Selenium is present in all the tissues of the body, but is concentrated most highly in the kidneys, liver, spleen, pancreas, and testes. People do not consume enough selenium both because of the type of diet they eat, and because of the low selenium content of the soil in which their food is grown. The selenium content of soil varies widely, with many areas showing serious depletion. In fact, there have been several reports of selenium deficiencies in livestock raised on selenium-depleted soil.
FUNCTIONS AND USES
Selenium’s best-known and perhaps most important biological function relates to its role as an antioxidant and anticancer mineral. As we have seen in other chapters, free radicals damage our cells, possibly leading to the development of cancer and other degenerative diseases. Selenium is an activating component of the enzyme glutathione peroxidase, which protects our cells from this damage.
Many animal studies have proven that selenium deficiency increases the incidence and rate of growth of cancers in animals that are either exposed to a variety of potent carcinogens or receive transplanted tumors. Companion studies have shown that high selenium intake protects against these cancers. For example, in one study in which rats were exposed to a potent carcinogen, only 15 percent of those who were also given selenium developed liver cancer as compared with 90 percent of the unsupplemented rats. In another study, the occurrence of cancer was 10 percent in the supplemented group versus 80 percent in the control group. In yet another animal study, selenium supplementation reduced colon cancer incidence by more than 50 percent. In another study, selenium protected against UV-induced skin damage and cancer, retarding the onset and number of skin lesions, and reducing inflammation, blistering, and pigmentation.
In. humans, there is ample epidemiological evidence that high selenium is correlated with a lower incidence of many types of cancer. For instance researchers have found that cancer risk is significantly lower in people living in areas with selenium-rich soil, in people with a high-selenium food supply, and in people with higher blood levels of selenium, when compared with people with lower intakes and blood levels. Selenium intakes in the people studied were close to 750 micrograms per day, with no toxic side effects noted. In a survey that spanned twenty-seven countries, including the United States, it was found that the cancer death rate was lower in those people whose typical diets were high in selenium. This and other cancer studies indicate that selenium is especially protective against cancer of the breast, colon, and lung. Data also suggests protection against tumors of the ovaries, cervix, rectum, bladder, esophagus, pancreas, skin, liver, and prostate, as well as against leukemia.
Since 1969, it has been known that the blood levels of cancer patients are low in selenium. In general, cancer patients with lower-than-average selenium levels have a greater number of primary tumors, more recurrences, more distant metastases (tumors that have spread to distant parts of the body), and a shortened survival time. In a study of 12,000 people conducted in Finland, the risk of fatal cancer in people with the lowest levels of serum selenium was nearly six times higher than that in people with the highest selenium concentrations.
Like other nutrients, of course, selenium cannot do its work alone. In several studies, it has been shown that selenium and vitamin E-and perhaps vitamin A, too—have a synergistic effect. For example, in one study, male smokers who died of cancer had lower levels of serum selenium, vitamin A, and vitamin E, when compared w