer. 2019 (the original UK guidance, since replaced by reference 18). https://cdn.macmillan.org.uk/dfsmedia/1a6f23537f7f4519bb0cf14c45b2a629/1532-10061/prehabilitation-for-people-with-cancer-tcm9-353994 (accessed 12 Oct 2026)
2. Davis J. Prehabilitation: transforming cancer care before treatment begins. Macmillan Cancer Support blog, 22 December 2025. https://www.macmillan.org.uk/healthcare-professionals/news-and-resources/blogs/prehabilitation-transforming-cancer-care-before-treatment-begins (accessed 12 Oct 2026)
3. Arends J, Bachmann P, Baracos V, et al. ESPEN guidelines on nutrition in cancer patients. Clinical Nutrition. 2017;36(1):11-48. https://doi.org/10.1016/j.clnu.2016.07.015
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