Introduction

Selenium as an Antioxidant
 

Selenium as a Cancer Preventive Agent

Sources of selenium

Brassica Vegetables

Selenium studies

Animal studies with Se- garlic

Se-enriched garlic has been well researched and documented for its cancer preventive properties for 6 years without an appearance in the commercial market (Ip et al. 1992; Ip and Lisk 1993; 1994a; 1994b; 1995; 1996). Ip and coworkers (1996) carried out a study of long-term versus short-term dosage in mammary cancer prevention in the rat 7,12-dimethylbenz[a]anthracene (DMBA) model. They found that a short-term exposure to Se-garlic (1 month) in rats, implemented immediately following the carcinogenic insult (DMBA), was able to prevent subsequent mammary tumor development that usually occurs months later. In fact a 1 month administration had the same results as Se treatment for 5 months. However, Se administration 13 weeks after a carcinogenic exposure had no impact on the growth or number of mammary tumors developing. Another study conducted by Ip and Lisk (1995), established whether the selenium in Se-enriched garlic was responsible for the cancer prevention and not other compounds found in garlic. It is not known why transformed cells in the early stage of carcinogenesis are more sensitive to selenium intervention than those in the late stage. It is possible that Se-garlic in part via the action of MSC blocks clonal expansion and/or alters clonal selection of tranformed cells by inducing apoptosis and by interfering with cell cycle transit.

Human Intervention Studies

Unfortunately, there have been very few human trials to measure the effect of selenium supplementation on the incidence of cancer. A randomized controlled trial funded by the National Cancer Institute and led by Larry Clark and numerous other cancer researchers (Clark et al. 1996) to determine whether selenium supplementation would effect the incidence of carcinoma of the skin was initiated in 1983. The researchers used a 200 *g dose of Se as high-Se brewer's yeast or a placebo yeast. The blinded phase of the study was stopped early because of secondary end point findings. Whereas there was no significant difference between placebo and selenium for the primary indication of skin carcinomas, it became evident there were significant differences for secondary endpoints of total cancer mortality, total cancer incidence, and the incidences of lung, prostate, and colorectal cancers (Clark et al. 1996, Combs et al. 1997). Total cancer incidence was 37% lower with a 50% reduction in cancer mortality in the Se group. The researchers cautiously concluded that supplemental selenium may reduce the incidence and mortality due to cancer and further recommend an independent trial for confirmation. Not all studies show a correlation between selenium levels and cancer incidence. One study of 60,000 nurses compared levels of selenium in toenail clippings and cancer incidence and did not find any apparent benefit in higher selenium levels (Garland et al. 1995)