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Selenium and Cancer: What kind should we use?

Jacob Schor ND

November 2001

 

Without question selenium plays a major role in cancer prevention and treatment.  The question remains though as to what chemical form of selenium works best.

 

A 1996 double blinded, placebo controlled study on selenium supplementation in “free living” people  (the human equivalent of free range chickens)  showed a nearly 50% decrease in cancer morbidity and mortality    Prior to this study there was already good evidence that selenium was cancer protective in humans.  Geographic data from around the world and in the United States shows a relationship between selenium in the food supply and cancer rates. , , ,  

 

Other earlier studies demonstrated that people with low blood selenium have an increase risk of cancer. , ,  , ,   Compared to Vitamin A, beta-carotene, and Vitamin E , selenium clearly has the greatest protective effect against cancer.

Clark’s study used selenium in the form of selenized brewer’s yeast.  To manufacture this product, brewer’s yeast is grown in a nutrient media heavily enriched with selenium.  The yeast absorbs the selenium and converts it into a variety of chemical forms.  All it took was 200 mcg of selenium a day to cause the dramatic shift in cancer occurrence in Clark’s test subjects.

 

Are all the chemical forms of selenium equally active?  If not, which form of selenium is most active?  Clark’s study doesn’t answer this.  There are several different approaches to compensate for not knowing these answers.  One is to look at the proportion of different compounds in the yeast. The majority of the selenium, about 80 to 85%, is in the chemical selenomethionine.   Some manufacturers have concluded therefore that selenomethionine is the active compound that fights cancer. Yet is this conclusion justified?  There are others who disagree.

 

In past studies most (over 90%) have used either selenomethionine or sodium selenite because these chemicals are commercially available.  Both chemicals work in animals producing tumor inhibition in breast, liver, skin, pancreas, esophagus, colon and other sites. Selenomethionine raises tissue selenium levels higher than the sodium selenite but was less effective at preventing tumors.

One researcher has done a great deal to answer these questions.  For the last decade Clement Ip has focused on selenium enriched garlic.   In 1992, he and his colleagues showed that garlic grown with a selenium fertilizer worked better than regular garlic at preventing breast cancer in rats exposed to known chemical carcinogens.

In 1994 he repeated the experiment with onions which didn’t work as well. In 1995 he went back to garlic and showed that the cancer protection varied with the selenium levels in the garlic; low selenium garlic was less effective than the high selenium garlic.     Now he began to ask the important questions, what form of selenium is most active?

 

In 2000 Ip published a study which compared the effect of the selenium enriched yeast used in Clark’s human trials against selenium enriched garlic with interesting results.  The yeast contains six times more selenium than the garlic, 1922 mcg/g  to 296 mcg/g.  The dominant chemical forms of selenium present in the garlic and yeast were also different.  In garlic the majority of the selenium (73%) was present as gamma-glutamyl-Se-methylselenocysteine (which I will just call “methylselenocysteine” for obvious reasons).  In yeast, the majority was, as mentioned, selenomethionine (85%).  When rats were fed garlic, far less selenium accumulated in their tissues than when fed yeast.  Yet in this comparison study, the garlic worked better than the yeast at preventing cancer.

It may be time to rethink our earlier assumptions. Selenium enriched brewer’s yeast works, but it may be in spite or the selenomethionine that it works.  One recent reviewer writes:  “.....the most abundant naturally-occurring chemical form of dietary selenium, selenomethionine is the least potent in the prevention of cancer.”  

Selenium yeast though does contain some methylselenocysteine.  Although less than 15% of the total selenium in yeast is selenomethionine, perhaps this small amount is enough to produce the anticancer effect seen in Clark’s study.

So what are we to do?  First we are not rats.  We do not know that this rat research will hold true for humans.  Converting rat research to human findings is unpredictable. As the best human trial used selenium yeast, simple logic says we should still use selenium yeast.  The assumption that selenomethionine could be used as an equivalent substitute for selenium yeast has been brought into doubt.

 

Although the most attractive selenium source based on the rat research appears to be the selenium enriched garlic, at this time nobody sells it.  So unless you are a rat in Clement Ip’s laboratory you are out of luck. 

[Note: several years after writing this review we did find a source and have been purchasing selenium from Applied Orthomolecular Research,  www.AOR.CA ]

I am putting these questions about selenium to a number of knowledgeable people who may help guide us through the confusion.  At this point here is my bottom line

1.  Selenomethionine can not be depended on to be as useful as selenium yeast which we have promoted in the past.  Some of the multivitamins we sell use this form of selenium and until we have information to the contrary we can not rely on them to have as significant a protective effect against cancer as we have previously thought.

