Selenium is considered as an essential trace mineral. It is thought to incorporate into protein to form selenoproteins which help prevent cell damage from free radicals- natural by-products of oxygen metabolism that may contribute to health problems like cancer and heart disease. Selenium was discovered in 1817 by J.J Berzelius. Until 1947, this element was only known for its toxic properties. Thanks to the continuous effort of scientists because at present-day, selenium’s beneficial effects are linked to prevention of serious diseases like cancer- at least that’s what nutritional supplement companies always say. However, take a closer look at those studies, and you will find out that their results don’t always agree. Some studies suggest that selenium may have preventive properties against cancer while the findings of other trials claim otherwise.
In December 1996, the result of a research study on the effects of selenium supplementation for cancer prevention in patients with skin carcinoma was published in the Journal of the American Medical Association. This was conducted by researchers from the University of Arizona and was led by Dr Larry Clark. This randomized controlled trial was the the earliest evidence to prove that while supplemental selenium didn’t give protective effect in skin cancer development, it may decrease the incidence of carcinomas of lung, colorectal, and prostate cancers. A second article about selenium’s potential for decreasing prostate cancer risk was published in the British Journal of Urology in 1998. It was a double blind cancer prevention trial, 974 men with a history of either a basal cell or squamous cell carcinoma were given either a daily supplement of 200 microgram of selenium or a placebo. The selenium-treated group had 63% reduction in the incidence of prostate cancer.
Doctor Larry Clark died of prostate cancer in 2000. The result of his works has become the basis of nutritional cancer prevention trials. In 2001, enrollment for the largest ever prostate cancer prevention trial commenced. This research study is called Selenium and Vitamin E Cancer Prevention Trial (SELECT). The objective of this study was to find out whether Selenium given alone or in combination with vitamin E could produce a significant protective effect against cancer prostate cancer, 35 533 healthy men aged 55 and above took part in SELECT. The initial result of this study was published 3 weeks ago, it didn’t quite agree with previous studies on selenium. The lead author of SELECT, Dr. Scott Lippman, said that they were very disappointed because neither supplements provided preventive effect for prostate cancer and other cancers. He also mentioned that the high consumption of Vitamin E and selenium due to the assumption that they can prevent prostate cancer. Initial findings also suggest that selenium may increase the chance of getting diabetes mellitus. The participants of the SELECT will still be evaluated for long term effects of Selenium and Vitamine E in prostate cancer risk as well as other cancers and cardiovascular diseases. The researchers stated that the supplements may still have biologic effects even after discontinuation of their use.
So if SELECT suggests that selenium has no preventive properties on prostate cancer, does this necessarily mean that it may be deemed worthless in other cancer types as well? Researchers from Dartmouth Medical School would disagree because their latest study on selenium suggests that it may prevent bladder cancer among women, moderate smokers and patients with p53 positive bladder cancer. Bladder cancer cases that stem from p53 alterations are associated with increased risk of recurrence and death. Although there was a previous study linking selenium with bladder cancer, this trial is the first to determine the link between p53 positive bladder cancer and selenium. To the researchers, these findings are remarkable. According to the correspondent author, Margaret Karagas, Ph.D, if selenium can prevent bladder cancer in certain groups like women and prevent certain types of tumors, such as those that form via the p53 pathway from advancing, then it will help determine how tumors can be prevented in the future. The researchers aspire to simulate their findings in a larger study to further investigate the link between selenium and bladder cancer in women and patients with p53 tumors.
From the study of the late Dr Clark up to the latest clinical trial on selenium, something is still unchanged: selenium seem to affect the progression of certain forms of cancer and not others. Quite intriguing, isn’t it? How could that be possible? Our inquiring minds would have to wait for familiar refrains: further studies that will validate previous results.
References:
Goldhaber SB. Trace element risk assessment: essentiality vs. toxicity. Regulatory Toxicology and Pharmacology. 2003;38:232-42.
Combs GF, Jr and Gray WP. Chemopreventive agents: Selenium. Pharmacol Ther 1998; 79:179-92.
Decreased incidence of prostate cancer with selenium supplementation: results of a double-blind cancer prevention trial. Br J Urol. 1998 May;81(5):730-4.
Clark LC, Dalkin B, Krongrad A, Combs GF Jr, Turnbull BW, Slate EH, Witherington R, Herlong JH, Janosko E, Carpenter D, Borosso C, Falk S, Rounder J.
Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. A randomized controlled trial. Nutritional Prevention of Cancer Study Group
L. C. Clark, G. F. Combs Jr, B. W. Turnbull, E. H. Slate, D. K. Chalker, J. Chow, L. S. Davis, R. A. Glover, G. F. Graham, E. G. Gross, A. Krongrad, J. L. Lesher Jr, H. K. Park, B. B. Sanders Jr, C. L. Smith and J. R. Taylorh
Arizona Cancer Center, College of Medicine, University of Arizona, Tucson, USA.
Larry Clark’s legacy: Randomized controlled, selenium-based prostate Cancer chemoprevention trials
MARSHALL James R. Arizona Cancer Center, University of Arizona, Tucson, AZ 84724, ETATS-UNIS
Effect of Selenium and Vitamin E on Risk of Prostate Cancer and Other Cancers: The Selenium and Vitamin E Cancer Prevention Trial (SELECT)
Early Release Article, posted December 9, 2008 JAMA
American Association for Cancer Research. 2008, December 14
Accumulation of Nuclear p53 and Tumor Progression in Bladder Cancer. The New England Journal of Medicine Volume 331:1259-1264 November 10, 1994
David Esrig, Donald Elmajian, Susan Groshen, John A. Freeman, John P. Stein, Su-Chiu Chen, Peter W. Nichols, Donald G. Skinner, Peter A. Jones, and Richard J. Cote
Tags: bladder cancer, Cancer, prostate cancer, selenium, vitamin e
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