Selenium

Stephen B. Strum M.D. 1998

Selenium is an antioxidant and a cofactor for glutathione peroxidase, an enzyme that is a potent scavenger of oxygen-free radicals. Epidemiological studies show a consistent tend for populations residing in geographic areas that have low selenium levels in the soil to have a higher mortality from cancer. Other studies showed a lower risk for cancer in individuals with higher serum selenium levels. These include cancers of the bladder, pancreas, thyroid, stomach, lung, esophagus, melanoma, head and neck tumors, and brain tumors. Other studies have suggested protective effects against prostate cancer.

The "Nutritional Prevention of Cancer Project" (NPC) was a controlled, randomized cancer prevention trial in which 1,312 patients received a daily 200 mcg dose of selenium or a placebo for up to 10 years. There were 13 cases of prostate cancer in the treatment group vs 35 cases in the placebo group during 7,818 person-years of follow-up. This was a statistically significant 63% reduction in the incidence of prostate cancer (p < 0.001). Given the slow rate of prostate cancer progression, the authors surmised that many patients in this study had disease that was undiagnosed at the time they entered the trial. They concluded that selenium supplementation may be useful in preventing the progression of occult prostate cancer (see discussion in abstract below).

We recommend selenium supplements be given as an organic, rather than an inorganic form. Organic sources of selenium such as selenomethionine, selenocysteine or mixtures of organic forms found in brewer’s yeast have a better safety profile. Recent research indicates higher doses of selenium can be safely given and may possess additional anticancer activity. We currently use daily selenium doses in the 400-800 mcg range in our patients. Other investigators are studying the effects of selenium much higher doses (1,000-3,000 mcg/day) for prostate cancer and claim to have had little or no toxicity. Clearly, this area is controversial and requires further study.

Decreased incidence of prostate cancer with selenium supplementation: results of a double-blind cancer prevention trial.

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

Br J Urol 1998 May;81(5):730-4

OBJECTIVE: To test if supplemental dietary selenium is associated with changes in the incidence of prostate cancer.

PATIENTS AND METHOD: A total of 974 men with a history of either a basal cell or squamous cell carcinoma were randomized to either a daily supplement of 200 mcg of selenium or a placebo. Patients were treated for a mean of 4.5 years and followed for a mean of 6.5 years.
RESULTS: Selenium treatment was associated with a significant (63%) reduction in the secondary endpoint of prostate cancer incidence during 1983-93. There were 13 prostate cancer cases in the selenium-treated group and 35 cases in the placebo group (relative risk, RR=0.37, P=0.002). Restricting the analysis to the 843 patients with initially normal levels of prostate-specific antigen (< 4 ng/mL), only four cases were diagnosed in the selenium-treated group and 16 cases were diagnosed in the placebo group after a 2 year treatment lag, (RR=0.26 P=0.009). There were significant health benefits also for the other secondary endpoints of total cancer mortality, and the incidence of total, lung and colorectal cancer. There was no significant change in incidence for the primary endpoints of basal and squamous cell carcinoma of the skin. In light of these results, the 'blinded' phase of this trial was stopped early.

CONCLUSIONS: Although selenium shows no protective effects against the primary endpoint of squamous and basal cell carcinomas of the skin, the selenium-treated group had substantial reductions in the incidence of prostate cancer, and total cancer incidence and mortality that demand further evaluation in well-controlled prevention trials.

 

Prostate Cancer Research Institute (PCRI)

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