Cancer Prevention Studies

Some readers may be interested in the potential of selenium in cancer prevention as well as treatment. A number of studies have reported that low selenium levels are associated with increased cancer risk in hu-mans.52,53,54 Low levels have also been associated with increased risk of heart disease and reduced immune function.55 Associations with cancer risk appear to be particularly strong for breast, colon, and prostate cancers. Some examples follow, along with studies that suggest selenium supplementation can reduce cancer risk:

• In a study on 321 subjects, 111 of whom later developed cancer, those with the lowest selenium levels were twice as likely to develop cancer as those with the highest levels. The association was strongest for cancers of the gastrointestinal tract and prostate.23

• A cohort study of 33,700 men reported that prostate cancer risk was reduced in men who had higher dietary selenium intake.24

• In a study on women over 50, high blood levels of selenium were associated with a reduced risk of

breast cancer.

• In a seven-year study on 1,312 human subjects, 200 micrograms of selenium supplement per day (from yeast) reduced the incidence of prostate cancer by 63 percent, colon cancer by 58 percent, overall cancer incidence by 35 percent, and cancer mortality, 49 percent.57 The incidence of skin cancer was not significantly affected.

• In a study on 974 men with a history of basal cell or squamous cell carcinoma, oral administration of 200 micrograms of selenium (from yeast) for 4.5 years reduced the risk of secondary prostate cancer by 63 percent. In addition, selenium supplementation reduced the risk of total cancer mortality, as well as the incidence of lung cancer, colorectal cancer, and total cancers.16

• Plasma selenium levels were lower in patients with malignant oral cavity lesions as compared to healthy controls and to patients with premalignant lesions. Twenty-two patients with premalignant lesions were treated with 300 micrograms per day of selenium supplements (selenite or organic selenium). After 12 weeks, lesions improved in 39 percent of the sub-

jects.58

• Selenium supplementation of 200 micrograms (as selenite) significantly reduced the incidence of liver cancer in an area of China where selenium levels are low.59

In contrast to these studies, a few studies have reported no association between selenium deficiency and cancer

Figure 14.2 Redox Cycling of Iron electron source electron loss reduced antioxidant antioxidant oxidized antioxidant reduced ,. +2. iron (iron )

oxidized iron (iron+3)

electron loss electron source

or have reported that selenium increases cancer risk. Based on the majority of studies, however, it appears that selenium is generally protective against cancer.

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