Diet and nutrition

Cancer and your diet

Often, people diagnosed with cancer make changes to their diet because they want to support their health. 

Information on cancer and diet can be very confusing and contradictory.

The media sometimes report that certain foods are ‘cancer-fighting’ or increase the risk of particular cancers. So far, research hasn’t established a conclusive link between any particular food or diet and the risk of ovarian cancer. The general advice  is to follow a healthy, balanced diet including vegetables, starchy foods, some dairy or dairy alternatives and healthy proteins such as lean meat, fish or eggs. If you would like individual advice about any dietary changes that could be helpful for you, you can ask your team. If necessary, you may be referred to a dietitian for specialist advice.

No conclusive evidence has been found that any dietary regime helps to shrink or stabilise cancer.

There is known to be a link between consumption of red and processed meats and the risk of bowel cancer. There have also been media reports in the past suggesting that dairy products can increase the risk of cancer, but so far the evidence does not support this.

Each person is different, and your dietary needs may change as you go through your treatment and after your treatment ends. For example, during chemotherapy you may eat differently if you have side effects such as nausea, mouth ulcers or taste changes.

It’s important to take in enough calories during chemotherapy to maintain your energy levels. If there are some foods that are easier for you to eat your team may advise you to eat enough of what you can enjoy or tolerate rather than trying to eat foods that you feel you ‘should’ have.

If your blood tests show any deficiency, such as iron, you may be given supplements or advised to eat specific foods. If you have a stoma, it’s likely that certain foods will affect how it functions and you will find that you need to adapt your diet. Your stoma nurse will be able to give you advice about this. You can view our information on stomas here.

If you have experienced or are at risk of a blocked bowel, you may be advised to eat less fibre.  This is known as a fibre-restricted diet or a low residue diet. You can find more information about diet for a blocked bowel here.

Watch: 'Diet and ovarian cancer', a webinar with oncology dietitian Selin Ramadan

Intermittent fasting

Intermittent fasting involves drastically reducing (or stopping) intake of calories for a defined period of time. 

Examples include consuming no calories for 12 or more hours per day and eating normally for the rest of the day or reducing intake of calories to a fraction of normal on alternate days or two days per week (as in the well-known ‘5:2’ diet) and eating normally on the other days. You can find more information about these plans here.

The theory behind this is that our bodies evolved over thousands of years to survive periods when food was scarce. Our early ancestors evolved to go into ‘growth’ mode when food was available and ‘repair’ mode in times of scarcity. Our bodies haven’t yet ‘caught up’ to life where food is always available.

The theory suggests that if we eat enough calories to stay in ‘growth’ mode all the time, it doesn’t give our bodies time to rest and repair, which isn’t the best way for us to stay healthy. 

In terms of cancer, one theory is that ‘repair’ mode involves slowing the rate at which our cells multiply and finding and destroying damaged cells that could otherwise develop into a tumour. So far, most of the evidence for this approach has come from animal studies or looking at test results in humans rather than actual health outcomes and it hasn’t been conclusively demonstrated in humans that fasting reduces cancer risk.

There is also a theory that fasting around the time of receiving chemotherapy can help to reduce some of the side effects. Chemotherapy works by damaging cells that are dividing, causing them to die, including cancer cells. This also includes other rapidly-dividing cells including those in the hair follicles and the lining of the digestive system, causing side effects like hair loss and diarrhoea. The theory suggests that if the healthy cells are in repair mode and not dividing quickly, they can avoid some of the chemotherapy side effects, while the cancer cells, which can’t slow, are still damaged. 

Intermittent fasting can efficiently decrease body mass and has a meaningful impact on the way cancer cells function. Because of this, research looking in to the role of intermittent fasting in the prevention and treatment of cancer is ongoing but there is no conclusive stance on the role of fasting. A recently published review summarises the evidence we have surrounding intermittent fasting in patients with cancer.

This review suggests that intermittent fasting alone, without any reduction in body weight, eating balanced diet and/or exercise, is unlikely to improve the effects of cancer treatment. If you have a poor appetite, reduced intake of food or have suffered unintentional weight loss, it is possible that intermittent fasting could cause you to become malnourished.

Making large changes to the number of calories we consume can affect our intake of the nutrients that we need to maintain our health. If you are going to reduce the amount that you eat, it’s important to make sure that you’re drinking enough water and getting the carbohydrates, proteins, fats and other nutrients that you need. In particular, when you’re having chemotherapy you may experience difficulty eating due to nausea, taste changes or mouth ulcers, for example, and it’s important to maintain your weight.

If you are considering making any significant changes to your diet, it's really important to get medical advice about whether this is appropriate for you at this time. 

Vegetarianism, veganism and soy

The NHS recommends that two thirds of our diets should consist of fruits, vegetables and starchy foods with the rest made up of dairy and/or dairy alternatives, proteins and a small amount of healthy fats. Therefore, most recommended food types are plant-based and the others can come from both animal and non-animal sources.

If you’re eliminating meat, fish or other animal products from your diet, it may take a little more planning to ensure that you’re getting the nutrition that you need. Meat is a good source of iron, but it is also possible your necessary iron from foods such as eggs, green leafy vegetables and pulses such as lentils and beans. Milk and dairy products contain calcium, but other sources include fortified plant ‘milks,’ almonds and bread. You can find more information about getting the nutrients that you need from a vegetarian or vegan diet here.

Two specialist oncology dietitians gave a presentation for us, in which they recommended aiming to meet your nutritional needs through diet. You can view this presentation here.

Although a well-balanced plant-based diet can provide most of the needed nutrients without supplements, they might be recommended in some situations, particularly vitamins D and B12.

