"Emerging evidence has shown that high doses of certain supplements can actually increase the risk of cancer" wrote Maria Elena Martinez, PhD, of the University of California San Diego, and co-authors in a commentary just published online in the Journal of the National Cancer Institute.
Except for their value in treating a few uncommon nutrient deficiencies, Martinez and co-authors also wrote that "Dietary supplements have minimal supporting data for health benefits in disease prevention, particularly cancer. Despite this evidence [of potential harm], marketing claims by the supplement industry continue to imply anti-cancer benefits. Insufficient government regulation of the marketing of dietary supplement products may continue to result in unsound advice to consumers. Both the scientific community and government regulators need to provide clear guidance to the public about the use of dietary supplements to lower cancer risk."
Laboratory and observational data had suggested a protective effect of folic acid against cancer, particularly colorectal cancer. However, a meta-analysis of randomized trials showed no effect of folic acid supplementation on development of colorectal adenomas (Int J Cancer 2011; 129: 192-203).
Two different randomized trials showed an increased risk of cancer (prostate) and precancerous lesions (colonic adenomas) in participants on long-term folic acid supplementation.
The Institute of Medicine recently updated its recommendations on vitamin D and calcium intake and found "there was not enough evidence to state that there is a causal association between low vitamin D intake and increased cancer risk."
Epidemiological studies have shown inverse associations between serum levels of 25-hydroxy (OH) vitamin D and several types of cancer. Three short-term randomized trials failed to demonstrate an effect of vitamin D supplementation on cancer incidence or mortality.
Observational studies have yielded conflicting data on the association between vitamin D concentrations and the risk of pancreatic cancer."
Time and again, a review of the evidence shows that supplements may not only lack clinical value, but may actually be harmful. The risk/benefit and the cost/benefit analyses therefore recommend against willy nilly usage.