The safety of vitamins and diet supplements | Cancer Research UK
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Vitamins, minerals and other nutrients

We need certain nutrients for our bodies to work properly, such as vitamins, minerals, essential fats and amino acids. Some examples are:

  • Vitamins A, C, or D 
  • Minerals like zinc, calcium, selenium and magnesium
  • Essential amino acids – phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, and lyseine
  • Essential fats
  • Some plant compounds (phyto nutrients or botanicals) – for example, carotenoids, flavonoids, and isoflavones

The best way to get the vitamins, minerals and other nutrients you need is through a balanced and varied diet, with plenty of fruit and vegetables. The Food Standards Agency has information about how to eat a healthy, balanced diet. It includes details of the amounts of particular nutrients you need each day.

But you may find that your cancer or its treatment makes it difficult to eat or drink properly. Or your cancer may stop you absorbing some nutrients from your food or drink. So you may have low levels of particular nutrients. 

Low levels of some nutrients in the body can make us ill. For example, if you don’t have enough vitamin C in your diet you will develop a condition called scurvy. Scurvy causes bleeding gums, extreme weakness and bleeding under your skin.

You can read about how cancer and treatment can make it difficult to get enough calories and nutrients from food on our coping with diet problems pages.


Dietary supplements and cancer

If you have low levels of particular nutrients, you may need to take dietary supplements. They are also called nutritional supplements. Vitamins and dietary supplements come as pills, tablets or a liquid. Some complementary or alternative therapists use injections of dietary supplements. 

Your doctor may prescribe particular nutritional supplements in certain situations. For example, if you take hormonal therapy that could weaken your bones, your doctor may prescribe calcium and vitamin D supplements. If your cancer stops you from absorbing nutrients easily from food your doctor may prescribe a daily multi vitamin and multi mineral supplement. 

Dietary supplements are widely used by people with cancer to help fight their cancer or make them feel better. If you are thinking of taking nutritional supplements, you need to get advice from a health professional. Most people use these products as well as their conventional cancer treatment. But others choose to use them as an alternative therapy instead of having conventional treatments such as radiotherapy or chemotherapy. Using nutritional supplements instead of conventional cancer treatment could be harmful to your health and may greatly reduce the chance of curing or controlling your cancer.


Dietary supplements and preventing cancer

At the moment there is no reliable evidence that any type of dietary supplement can help to prevent cancer. But there is evidence that a healthy diet with plenty of fruit and vegetables can reduce your cancer risk. 

You can find out more about about a healthy diet

Some research has looked at whether particular vitamins and dietary supplements can help to prevent cancer in certain groups of people. One large study in the USA found that giving vitamin E supplements to male smokers reduced their risk of prostate cancer. It also found that giving beta carotene supplements to men with low levels of beta carotene in their diet reduced their risk of prostate cancer. Beta carotene is a substance the body uses to make Vitamin A. But the supplements had no effect for non smokers or men who got normal levels of beta carotene from their diet. 

Although eating foods that contain beta carotene can help to reduce lung cancer risk, taking beta carotene supplements does not seem to have the same effect. In 2012 a review of 9 studies looked at the effect of various vitamins and minerals on lung cancer risk. It found that for people not at high risk of lung cancer none of the supplements of vitamins or minerals or their combinations increased the risk. But for people at high risk of lung cancer, such as smokers or people exposed to asbestos, taking beta carotene supplements gave a small increase in lung cancer risk.


Dietary supplements during cancer treatment

Most supplements are safe for people to use alongside conventional cancer treatments. But we don't know very much about whether some types of supplement could interact with particular types of cancer treatment. Guidance by the Royal College of Radiologists states that very little is known about the interactions between cancer drugs and nutritional supplements. They advise that if you are not able to eat a normal, balanced diet and want to take vitamin supplements you can take a good quality, once a day, multivitamin and minerals pill, making sure not to have more than the stated dose.

It is especially important to get advice from your doctor, specialist nurse, or dietician if you want to take supplements and are having any kind of cancer treatment. 

You can find out about cancer treatments.

Possible risks of nutritional supplements

Some dietary supplements can cause skin sensitivity and severe reactions when taken during radiotherapy treatment.

Some vitamins or minerals could interfere with how well cancer drugs work but we don't know much about this yet. Some antioxidant supplements such as Co enzyme Q10, selenium and the vitamins A, C and E can help to prevent cell damage. Some doctors think that this may stop chemotherapy working so well. But at the moment there is so little research that we just don't know for sure. The Royal College of Radiologists advises that patients should avoid high doses of antioxidant supplements during their cancer treatment.

Reducing symptoms or side effects

While there is no evidence that nutritional supplements can help to treat any type of cancer, some studies have shown that they may help to control the symptoms of cancer or the side effects of treatment. For example, omega 3 fatty acids in fish oil can help to control extreme weight loss due to cancer (cachexia). But we need a lot more research in this area before we will know for sure which vitamin and diet supplements may play a role in helping to control treatment side effects and which are safe to take. 

A systematic review by the Cochrane Collaboration in 2006 (revised in 2009) looked at the research that has been carried out into people taking selenium to ease the side effects of chemotherapy, radiotherapy and surgery. This review found that selenium did not help to reduce the treatment side effects. The selenium caused some side effects in some people. We need more research to look at the dose that people should take and to find out what effects it has. 

You can read the review into selenium for cancer treatment side effects on the Cochrane Library website.


Dietary supplements and advanced cancer

There is currently no strong evidence that dietary supplements can help to treat or control the growth of cancer. But some small pilot studies have seemed to show that nutritional supplements may help to control the growth of advanced cancer for some people. For example, one study of 41 patients gave people with advanced cancer supplements of coenzyme Q10 and a mixture of other antioxidants (e.g. vitamin C, selenium, folic acid and beta carotene). 

In the study the patients were estimated to live for an average of a year, ranging from 3 to 29 months. But in the study they lived for an average of 17 months, ranging from 1 to 120 months. So the patients lived more than 40% longer than the researchers predicted. The treatments caused very few side effects. This study seemed to show that dietary supplements may be helpful for some people with advanced cancer but we need more studies to confirm these results.


European Union (EU) laws on supplements

The European Union Food Supplements Directive (The Directive) deals with the regulation of vitamin and mineral food supplements. It was revised in 2011. The Directive is a European safety measure designed to help to protect the public and governs

  • Which vitamins and minerals you can buy in the UK and other European Union states
  • The ingredients that food supplements can contain
  • The doses in which they can be present in each supplement

Supplements can only contain vitamins and minerals taken from the approved list. You can see information about the European Union Food Supplements Directive and see the approved substances.


More information about vitamins and minerals

The MedlinePlus website in the USA has information about many of the vitamins and minerals used as dietary supplements. It gives information about their possible side effects, interactions and research evidence on their use in cancer and other illnesses.

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Updated: 24 October 2014