Many of us take daily vitamin supplements in the belief that, even if it they don't do us much good, at least they will not cause us any harm.
However there is increasing evidence to show that this sentiment is misguided; a growing body of research shows that those who regularly take vitamin and mineral supplements for "insurance purposes" may indeed be putting themselves at increased risk of developing the very diseases for which they are taking these products.
Vitamins not effective in preventing cancer and heart disease Furthermore it is important to note that there are very few studies to show whether supplementary vitamins and minerals do actually help prevent degenerative diseases such as cancer, heart disease and Alzheimer's disease.
Where these nutrients are undoubtedly essential is in the prevention of specific deficiency diseases and abnormal developmental conditions such as anemia, spina bifida and rickets.
(The exception to this seems to be vitamin D which is attracting a lot of attention as a possible factor that affects diseases as diverse as multiple sclerosis, heart disease and kidney failure - however caution is advised here too).
As far as cancer is concerned some clinical trials have even had to be ended prematurely because the patients taking a supplement (such as beta-carotene) developed cancer at a higher rate than the control groups taking the placebos.
Breast and prostate cancer Recently published research shows that women who take multivitamins appear to be at an increased risk of breast cancer than those who do not take the supplements.
An unrelated study reported similar epidemiological evidence showing that while, in general, woman who took regular folic acid supplements reduced their breast cancer risk, others had a greater than average chance of developing certain breast cancer subtypes! And yet another study found that men who took the mineral supplement, selenium were at a higher risk of falling victim to prostate cancer.
Folic acid supplements aggravate B12 deficiency The argument for the fortification of staple foods with folic acid has received another setback.
Although folic acid is vital in the prevention of birth defects and can mitigate some of the effects of Alzheimer's disease, it does have another negative effect.
Vitamin B12 deficiency is relatively common in the elderly and can lead to anemia and nervous system damage.
As folic acid supplementation can exacerbate the damage to the nervous system in those with vitamin B12 deficiency it is important to administer B12 supplementation before those containing folic acid.
Unfortunately many people are unaware that they have a B12 deficiency as this vitamin is not used to fortify food and often has to be administered by injection anyway.
Foods the best source of beneficial bioactive compounds The answer to this dilemma is not to rely on vitamin and mineral supplements unless you have a demonstrable deficiency.
Rather eat foods that are rich sources of the vitamins you think you need.
For example cheap, tasty lentils are loaded with naturally occurring folic acid as are green leafy vegetables, asparagus and others.
It is also important to note that there are many other non-vitamin/mineral compounds in fruit, vegetables, nuts and, particularly culinary herbs and spices, that have very potent disease preventing properties.
If you eat a wide range of these foods you are more likely to substantially reduce, rather than increase, your risk of falling victim to one of these dread diseases.
References:

1. Multivitamin use and breast cancer incidence in a prospective cohort of Swedish women Am J Clin Nutr (March 24, 2010).
   Susanna C Larsson, Agneta Åkesson, Leif Bergkvist and Alicja Wolk
2. Vitamins C and E and Beta Carotene Supplementation and Cancer Risk: A Randomized Controlled Trial.
   ; J Natl Cancer Inst, 2009:101:14-23; Lin J et al.
   
3. Plasma Selenium, Manganese Superoxide Dismutase, and Intermediate- or High-Risk Prostate Cancer Journal of Clinical Oncology, February 2, 2009; June M.
   Chan,et.
   al.