The body makes vitamin D when skin is exposed to sunlight. It’s difficult to get enough from food alone. The Department of Health recommends that you consider taking a vitamin D supplement during autumn and winter, and at other times of the year if you're rarely outdoors or if you wear clothes that cover up most of your skin when you’re outside. Find out more about this here.

Vitamin B12 is only available naturally from animal sources. Vegetarians who eat dairy products and eggs usually get enough from these sources, but vegans will need to make sure that they eat plant-based foods fortified with vitamin B12 or take a supplement. You can find more information about vitamin B12 on the Vegan Society’s website here.

Information on other important minerals such as Iodine and Selenium, which are unlikely to be found in vegan diets can be found here.

If you’re pregnant or have a health issue such as a vitamin or mineral deficiency or difficulty in absorbing nutrients from your food, you may be advised to take a supplement. If you’re thinking of using supplements, it’s important to discuss this with your team first to make sure that they’re safe for you.

Soy can be made into products similar to meat items such as sausages and dairy products such as milk. However, some media reports have suggested that soy can increase the risk of certain cancers or not be suitable for people with cancer. This arises from the fact that soybeans contain isoflavones, which are often defined as phytoestrogens (chemicals found in plants similar to oestrogen). As some ovarian and breast cancers are driven by oestrogen, we are sometimes asked whether consuming products that contain isoflavones is safe for those diagnosed with these cancers.

The evidence currently says that there are health benefits to consuming soy in its various forms, and that eating moderate amounts of soy is safe, including for those diagnosed with cancer. It can also help to alleviate hot flushes associated with surgical menopause.

Although there are similarities between isoflavones and oestrogen, they don’t behave in the same way in the body and their effect is much weaker. The studies that suggested a possible connection between soy and oestrogen-driven cancers were carried out in laboratories or on animals using higher doses of isoflavones than are found naturally in soy.

Specialist cancer dietitian Rachel White wrote about soy in the Ovacome Magazine Autumn/Winter edition in 2018 (pages 10-11). You can also read more about soy as part of a healthy diet, including for people with cancer here.

You can also read more about meat, fish and dairy products and cancer here.

Watch: 'Q&A with oncology dietitian Selin Ramadan'

Sugar and carbohydrates

Carbohydrates fall into two categories:

• Simple carbohydrates (also called free sugars) found in food like sweets and ice cream, which contain one or two carbohydrate molecules joined together.
• Complex carbohydrates such as starch (made of longer chains of molecules) found in foods such as bread, pasta and potatoes. Complex carbohydrates can be divided into processed or refined grains that have had their fibre removed (such as white bread) and wholegrain carbohydrates (such as wholemeal pasta)that contain fibre and other nutrients.

You can find more information about carbohydrates here.

The NHS Eatwell guide recommends that a third of our diets should consist of starchy foods such as potatoes and grains, and another third of fruit and vegetables, which all contain carbohydrates. As well as carbohydrates, which are our bodies’ main source of energy, these foods also contain other nutrients such as vitamins, minerals, fibre and amino acids (the building blocks of proteins). Therefore, reducing or eliminating carbohydrates altogether will also have significant effects on the rest of your diet and nutrition.

All of the carbohydrates that we eat (apart from insoluble fibre, which passes through our bodies and helps waste to move through the bowels) are broken down through the process of digestion into glucose molecules that our cells use to produce energy.

Free sugars are digested and reach the bloodstream as glucose more quickly, whereas complex carbohydrates raise blood glucose more gradually. Excess glucose that the cells don’t need for energy is stored as a molecule called glycogen. If the body’s glycogen stores are full, any extra glucose is converted to fat. You can read more about carbohydrates and their role in a healthy balanced diet here.

Cancer cells undergo a change in their cellular machinery meaning that they produce their energy differently to normal cells. This means that they need a lot of glucose to continue growing and multiplying. This is why some people believe that cutting down or eliminating sugar from their diets will prevent cancer from growing.

However, as we mentioned above, all the carbohydrates that we digest ultimately enter the bloodstream as glucose and there isn’t a way to stop only cancer cells from accessing glucose while maintaining the essential supply to healthy cells. You can read about this in more detail here.

There are other reasons to reduce our intake of foods that are high in sugar and/or processed or refined carbohydrates. The Eatwell guide says that foods high in salt, fat and sugar such as crisps, sugary drinks and chocolate aren’t needed in our diet and recommends eating these foods less often and in small amounts as a treat.

These foods have limited nutritional value and eating too much of them can lead to conditions such as obesity, high blood pressure and type II diabetes. However, it doesn’t mean that all sugars or carbohydrates are bad for us. As mentioned earlier, many foods containing carbohydrates such as vegetables and whole grains also contain other important nutrients that our bodies need to stay healthy.

There are several different ‘low-carb’ diet programmes.The ‘ketogenic diet,’ for instance, is high in fat and very low in carbohydrates, aiming to cause cells to ‘switch’ from using glucose as their energy source to ‘ketones’ produced in the liver. It is sometimes used to treat intractable epilepsy in children and there is a current clinical trial investigating whether it may be helpful for glioblastoma, a type of brain tumour.

The ketogenic diet is very different from the healthy diet recommended by the NHS and can be challenging to follow. It requires careful planning to ensure that you get enough nutrients that would otherwise be obtained from carbohydrate-rich foods. This can be complex and time-consuming and may make dining out challenging if menus don’t include suitable dishes. Many people experience side effects, especially when they first start the diet and their bodies aren’t used to it, sometimes called ‘keto ‘flu.’ You can read about the experiences of a paediatric dietitian who tried the diet here.

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