Selenium supplementation associated with reduced colorectal cancer risk
The April 2006 issue of the International Journal of Cancer published the findings of researchers from Roswell Park Cancer Institute in Buffalo, New York that supplementing with selenium significantly reduced the risk of colorectal adenomas (polyps) among smokers or those whose levels of the mineral were low. Polyps can be a precursor to colorectal cancer (CRC).
The study population consisted of participants in the Nutritional Prevention of Cancer trial follow up who were randomized to receive a 200 microgram per day selenium supplement or a placebo for an average of 7.9 years. Plasma selenium levels, body mass index and smoking status were ascertained upon enrollment. Five hundred ninety-eight subjects received one or more sigmoidoscopic or colonoscopic examinations during the follow up period. There were 99 adenomas found during the initial examinations, which were classified as prevalent, and 61 identified at subsequent examinations, classified as incident.
For prevalent and incident adenomas, there was a slight reduction in risk associated with selenium supplementation among all participants. Current smokers were more likely to have adenomas than those who never smoked. When current smokers were separately analyzed, there was a significant 73 percent reduction in risk found among those supplemented with selenium compared to those who received the placebo. When the participants were divided into thirds according to baseline plasma selenium status, those whose levels were in the lowest third and who received selenium rather than a placebo, also experienced a 73 percent reduction in risk.
Several mechanisms may be responsible for a chemopreventive effect of selenium against colorectal adenomas, such as depression of carcinogen bioactivation, cell proliferation and cell cycling, and an increase in programmed cell self-destruction.
The authors conclude, "These data show that selenium supplementation is associated with a decrease in prevalent adenomatous polyps, particularly in current smokers and subjects with a low baseline plasma selenium level. The chemopreventive effects of selenium supplementation appear [to] impact both the formation of colorectal tumors and the premalignant lesions for CRC. Prospective studies specifically designed to test the chemopreventive efficacy of selenium supplementation on CRC and adenoma incidence are needed to confirm these results."
There is a direct relationship suggested by epidemiological studies between total fat intake in the diet and increased risk of cancer in the colon and rectum. Animal fat, particularly dairy products, and red meat are associated with colon cancer risk, whereas there is no association with vegetable fats, and fish oils appear to have a protective effect (Schloss et al. 1997). Lower cholesterol levels have been shown in patients diagnosed with colorectal cancer (Forones et al. 1998). Conversely, elevated levels of serum triglycerides have been associated with a higher risk of adenomatous polyps (Bird et al. 1996b).
The risk factor associated with alcohol consumption and cigarette smoking is startling. Daily alcohol intake has been associated with a twofold increase in colon carcinoma (Giovannuci et al. 1998). Smoking is an independent risk factor and long-term smoking is particularly damaging, increasing the relative risk by 1.6-4.5 fold for adenoma formation (Nagata et al. 1999) Smoking more than 20 cigarettes a day increases the likelihood of having polyps by more then 250%, while alcohol consumption increases likelihood by 87%. When combined, smoking and alcohol consumption increase the likelihood by an astonishing 400% (Martinez et al. 1995; Lieberman et al. 2003).
Increased vitamin D intake has been associated with reduced risk for colon carcinoma (Garland et al. 1999). Vitamin D3 causes differentiation of colon cancer cells. Cancer cells that are well differentiated are close to the original normal healthy colon cells in nature and are usually less aggressive cancer cells. Poorly differentiated cells have changed more from the normal healthy cells and are usually more aggressive cancer cells.
In high-risk individuals, the use of multivitamins has been shown to reduce the risk of adenoma formation (Whelan et al.1999). A reduced risk of colon cancer is associated with the use of vitamin C (Howe et al. 1992). Vitamins C, E, and A showed protection against the risk of developing colorectal cancer (Newberne et al. 1990). Low levels of selenium correlated with the presence of adenomas (benign tumors), whereas increased levels were associated with reduced risk of adenomas (Russo et al. 1997). Intervention trials have found a beneficial effect of selenium supplementation. There is an association between iron exposure and colorectal polyps (Bird et al. 1996a).
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