In 1993, a cross-sectional study of 48 individuals in the United States by Clark et al. found an almost 4-fold increased risk of colorectal cancer for plasma selenium concentrations above 128 mcg/l versus those below 128 mcg/l (95% CI = 1.02–15.71). (87)
In 1995, Garland et al. found low selenium levels in the toenails of women in the Nurses study was associated with increased risk of colorectal cancer. (88) In 2000, Ghadirian et al. reported a statistically significant inverse association between toenail levels of selenium and colorectal cancer. (89) In 2002, Fernandez-Banares et al. found selenium blood levels to be inversely associated with colorectal cancer risk. (90)
In 1996, the NPC Intervention Trail found that colorectal cancer incidence was reduced by 54% (RR=0.46) (46)
In 1998, Bonelli et al. reported a reduction in colorectal adenoma of 31% (RR= 0.69, 0.41–1.2) by supplements of selenium along with antioxidants vitamins A, C and E. (91)
Also in 1998, Hofstad et al. reported a reduction of colorectal cancer by 42% (RR= 0.58, 0.39–0.87) with supplementation of beta-carotene, vitamin C, vitamin E and selenium. (92) The researchers concluded, “Our data may support a protective role of calcium and antioxidants on new adenoma formation.”
In a 2007 review of the chemoprevention of colorectal cancer, Das et al note that “selenium compounds have also been shown to inhibit the development of adenocarcinomas in animal models of colorectal carcinogenesis, and there is evidence from epidemiological studies showing an inverse relation between cancer mortality and selenium content in diet.” (93)
Das et al discuss a pooled analysis of data from 3 randomized trials that tested the effects of various nutritional interventions for colorectal adenoma prevention (the Wheat Bran Fibre Trial, the Polyp Prevention Trial and the Polyp Prevention Study) in subjects that had recently undergone adenoma removal, it was shown that the subjects with baseline serum or plasma selenium in the highest quartile (median = 150 ng/l), when compared with those in the lowest quartile (median = 113 ng/l), had a significantly lower risk of adenoma recurrence (OR = 0.66, 95% CI = 0.50–0.87).
Das et al also include the NPC Trial in their review. They note that in the NPC Trial, a chemoprevention trial in 1,312 patients using 200 mcg of selenium daily (n= 653) versus placebo, the following results were reported. ”There was a significantly reduced risk of developing CRC in the selenium group versus placebo (RR = 0.42; 95% CI = 0.18–0.95; p = 0.03).
These studies provided credible scientific evidence that selenium is protective against colorectal cancer.