Selenium prevents cancer. This is common knowledge which is only awaiting conclusive confirmation. It received recognition when, in 1996, an American researcher (Clark) showed in a randomised study that the frequency of cancer fell by 38%, and that the fatality rate of those with cancer fell by 50%, in participants who received daily supplement of 200 micrograms selenium.
The supposition that selenium is preventative for cancer is in fact much more extensively backed. This has been shown by a leading selenium expert, Margret Rayman from the University of Surrey in England, in a thorough, but also complicated, summary. She has also illuminated who selenium prevents cancer and, even more importantly, how much is needed.
Rayman reviews the many geographical studies that have, since the 1960’s, consistently shown that the populations who received the least amount of selenium also had the highest cancer rates. Animal studies are also discussed. If one gives selenium to a male dog, not only is there less damage to the DNA of prostate cells, but the damaged cells that remain also die normally instead of living on as cancer cells.
A certain pattern emerges when one looks at studies where the blood concentration of selenium is compared to the cancer rate in groups of people. In a French study of this type from 2005, the death rate from cancer after nine years was four times greater in the 25% of the study-group who received the least selenium than in those who received the most selenium. Typically the French receive as little selenium in their diets as the British. Many studies from many countries have shown similar results for lung cancer (a 26% lower risk of cancer was reported in those with diets rich in selenium), oesophagus cancer, stomach cancer, and not in the least, prostate cancer as well as possibly cancer of the large intestine.
How much is enough
The best evidence can always be found in randomised studies where neither the patients nor the doctors know who receives what. A Chinese study of this type has shown that selenium has an especially effective against liver cancer. As mentioned before, Clark’s study had similar results. It is intriguing that, even though Americans receive an average of 200 micrograms more selenium daily than us, an additional 200 micrograms was beneficial to most. The effects were nevertheless minimal in those who received the most selenium beforehand. These individuals already received close to the optimal dose. But third of the population who received the least beforehand, had their cancer risk halved, and their prostate cancer risk decreased by 86%, after taking supplementary selenium.
Typical Europeans receive too little selenium while Americans receive double as much and Japanese receive almost three times as much. It has become apparent that 70 micrograms of selenium is needed in the daily diet to maintain levels of the selenium based antioxidant GSHpx in the body. The Japanese and most Americans receive this amount in their diets while we do not. But why is their cancer risk reduced when they receive supplementary selenium? The reason cannot be GSHpx and may not even be the anti-oxidizing effects alone.
Rayman examines many possible explanations. One is that high doses of selenium lead to the formation of the simple selenium compound methylselenol; which can kill cancer cells, counteract the formation of blood vessels (which the cancer cells need to survive) and can inhibit cancer in other ways. But selenium is naturally an antioxidant, an immune system stimulant, an activator for cancer inhibiting genes, an inhibitor for growth factors, etc. There is not one, but many, mechanisms of action.
Unchecked amounts of selenium should not be taken. Studies indicate that sufficiently high doses of selenium can increase the risk of cancer as much as insufficient amounts. Clark’s study, as well as others, suggests that a daily supplement of 200 micrograms is optimal.
The reward can be large, but more research is needed. Currently a large clinical trail (called SELECT) is being undertaken in the U.S.A., but a study in the more selenium poor Europe would be better. Rayman believes that such a study should be undertaken. But who wants the placebo!?
- Rayman M P. Selenium in cancer prevention: A review of the evidence and mechanism of action. Proceedings of the nutrition society 2005;64:527-42.
- Clark LC et al. Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. A randomized controlled trial. Nutritional Prevention of Cancer study group. JAMA 1996;276:1957-63.
- Akbaraly NT et al. Selenium and mortality in the elderly: Results from the EVA study. Clin Chem. 2005;51::2117-23.