2.  Selenium enriched yeast remains the best proven source of selenium in human trials so we should use it when we want cancer protection.

3.  Selenium enriched garlic is very promising. [Several years after writing this review we did find a source and have been purchasing selenium from Applied Orthomolecular Research,  www.AOR.CA ]

References:

Clark L. C., Combs G. F., Turnbull B. W., Slate E. H., Chalker D. K., Chow J., Davis L. S., Glover R. A., Graham G. F., Gross E. G., Krongrad A., Lesher J. L., Park K., Sanders B. B., Smith C. L., Taylor R. Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. J. Am. Med. Assoc. 1996; 276:1957-1985

Clark L. C., Cantor K., Allaway W. H. Selenium in forage crops and cancer mortality in U.S. counties. Arch. Environ. Health 1991; 46:37-42

Schrauzer G. N., White D. A., Schneider C. J. Cancer mortality correlation studies. III. Statistical associations with dietary selenium intakes. Bioinorg. Chem. 1977; 7:23-24

Shamberger R. J., Tytko S. A., Willis C. E. Antioxidants and cancer. Part VI. Selenium and age-adjusted human cancer mortality. Arch. Environ. Health 1976; 31:231-235

Yu S. Y., Chu Y. J., Gong X. L., Hou C. Regional variation of cancer mortality and its relation to selenium levels in China. Biol. Trace Elem. Res. 1985; 7:21-29

Clark L. C., Graham G. F., Crounse R. G., Grimson R., Hulka B., Shy C. M. Plasma selenium and skin neoplasms: a case control study. Nutr. Cancer 1984; 6:13-21

Clark L. C., Hixson L. J., Combs G. F., Jr. , Reid M. E., Turnbull B. W., Sampliner R. E. Plasma selenium concentration predicts the prevalence of colorectal adenomatous polyps. Cancer Epidemiol. Biomark. Prev. 1993; 2:41-46

Salonen J. T., Alfthan G., Huttunen J. K., Puska P. Association between serum selenium and the risk of cancer. Am. J. Epidemiol. 1984; 120:342-349

Salonen J. T., Alfthan G., Huttunen J. K., Puska P. Association between serum selenium and the risk of cancer. Am. J. Epidemiol. 1984; 120:342-349

Willett, W. C., Polk, B. F., Morris, J. S., Stampfer, M. J., Pressel, S., Rosner, B., Taylor, J. O., Schneider, K. & Hames, C. G. (1983) Prediagnostic serum selenium and risk of cancer. Lancet ii: 130-134.

Comstock G. W., Bush T. L., Helzlsouer K. Serum retinol, beta-carotene, vitamin E and selenium as related to subsequent cancer of specific sites. Am. J. Epidemiol. 1992; 135:115-121

I p C, Birringer M, Block E, Kotrebai M, Tyson JF, Uden PC, Lisk DJ.Chemical speciation influences comparative activity of selenium-enriched garlic and yeast in mammary cancer prevention.Journal of Agricultural and Food Chemistry 48(6):2062-70. 2000

Combs, G. F., Jr. (1997) Selenium and cancer. In: Antioxidants and Disease Prevention (Garewal, H. S., ed.), pp. 97-113. CRC Press, New York, NY.

Ip C., Hayes C. Tissue selenium levels in selenium-supplemented rats and their relevance in mammary cancer protection. Carcinogenesis 1989; 10:921-925

Ip, C. & Ganther, H. E. (1992) Relationship between the chemical form of selenium and anticarcinogenic activity. In: Cancer Chemoprevention (Wattenberg, L., Lipkin, M., Boone, C. W. & Kelloff, G. J., eds.), pp. 479-488. CRC Press, Boca Raton, FL.

Ip C., Lisk D. J. Characterization of tissue selenium profiles and anticarcinogenic responses in rats fed natural sources of selenium-rich products. Carcinogenesis 1994a; 15:573-576

Ip C., Lisk D. J. Efficacy of cancer prevention by high selenium-garlic is primarily dependent on the action of selenium. Carcinogenesis 1995; 16:2649-2652

Ip C, Birringer M, Block E, Kotrebai M, Tyson JF, Uden PC, Lisk DJ.Chemical speciation influences comparative activity of selenium-enriched garlic and yeast in mammary cancer prevention. Journal of Agricultural and Food Chemistry 48(6):2062-70. 2000

Drake, ED Selenium Are you getting enough to reduce your risk of cancer.WriterÆs Showcase, Lincoln, NE 2001