Abstract
According to the Nutritional Prevention of Cancer (NPC) trial, a selenized yeast supplement containing selenium, 200 mcg/day, decreased the incidence of total cancer, cancers of the prostate, colon and lung, and cancer mortality. The active agent in the selenized yeast supplement was assumed to be selenomethionine (SEMET), although the supplement had not been well speciated. The SELECT study, largely motivated by the NPC trial, enrolling nearly 40 times as many subjects, showed unequivocally that selenium 200 mcg/day, with selenium in the form of SEMET, does not protect selenium-replete men against prostate or other major cancer. The agent tested by SELECT, pure SEMET, could have been different from the selenized yeast tested in NPC. One of the selenium forms suspected of having chemopreventive effects, and which may have been present in the NPC agent, is methyl selenocysteine (MSC). This study, with 29 selenium-replete patients enrolled in a randomized, double-blind trial, compared the multiple-dose toxicity, pharmacokinetics and pharmacodynamics of MSC and SEMET. Patients were on trial for 84 days. No toxicity was observed. Although SEMET supplementation increased blood selenium concentration more than MSC did, neither form had a more than minimal impact on the two major selenoproteins: selenoprotein P(SEPP1) and glutathione peroxidase(GPX).
Highlights
Selenium (Se) is an essential nutrient [1]; lower Se levels in toenails and blood have been associated in ecological and individual-based epidemiologic studies with increased risk of cancer and a number of other chronic diseases [2,3,4]
The agent tested by SELECT, pure SEMET, could have been different from the selenized yeast tested in Nutritional Prevention of Cancer (NPC)
One of the selenium forms suspected of having chemopreventive effects, and which may have been present in the NPC agent, is methyl selenocysteine (MSC)
Summary
Selenium (Se) is an essential nutrient [1]; lower Se levels in toenails and blood have been associated in ecological and individual-based epidemiologic studies with increased risk of cancer and a number of other chronic diseases [2,3,4]. Experimental evidence that selenium supplementation decreases the risk of cancer or of other chronic diseases is mixed at best. The NPC trial, which randomized 1312 patients with a history of nonmelanoma skin cancer to 200 mcg selenium per day in selenized yeast or to a yeast placebo, gave rise to great optimism regarding selenium’s chemopreventive properties. An important caveat for the NPC findings is that these endpoints were secondary to the primary endpoint: non melanoma skin cancer recurrence. Assignment to supplementation did not decrease, but slightly increased, non melanoma skin cancer recurrence [8]
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