In 1817, while attempting to isolate an entirely different element, Jons Jakob Berzelius accidentally discovered selenium. Although 30 years ago it was shown to be essential for animals, selenium's importance in human nutrition has attracted wide attention only in recent times.
Despite this relatively short period of study, claims are flying about the mineral's ability to prevent cancer and prolong life. In reality, more research is needed to illuminate the role of selenium in health and disease.
The first suggestion that it might prevent tumor formation surfaced toward the end of the 1940s. Since then, numerous animal studies have documented selenium's ability to inhibit tumor formation at a variety of sites, including skin, liver and colon. How these findings relate to humans is unclear.
Because of differences in soil concentration of the mineral, geographic location helps determine selenium status. Yet no detailed studies have been conducted relating soil selenium levels and cancer incidence worldwide. The data that do exist are a mosaic of inconsistencies. In New Zealand, where the soil is selenium-deficient and serum-selenium levels are low, statistics indicate that, except perhaps for colon cancer, inhabitants are no more susceptible to other forms of the disease.
Researchers have compared serum-selenium levels in cancer patients to those of individuals without the illness. A study of cancer patients in New Zealand found that their serum levels, although low, were no different from those without cancer who lived in the same area. A recent investigation in Finland, however, revealed significantly lower selenium levels in women with uterus and cervical cancers.
It can be argued that the selenium depletion was an effect of the disease rather than a cause. But, the investigators counter that the serum levels did not vary with the stage of cancer, as might be expected if depletion occurred over the course of tumor development. And conversely, elevated selenium levels have been reported in other types of cancer, leukemia for one.
Study Needed to Answer Question
What is needed to help answer the question is a study in which selenium levels of a large group of people are monitored over many years and compared to the incidence of cancer development.
Attention also has focused on the relationship between selenium and cardiovascular disease. The interest stems primarily from reports in the late 1970s describing the first example of a selenium-responsive disease in humans. Named Keshan's syndrome after the province in China where it was originally identified (a region extremely low in selenium), it is characterized by a degeneration of the heart muscle, or cardiomyopathy.
Because the occurrence of the disease shows a seasonal variation that cannot be explained by changes in selenium levels of the food supply or in the individuals themselves, it probably involves some other factor, like a virus, as well. A treatment program of sodium selenite, however, has dramatically reduced the incidence of the disease.
A similar cardiomyopathy has been observed in selenium-deficient animals, along with abnormal heart rhythms and blood-pressure changes.
Although a number of studies have linked low serum-selenium levels to coronary heart disease in humans, a recent report from Great Britain could find no correlation between selenium status and such coronary heart disease risk factors as cholesterol, triglyceride levels and blood pressure. The investigators did note a decrease in selenium in the serum of smokers, a group known to be at greater risk for coronary heart disease. It is still a mystery how this correlation relates to the disease itself.
Any Biochemical Basis?
Is there any biochemical basis for all the speculation about selenium? Most researchers point to the selenium-containing enzyme, glutathione peroxidase, which protects cell walls and the genetic material within from damage caused by the breakdown products of fat metabolism. This "antioxidant" property is similar to that of Vitamin E, and the two nutrients are thought to complement and, in some cases, to substitute for one another if need be. The ability of glutathione peroxidase to prevent cell damage is responsible for the exaggerated claims that selenium can ward off or retard the aging process.
Here in the United States, there is little cause for alarm about selenium intake. Unlike isolated areas of the world, regional differences in soil content are of limited concern in this country because we eat foods produced in many places with varying amounts of selenium in the earth. While as yet there is insufficient information to establish a Recommended Dietary Allowance, the Food and Nutrition Board suggests a "safe and adequate" intake of between 50 and 200 micrograms a day.