High Selenium Levels Can Be Harmful

Fifty years ago chickens in North Dakota were born with wing and leg deformities, or missing beaks and eyes. The same deformities were found last year among wild ducks using a water reservoir near Monterey, Calif. Selenium was the problem in North Dakota and it’s the problem for those wild ducks . . . the same selenium that’s essential to human life and may even help prevent cancer. Selenium is a mineral like sodium and calcium.

About 25 years ago physicians in central China encountered an epidemic of selenium poisoning among their patients. It came from corn grown locally and irrigated with water that was loaded with selenium leached from a nearby coal mine. After a year of consuming selenium in concentrations more than 25 times the safe range (200 micrograms a day is regarded as a safe upper limit of human intake) victims of selenium poisoning developed a scalp rash, brittle hair and fingernails, and easily blistered skin. More advanced symptoms were generalized numbness, paralysis, convulsions and death.

Plants appear to be indifferent to the presence or absence of selenium in their tissues. They take in as little or as much as the water carries, although some weeds (loco weed) are selenium accumulators and capable of poisoning animals that graze on them. It can cause hair loss, cracked hooves and a nerve condition called the blind staggers. The animals are duplicating human toxicities. These apparent poisonous effects of selenium have prompted nutritionists to caution people against excessive selenium dosing.

Geological Heritage

The amount of selenium in the soil depends on its geological heritage. In general, grain from the great plains of the Midwest is selenium rich and most produce grown in the eastern and western halves of the country is selenium poor. In North Dakota the soil is naturally rich in selenium so plants such as wheat are rich in it too. When such wheat is fed to chickens it causes deformities in their offspring. No comparable difficulties have occured among other inhabitants of North Dakota including humans, but toxic-feeding studies carried out in laboratory animals 40 years ago did show that very high doses of selenium can cause cancer.

For more than 30 years intensive laboratory study has gone into an evaluation of selenium’s role in human metabolism, and evidence has emerged that it plays a role in numerous human body functions. Ironically, most studies with selenium in small doses have demonstrated that the mineral can actually help prevent cancer by acting with an enzyme in all body tissues called glutathione peroxidase. The two constitute a kind of fire extinguisher, dosing potentially damaging hot spots created by irritating byproducts of metabolism called free radicals; so small amounts of selenium can be cancer protective.

The wide geographic sources of today’s diet assure us an adequate supply of selenium so that supplements are generally not recommended. For those readers with a family tendency to cancer and who are determined to put what is presently known into their daily life styles however, there is no evidence that a supplement of 25 or 50 micrograms of selenium a day is harmful. Indeed 2,000 American dentists are presently taking it in just those levels as participants in a long-term double-blind study to see if the supplement does in fact have an anti-cancer effect.