Nutrition for people with cancer: what to eat during treatment

What foods should a person with cancer avoid?

There is no general list, and that is the honest answer to the most frequently asked question. The European ESPEN and ESMO guidelines do not rule out any food group because someone has a tumour or is receiving chemotherapy [1][3]. What is adjusted is something else: the symptom you have this week. If you have mouth sores, leave out lemon and tomato while they last. If you have diarrhoea, you can avoid insoluble fibre and pulses for a few days. If the smell of meat puts you off, replace it with eggs or fish. None of this is permanent. For questions about the prostate, see “Prostate and diet”.

The difference matters more than it may seem. Keeping a long list of excluded foods for months reduces how much you eat and makes your diet less varied. It also speeds up the loss of weight and muscle [1]. So when someone gives you a list of twenty foods to avoid, a useful question to ask is for how long? and because of which symptom?.

There is one exception, and it is not dietary: foods your medical team has restricted because of your specific situation — recent digestive surgery, an obstruction, profound immunosuppression, or a medicine with a known interaction. That takes precedence over anything you read here.

Why do people lose weight with cancer, and how much should you eat?

Weight loss happens in two ways at once. The illness changes your metabolism: your body uses energy and breaks down protein in ways that do not depend on what you eat. In addition, treatments reduce appetite, alter taste, and cause nausea, mouth sores or diarrhoea. As a result, you eat less. This combination has a name — cachexia — and is why your team asks about your weight at every visit [3].

It helps to understand why they insist on this. The ESPEN practical guideline estimates that up to 10–20% of people with cancer die from consequences of malnutrition [1]. This figure explains why it is important to detect and address weight and muscle loss early.

The reference figures in the two European guidelines are similar. ESPEN recommends estimating energy needs as you would for a healthy person: 25 to 30 kcal per kilogram of body weight per day [1]. Protein should be above 1 g per kilogram per day and, if possible, reach 1.5 g [1]. ESMO sets a minimum of the same 25 to 30 kcal per kilogram and at least 1.2 g of protein per kilogram [3]. For someone weighing 65 kg, that is about 1,600 to 1,950 kcal and between 65 and 100 g of protein a day. An egg, a yoghurt, a serving of fish and a plate of pulses already provide a substantial part of that.

Protein alone is not enough. Both guidelines pair it with physical activity, because muscle is maintained by using it [1][3]. It is the same principle we work on in the longevity guide to maintaining muscle and bone, applied at a time when it is more difficult. And if you are already losing weight and cannot stop it, that is precisely what is addressed in a dietitian appointment to gain weight, always in coordination with your oncology team.

What should you eat when you feel nauseous or the smell of food puts you off?

Nausea and vomiting are common side effects of chemotherapy and can be treated with medicines. If the prescribed antiemetics do not work, tell your team: diet is not a substitute for them. Food can make things easier. These are the recommendations in the nutrition guide from the Spanish Association Against Cancer [2]:

• Eat small amounts every few hours. An empty stomach makes nausea worse.
• Have something dry before you get up properly: toast, breadsticks or plain biscuits.
• Have food lukewarm or cold. Heat is what releases the smell.
• Sip drinks throughout the day, preferably between meals rather than with them.
• Avoid fried foods, very fatty or very sweet foods, and heavily seasoned foods on treatment days.
• Ask someone else to cook, and stay out of the kitchen while the food is being cooked.
• Do not lie down straight after eating; rest with your upper body raised.

If you also have heartburn or an irritated stomach, many of these recommendations are similar to those in the diet for gastritis. If you have bloating and gas, the guide to a bloated abdomen explains why it happens and what may help.

What if food tastes metallic or has no taste?

This is called dysgeusia. It is one of the most discouraging side effects because it can make something that used to be pleasurable unpleasant. There is no trick that will solve it, but the AECC guide brings together several ways to make it easier to manage [2]:

• If you notice a metallic taste, use plastic cutlery and plates, and cook with a wooden spoon. Avoid letting vinegar, tomato or juice come into contact with metal containers.
• Rinse your mouth before eating: this almost always improves the taste of what follows.
• Cook with mild seasonings — basil, oregano, bay leaf or thyme — or marinate meat and fish in orange or lemon juice, if your mouth is not sore.
• If red meat feels impossible to eat, replace it with chicken, turkey, rabbit, eggs, fish or tofu. What matters is the protein, not where it comes from.
• Hide meat and fish in purées and soups, and boil them or cook them in the microwave; they smell less this way than when grilled or oven-roasted.

Taste usually returns gradually after treatment ends. In the meantime, the goal is not to enjoy the food, but to get it in.

What should you eat if you have mouth sores or a dry mouth?

Inflammation of the lining of the mouth is called mucositis and makes eating painful. Dry mouth, or xerostomia, is very common during radiotherapy to the head and neck. Both change what is suitable to eat, but in opposite ways: with mouth sores, leave out anything that irritates; with a dry mouth, add foods that lubricate and stimulate saliva [2].

If you have mouth sores, leave out coffee, tea, alcohol, spicy foods and acidic foods for a few days: tomato, lemon, orange, pineapple, vinegar and gazpacho. Also avoid fizzy drinks, very salty foods, and dry or fibrous foods such as nuts, toast, crisps and very mature cheeses. Rinse your mouth with chamomile, thyme or bicarbonate three or four times a day, never with mouthwash containing alcohol, and try holding very cold water or crushed ice in your mouth for a few minutes before eating [2]. If drinking hurts, a straw can keep the liquid from passing over the sore area.

