At first thought, it may seem obvious that you would want to take vitamins while undergoing treatment for prostate cancer. This might not always be the case however.
Men undergoing radiation therapy for their cancer are sometimes told by their physicians to stop taking vitamins during the treatment period. The thinking behind this advice has to do with the way vitamins work within the body.
Some vitamins (most notably, vitamin C) work within the body as antioxidants. Other antioxidants include beta-carotene, lutein, lycopene, selenium, vitamin A, and vitamin E. Oxidized molecules and "free radicals" are created when cells are damaged by harmful exposures and by some natural processes as well. These oxidized molecules can cause damage to a cell's DNA, resulting in its death. Antioxidants such as vitamin C work to inactivate oxidized molecules and free radicals, thus making them no longer dangerous to cells. Obviously, the less damage to your cells, the better. This is why, most of the time, antioxidants such as vitamin C are beneficial.
When undergoing radiation treatments, however, one of the primary ways in which radiation works to kill cancer cells is through the creation of oxidized molecules and free radicals. These then act to kill the prostate cancer cells nearby.
Some doctors think that, by taking vitamins and other antioxidants (such as vitamin C) while undergoing radiation treatments, the overall ability of the radiation to kill cells will be reduced. Put simply -- more antioxidants present in the body will result in less oxidized molecules and free radicals created and less damage to prostate cancer cells overall.
Other doctors argue, however, that anti-oxidants may be beneficial during radiation therapy because they may protect the healthy, non-cancerous tissue surrounding the prostate from radiation damage.
Both during and after your prostate cancer treatment, you should be sure to ask your doctor about what your diet should include. Nutrition, including the use of vitamins, during this time is an important piece of your overall health. In particular, be sure to discuss whether you should be taking vitamins or not during your treatment, especially if you are undergoing radiation therapy.
More information about how diet and nutrition may affect your risk of prostate cancer is available.

