Selenium compounds as establishd by in vitro and in vivo experimental models show than that selenium is an effective anticancer agent. Clinical trials, however, have not shown that selenium supplementation in humans is an effective way to prevent cancer.
The transsulfuration pathway metabolizes L-selenomethionine which is the food form of selenium. L-selenomethionine is stored in the body through replacing L-methionine in proteins. Selenium not exiting L- selenomethionine can explain why selenium supplemenation, heretofore, has not been an effective way to prevent cancer in humans.
Tte transsulfuration pathway metabolizes homocysteine. High homocsyteine levels point to the transsulfurtation pathway being dsyregulated. High homocysteine levels are associated with cancer. High homocysteine levels in cancer would then point to the transsulfuration pathway being dysregulated in cancer. With the transsulfuration pathway dysregulated in cancer L-selenomethionine is not metabolized. As L-selenomethione is not metabolized selenium supplementation in the form of L-selenomethionine is not an effective way to prevent cancer in humans.
Se-methylselenocysteine is a very effective anti-cancer agent. Se-methylselenocysteine is a form of selenium that is not metabolized via the transsulfuration pathway but rather is metabolized by kynurenine aminotransferase, which is not an enzyme in the transsulfuration pathway, so formation of selenoproteins from Se-methylselenocysteine is not stopped by dysregulation of the transsulfuration pathway. Se-methylselenocysteine could be an effective anti-cancer agent in experimental models and also in humans.
Choosing an appropriate dosing scheduls is a key to effective selenium supplementation. Supranutrional selenium can increase activity of thioredoxin reductase. There is an end of dosarge effect with selenium apparently due to declines, during the day, in activity of thioredoxin reductase. For the prevention of cancer 100 micrograms of Se-methylselenocysteine taken twice a day would be a more effective selenium supplmentation schedule than 200 micrograms of of selenium from Se-methylselenocysteine taken once a day.