Page Banner

United States Department of Agriculture

Agricultural Research Service

Selenium - An essential mineral for the body
headline bar
By Jennifer Watts

Selenium, a mineral found in the soil, is taken up into plants and animals and is necessary for proper body functioning.

The majority of the United States has soils that are very low in selenium, requiring the supplementation of all animal feeds to prevent deficiency related diseases.

Here in the Upper Midwest, there are pockets of soil that are exceptionally high in selenium, so plant and animal products from this area are higher in selenium than those in other areas.

The U.S. population consumes most of its selenium from wheat products, beef, chicken and several types of fish. There are at least 24 selenium-containing proteins. However, scientists only know what eight to 10 of these proteins really do and how selenium functions within those proteins.

The major selenium-containing proteins are involved in maintaining antioxidant status and working in conjunction with vitamins E and C. Selenium also is essential for the enzymes that activate thyroid hormones and for proper motility of sperm mitochondria.

The most interesting discovery in the last 30 years of selenium research was the finding that forms of selenium could reduce tumor formation and growth in rats. This further was investigated in a human clinical cancer prevention trial, which showed that there was a reduction in the risk of prostate, colorectal, and lung cancers with selenium supplementation of 200 micrograms per day.

This study was very significant; it showed the addition of a nutrient, at intakes above those necessary for proper functioning of the body ("supranutritional levels") had a distinct effect on the reduction of cancer risk. What was more interesting was the fact that only those patients who had blood selenium levels less than 106 nanograms per milliliter (ng/mL) saw the benefit of the supplemental selenium. Patients with blood levels greater than 120 ng/mL had no reduction of risk. Some researchers believe that if people have blood selenium levels higher than 120 ng/mL, that there is a protective effect of reducing cancer risk.

Current data show that about 45 percent of the U.S. population has blood selenium levels less than 120 ng/mL, and most are very close to that level. Since most are close to the concentration believed to be the threshold for protection, it would seem unnecessary to take 200 micrograms of selenium a day, which can raise blood selenium levels much higher than what is needed.

A selenium study in the Grand Forks Human Nutrition Research Center currently is under way and is designed to help determine how much selenium is necessary for normal, healthy Americans to take to raise their blood selenium levels to the concentration of 120 ng/mL. This is being done by giving volunteers one of four doses of selenium: 0, 50, 100 and 200 micrograms per day and tracking their blood selenium values for one year.

It has been determined that if a person increases his/her selenium intake, it takes about nine to 10 months for the blood concentrations to reach a new steady level.

We hope that by recruiting people of all different sizes that a variety of effective doses can be given; an effective dose is the amount of selenium given divided by body weight. By having a lot of variety, we can develop an equation that will help determine the amount of selenium people would need to take to raise their blood selenium levels.

Other aspects of selenium metabolism also are being investigated, such as: a noninvasive way of determining selenium status, effect of selenium supplementation on DNA repair and the effect of genotype on response of selenium-containing enzymes. All of these parts of the study are extremely important and previously have not been fully investigated.

A benefit of this study may be confirming what researchers believe that the amount of supplementation needed to raise blood selenium levels will be less than 200 micrograms a day. If that were the case, then it would not be necessary to take a supplement and instead, receive the benefits through diet. It may be possible to select foods that are high in or enriched with selenium and increase blood selenium through foods.

This is important for North Dakota agriculture, due to the areas of the state that have high levels of selenium in the soil.

Last Modified: 3/3/2006