Despite public enthusiasm for the use of nutritional supplements in those undergoing cancer treatment there is still no clear documented advantage of their use. Some patients with more advanced or aggressive cancers may develop nutritional deficiencies during the course of their illness and require nutrition support. However agents used in chemotherapy treatment may have anti-nutrient effects and the co-administration of nutritional supplements may thus be unwise as they might blunt the effects of medication in curbing the growth of the cancer itself.
Additionally some nutrients are potentially growth-promoting and this could apply to vitamin A, beta-carotene (which is converted into vitamin A), zinc and possibly some of the B vitamins. The rapidly dividing cancer cells may have a bigger appetite for these and other nutrients than your own healthy tissues. The Cochrane Review - Antioxidant Supplements for Prevention of Mortality in Healthy Participants and Patients with Various Diseases (April 2008) found that vitamin A, beta-carotene and vitamin E appeared to increase the risk of both cancer and an early death [internal link]. Thus a cautious approach seems reasonable until more definitive advice is available.
For further advice see www.bacup.co.uk and Cancer Research Campaign and BUPA website