A 10-year cancer prevention trial suggests that the trace element selenium might significantly lower the incidence of cancer in people with a history of skin cancer. The supplements did not, however, affect the incidence of basil or squamous cell cancers of the skin, which was the original hypothesis of the study.
The study included a total of 1,312 skin cancer patients who were seen at seven dermatology clinics in the eastern United States – an area where there is relatively low selenium levels in the soil and crops. The average age of the participants was 63. At the start of the study, the primary purpose was to see if the dietary supplement selenium could lower the incidence of basal cell or squamous cell skin cancers. Seven years later, three commonly occurring cancers – lung, colorectal, and prostate – were added to the list of diseases looked at in this study.
Participants in the randomized, double-blind study (in which neither the patients nor the doctors knew who was receiving selenium) took either a daily tablet containing 200 micrograms (µg) of selenium as brewer's yeast or a placebo for 4.5 years. They were followed for an additional 6.4 years. Three-quarters of the participants were men.
American diets generally include enough grain, meat, and fish – the primary sources of selenium – to meet the recommended dietary allowance (RDA): 70 µg per day for men and 55 µg per day for women. (The human toxicity levels for selenium have not been definitely established, although the Environmental Protection Agency established 350 µg per day as the maximum safe dose.)
Selenium can be toxic at very high doses, which was was first observed in animals that had eaten plants that contained toxic levels of selenium. Toxicity in humans has rarely been seen. However, toxicity has been reported in parts of the world where dietary selenium levels are extremely high. The reported symptoms of human toxicity from selenium have included increased nail and hair brittleness or crumbling, skin rashes, "pins and needles" sensations, irritability, nausea, vomiting, or garlic breath. These side effects were observed in approximately 10 percent of Chinese subjects who consumed a diet with over 1,000 µg of selenium daily.
What Does Selenium Do?
Selenium is a trace element that naturally occurs in the soil. It enters our bodies through our food (both plant and animal products) and, to a lesser extent, through the water we drink. Plants absorb selenium from the soil into their leaves, stems, seeds, and fruits. Animals eat plants containing the selenium which is stored in their tissues. Brazil nuts, wheat, corn, rice, soybeans (grown in selenium-rich soil), eggs, fish, and meat are particularly good sources of selenium.
The major functions of selenium in the body are:
- Selenium helps protect against cancer by encouraging cancer cells to undergo apoptosis – a form of programmed cell death that occurs when genetic damage is detected in the cell.
- An enzyme, Glutathione Peroxidase (GSH-PX) requires selenium for its formation. This enzyme helps prevent a process called oxidation, which can cause injury in cells.
- Selenium appears to improve the functioning of the immune system and its response to infections.
- Selenium appears to cause the formation of natural killer cells, which destroy foreign bacteria that enters the body.
- P450 enzymes might be induced by selenium. These enzymes help detoxify some cancer-causing substances.
- Male fertility might be enhanced by selenium due to increased sperm motility.
- At high doses, selenium might decrease the rates of tumor cell growth.
Additional Study Details
The results of the study showed that total cancer incidence was significantly lower in the selenium group than in the placebo group (77 cases versus 119), as was the incidence of some specific cancers: the selenium group had fewer lung cancers (17 versus 31), fewer colorectal cancers (8 versus 19), and fewer prostate cancers (13 versus 35). These differences were statistically significant.
The results also showed that overall mortality was 17 percent less in the selenium versus the control group (108 versus 129) with this difference largely due to a 50 percent reduction in cancer deaths (29 versus 57). Lung cancer deaths were lower in the selenium treatment group than in the placebo group (12 versus 26).
The National Cancer Institute points out that the study population: 1) was relatively small, 2) consisted of people who live in low-selenium regions, and 3) were at high risk for non-melanoma skin cancer. They recommended that the results should be confirmed in a larger population more representative of the entire United States.
Other Selenium Research
Because of the significant rates of prostate cancer reduction in the selenium trial, the National Cancer Institute began a much larger clinical study with over 35,000 men. The study is called SELECT (which stands for the Selenium and Vitamin E Cancer Prevention Trial) and is designed to see if one or both of these dietary supplements could prevent prostate cancer. More than 400 sites in the United States, Puerto Rico, and Canada are taking part in the study.
In animals, selenium administration has been shown to have anti-tumor activity, but at levels several times greater than the nutritional needs. Likewise, in tissue culture experiments, supplementation of cultured tumor cells with selenium at much higher doses than the cells normally require has been shown to inhibit tumor growth and stimulate apoptosis.
Two additional clinical trials took place in China. In one trial, a daily supplement containing 50 µg of selenium plus three other minerals and vitamins had no effect on the high incidence of esophageal cancer or total cancer incidence or mortality. The second and larger trial showed a significant reduction in stomach cancer incidence (16 percent) and stomach cancer mortality (21 percent) using a daily mixture of antioxidants, one component of which was selenium. |
