There are several well-established links between diet and the causes of cancer. It is thought that diet plays a part in the development of up to a third of all cancers. However, there is no scientific evidence that eating a particular diet can treat the cancer once you have got it or reduce the risk that it may come back if you are in remission.

In general, eating fresh fruit, vegetables and foods that are whole grain and high-fibre is good for you and may reduce the risk of getting cancer. Diets based more on fish (especially oily fish like salmon or mackerel) and chicken, and less on red meat and animal fats, may also reduce the risk. A high intake of smoked food may slightly increase the risk of some cancers.

Although diet cannot cure or control cancer, eating a healthy diet can improve general health and can help to give people a sense of control over their life and their future, which can be very beneficial. Your doctor, specialist nurse or dietitian can give you advice on healthy eating.

Whether or not you decide to change your diet may also be affected by your family's views. Families who are coping with the stress of a relative with cancer may add to the pressures on themselves by suddenly becoming vegetarians, or trying to get used to herbal teas. It is important to balance your doctor's or dietitian's advice with common sense.

The basic principles of a healthy diet are:

Watch your weight

Try to maintain your weight within the normal range for your height (your GP can advise you on your ideal weight) by using the following guidelines:

• eat plenty of fresh fruit and vegetables
• eat foods that are high in fibre – such as wholemeal bread and grains
• eat more fish and chicken and less red meat
• eat less fat
• eat less sugar and salt
• have only moderate amounts of tea and coffee.

It is also important to take time to eat slowly, peacefully and with enjoyment.

Be sensible with alcohol

Current sensible drinking guidelines recommend that:

• men drink no more than three units of alcohol per day, or 21 per week
• women drink no more than two units per day, or 14 per week.

One unit of alcohol is the equivalent of a small glass of wine (125ml), half a pint of standard strength beer or lager, or a single measure (25ml) of a standard strength spirits.

The guidelines on healthy eating do not apply to people who have lost a lot of weight. They may need to eat whatever foods they can manage in order to get enough calories and protein. There may be times when someone with cancer wants to eat foods that are considered unhealthy but are very enjoyable. In this situation, it is sensible not to be too rigid about a healthy diet.

Our separate section on diet and cancer may be helpful to read if you want more information on dealing with weight loss or eating problems due to cancer or its treatments.

Many complementary therapists do not recommend strictly vegan or vegetarian diets but are convinced that a good diet can be helpful. They create special diets, with the help of the person with cancer, which are as nutritious and enjoyable as possible.

The Bristol diet, developed at the Penny Brohn Cancer Centre (formerly the Bristol Cancer Help Centre), is a type of complementary diet therapy. It is perhaps the best known diet in the UK for people with cancer. The emphasis of this diet is on whole foods; fresh fruit and vegetables; raw cereals; and organic fish, poultry and eggs. The diet stresses the avoidance of dairy produce (such as milk, cheese and yoghurt), red meat, salt, sugar and caffeine (eg tea and coffee) and encourages the use of organically grown produce.

The diet can be modified to a person's individual needs, and the guidelines recommend that changes should be made gently and without pressure. They emphasise that eating should be enjoyable and not stressful. You can get information about the this diet from the Penny Brohn Cancer Centre.

There has been a lot of publicity about alternative diets for treating cancer over the past few years. Some alternative diets claim to rid the body of toxins that accumulate from the food we eat in the Western world. Many of these dietary therapies are strictly vegan or vegetarian. They involve eating food that is raw, sugar-free and low in salt. Sometimes, vegetable/fruit/liver juices and high doses of vitamins/minerals/enzymes may be used. Some recommend taking large doses of vitamins.

Many dramatic claims for cures of people with advanced cancer have been made and it is completely understandable that people with cancer should be attracted to diets which seem to offer the hope of a cure. However, there is no reliable evidence that alternative diets can extend the life of a person with cancer. As these diets have not as yet been properly studied, their real effect is uncertain.

There is increasing evidence that healthy eating can help to reduce the risk of cancer. However, there is no scientific evidence that cutting out key elements of a normal diet, as some diet therapies advise, will improve the outcome for people once they have cancer.

