The Durk Pearson & Sandy Shaw®
Life Extension NewsTM
Volume 20 No.
1 • February 2017
If You Take Selenium to Help Reduce Your Risk of Cancer...
...You need to know that not all forms of selenium are equally effective in doing so and that selenite is likely to be more effective than some other forms, including selenomethionine.
DIFFERENT FORMS OF SELENIUM CAN HAVE VERY DIFFERENT EFFECTS
Selenium has anticarcinogenic effects, which is a major reason why the mineral is taken as a supplement by so many people. A recent paper (Olm, 2009) explains why, in terms of cancer prevention, selenate, selenocysteine, or selenomethionine may be less preferable than selenite. There, the authors explain what they have discovered about how different forms of selenium can exert different cytotoxic effects against cancer cells.
The researchers found that there is a reductive (antioxidant) microenvironment around cancer cells due to the secretion from cancer cells of cysteine that occurs as part of a cystine/cysteine redox cycle. This cycle is under the control of multidrug resistance protein pumps that operate in cancer cells to pump out intracellular molecules cytotoxic to cancer cells such as cysteine. This reductive extracellular microenvironment, in turn, activates selenium into forms that are cytotoxic because they are taken up into the cancer cells where they release deadly hydroxyl radicals. Selenate and selenomethionine were reported (Olm, 2009) to be less able to enter the cancer cells (less redox active). While selenocysteine is redox active, it has to be enzymatically degraded by the enzyme beta-lyase to selenide to achieve its full toxic potential. Moreover, selenocysteine may, the authors suggest, be partly secreted back to the extracellular compartment along the same pathway as cysteine.
The bottom line of this complex mechanism of selenium cancer protective effects is, as proposed in this paper, that there might be a “... possibility of a cancer-specific high-affinity selenium uptake mechanism that might explain cancer-specific selenite cytotoxicity at therapeutic selenite concentrations (μM range) (Olm, 2009).”
We hope that research is done on the use of sodium selenite as an adjunct to cancer chemotherapy. At least some of the cancer cells that are most resistant to chemotherapy will be particularly susceptible to selenite. The reason that chemotherapy so often eventually fails is that the cells that remain alive after that therapy are those that are able to pump the drugs out and, hence, are resistant and can continue to proliferate. The new resistant population is no longer treatable with the original chemotherapeutic regimen. These cells may be especially susceptible to the effects of selenite.
Meanwhile Back at the FDA the Selenium Evidence is Receiving Short Shrift
While considerable evidence supports a role for selenium in cancer risk reduction, the FDA has denied qualified health claims that selenium MAY have anticancer effects and MAY reduce the risk of several types of cancer. In the rarified atmosphere at the FDA, the belief persists that nothing can be communicated to the public as true about dietary supplements or foods unless the FDA first gives you their permission.
RARER THAN GOLD
Interestingly, the concentration of Se in the Earth’s crust is actually lower than that of gold, making it rarer than gold (Whanger, 2004). It should be no surprise, then, that there are many areas of the world where people live in a low-selenium environment.
UPPER TOLERABLE (SAFE) LIMIT OF SELENIUM SET BY THE INSTITUTE OF MEDICINE
The Institute of Medicine of the National Academy of Sciences (2000) has set 400 µg of selenium as the upper tolerable (safe) limit.
Olm et al. Extracellular thiol-assisted selenium uptake dependent on the xc-cystine transporter explains the cancer specific cytotoxicity of selenite. Proc Natl Acad Sci U S A. 106(27):11400-5 (2009).
Whanger. Selenium and its relationship to cancer: an update. Br J Nutr. 91:11-28 (2004).