If you have a dry mouth, the recommendation is the opposite: dishes with sauce, soups, purées, rice in broth, well-cooked pasta, fresh cheeses, yoghurts, gelatine and ripe fruit, as well as small sips of water throughout the day. Acidic foods, which can irritate mouth sores, can help here because they stimulate saliva [2]. That is why the two sets of recommendations should not be combined: choose the one that matches what you are experiencing.

Radiotherapy to the head and neck, chest and abdomen can have a major effect on eating. For this reason, ESPEN recommends individualised nutritional advice from the outset, to prevent weight loss from interrupting treatment [1].

What should I do about diarrhoea and constipation?

Both can occur, sometimes in the same person and in different weeks, and they are managed in opposite ways.

For diarrhoea, hydration comes first: drink more than usual —water, mild herbal infusions, defatted broths— and eat small amounts every two or three hours. Boiled rice, apple or potato purée, boiled white fish or chicken, white bread and pasta are usually better tolerated. So are a plain omelette, banana, peeled apple and yoghurt. For a few days, avoid vegetables that cause flatulence, irritating seasonings, chocolate, alcohol, wholegrain foods, pulses and nuts [2]. If the diarrhoea is heavy and lasts more than a day, tell your care team: you may need oral rehydration solution from a pharmacy or an antidiarrhoeal, which your doctor will prescribe [2].

For constipation, which is often caused by medication, eating little and moving less, drink more fluids and switch refined grains for wholegrain ones. Eat pulses three or four times a week, five portions of fruit and vegetables, and a handful of nuts. Go for a daily walk [2]. This is covered in more detail in the constipation diet guide.

Is a special diet needed when your immune defences are low?

This is where recommendations have changed the most, and it is worth being clear about it. For decades, people were prescribed what was called a low-bacterial-content diet or a “neutropenic diet”. It excluded yoghurt, cheese, kefir, fruit that cannot be peeled, nuts, mushrooms and raw vegetables. There is no supporting evidence for this long list of exclusions. ESPEN reports that a Cochrane review found no evidence that these diets prevent infections during chemotherapy-induced neutropenia [1]. It also found no consistent clinical data to recommend them beyond the first month after an allogeneic progenitor cell transplant [1]. A later meta-analysis of six studies involving more than 1,100 people reached the same conclusion [5]. The neutropenic diet did not result in fewer infections or lower mortality than a diet without these restrictions.

Food hygiene measures remain in place and should be followed more carefully. Wash your hands before cooking and eating. Cook meat, fish and eggs thoroughly. Do not defrost food at room temperature. Disinfect fruit and vegetables that you plan to eat raw and unpeeled. Follow use-by dates and maintain the cold chain [2]. These are food safety measures, not a restrictive diet.

And the decision is not ours to make. If you are in hospital, undergoing a progenitor cell transplant or experiencing profound immunosuppression, your hospital sets the rules, and they take priority over any general guide. You can ask at your appointment how long the restriction you were given should last, because keeping it in place longer than necessary contributes to weight loss and reduces variety.

Do alkaline diets, fasting, cutting out sugar or supplements help?

No, and it is worth explaining why, because these are the four things that come up most often.

• Alkaline diet. It is based on the idea that certain foods make the blood more acidic. The nutrition guide from the Spanish Association Against Cancer sums it up in two words: it is false [2]. Your body regulates blood pH through your kidneys and lungs, not through your menu.
• Sugar “feeds” the tumour. Almost all the cells in your body use glucose as fuel, so cutting out sugar does not leave the tumour with nothing; it leaves you with less energy at the worst possible time. It is sensible to limit pastries and soft drinks because of their association with excess weight, which is linked to several types of cancer, and for no other reason [2].
• Ketogenic diet and fasting. There are no clinical trials showing a benefit of the ketogenic diet in people with cancer, and its poor palatability may lead to eating less and losing weight [1]. On fasting, ESPEN is explicit: it does not recommend fasting before, during or after chemotherapy [1].
• Supplements. ESPEN recommends providing vitamins and minerals in the recommended amounts and advises against high doses without a documented deficiency [1]. The AECC goes a step further and warns that vitamin, antioxidant or herbal extract supplements may interfere with treatments and worsen clinical outcomes [2]. Oral nutritional supplements —high-calorie pharmacy shakes— may help when food intake is not enough [3]. Your doctor or dietitian-nutritionist should prescribe any supplement.

The fact that a diet does not cure cancer does not mean that food makes no difference. It means its role is different: supporting your weight, preserving muscle and helping you reach each cycle in better condition. If you were looking for general guidance on eating better without promises, the anti-inflammatory diet guide explains what to expect from it and what not to expect.

Are there foods that cause cancer, and what should you eat after treatment?

No single food causes cancer on its own, just as no single food cures it. The World Cancer Research Fund reviewed its cancer prevention recommendations in 2018 [4]. They are also advised, as far as possible, after a diagnosis: maintain a healthy weight and stay active. It suggests eating wholegrains, vegetables, fruit and pulses. It advises limiting fast food, processed foods high in fat or sugar, red meat and sugary drinks. It recommends eating little or no processed meat and not drinking alcohol [4]. The same report adds two points that should be read together: do not use supplements to prevent cancer, and, after a diagnosis, follow these recommendations as far as possible [4].

That “as far as possible” is the key to what comes after treatment. When treatment ends, you do not suddenly go back to the way you used to eat. First, you regain the weight and muscle you lost, with enough protein and strength exercise [1][3]. Then you adjust the rest. If you have gained weight during treatment, as happens with some tumours and hormone therapies, you can follow the weight-loss diet, without rushing and without crash diets.

An oncology dietitian can help you put all of this in order. They review your report, treatment and symptoms this week and turn them into a plan you can follow.