Cancer doctors and dietitians are concerned that many alternative diets are completely inappropriate for most cancer patients. They can be high in bulk, and low in calories and protein, which can cause weight loss. Weight loss may be a particular problem if people have already lost weight from the cancer itself or its treatment. They then need all the protein and calories they can get, from whatever source.

Some people get pleasure and satisfaction from preparing these special diets, but others find them quite boring, time-consuming to prepare and even unpleasant to eat. Some of the alternative diets are very expensive. Some may even be harmful to people with cancer.

People can also feel very angry and let down if they are led to believe that a particular diet will cure or control their cancer and this does not happen. They can also feel that they are to blame for not following the diet properly or because they did not believe in it enough.

It can be very confusing to be faced with conflicting advice about what to eat, but most doctors and specialist nurses recommend a well-balanced diet and one that you enjoy.

If you have any queries about alternative diets, or are thinking of following one, get further advice from your doctor, specialist nurse or dietitian.

Gerson therapy was created by a German doctor, Max Gerson. You may also hear it called the Gerson Regime or the Gerson diet. Gerson therapists claim that the diet works by cleansing the body of toxins, boosting the immune system, and stimulating the body’s metabolism.

The diet itself is strictly based on organic fruit and vegetables, taken mainly as juice prepared with a juice extractor. In addition a number of nutritional supplements are given, and coffee enemas are used to 'flush out toxins'. This diet needs a lot of time and commitment and may be expensive to follow.

It has been claimed that the Gerson diet is effective in treating cancer but evidence from research studies so far does not support these claims.

People who have used the Gerson diet have reported the following effects: nausea, vomiting, stomach cramps, a high temperature and headaches. Other possible side effects include perforation or rupture of the large intestine (colon) from using enemas, infections from the enema devices and serious fluid, electrolyte, and vitamin imbalances. The high levels of hormones and extracts used can sometimes lead to toxicity and may disrupt the metabolic balance in the body.

As the body cannot make vitamins in large enough amounts, they are an essential part of a healthy diet. Tiny amounts of certain vitamins help to protect the body from cancer. Therapists who recommend very large doses of vitamins (megavitamin therapy) in both preventing and treating cancer have tended to assume that if a little of a vitamin will do you good, a lot will be even better. However, there is no evidence that taking large doses of vitamins is helpful in treating cancer, and recent research has shown that high doses of vitamins may be harmful.

The best way to get the vitamins and minerals you need is by eating a healthy diet.

Selenium is a mineral trace element which can be found in brazil nuts, grain, fish and meat. There is some suggestion that selenium may protect against cancer. However, it is poisonous if taken in large doses.

Vitamins used in megavitamin therapy:

• Vitamin A – There is some evidence to suggest that vitamin A may have a protective action in some forms of cancer. However, very high doses can be harmful.
• Vitamin C – Vitamin C is the most popular of the vitamin supplements used for cancer. Studies have shown that eating foods rich in vitamin C reduces the risk of developing some cancers. But a review of studies in which vitamin C was given as a supplement concluded that it is uncertain whether taking greater amounts of vitamin C than you could get through a healthy diet helps to prevent cancer occurring.
• Vitamin E – This is also thought to possibly help prevent cancer. But as yet there is no reliable evidence to suggest it can help treat cancer.
• Vitamin Q10 is also sometimes used in this treatment. It has been suggested that molecules in the body called free radicals, may damage cell membranes, and that these changes to the cell membrane are part of the development of cancer. It has been claimed that vitamin Q10 prevents this damage. However, there is no good scientific evidence to support these claims.

Nutritional therapists focus on using diet as part of the healing process. They do not aim to cure a cancer, but try to improve the natural health of the body. A nutritional therapist will assess your general health and recommend a diet that is specific to your needs. There is no evidence that nutritional therapy can cure a cancer or reduce the chance of it coming back.

You can get more information about nutritional therapy and find a registered therapist from the British Association for Nutritional Therapy.