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United States Department of Agriculture

Agricultural Research Service

Selenium-enriched food fights cancer
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By Huawei Zeng

Selenium, which is an essential micronutrient for humans, first was discovered in 1817. Because of selenium's silvery color, it was named for Selene, the ancient goddess of the moon.

In the U.S. population, the recommended daily allowance for selenium is 55 micrograms per day. Consuming amounts above this level is considered “supranutritional.” For example, doses of 100 to 200 micrograms per day inhibit genetic damage and cancer development in humans, and a dose of 400 micrograms per day is considered a safe upper limit.

Cancer affects one out of every three Americans during their lifetime. The number of new cancer cases has steadily increased over the past few decades.

The National Academy of Sciences, the U.S. Department of Health and Human Services, the National Cancer Institute and the American Cancer Society all have indicated that 80 percent to 90 percent of all cancers are produced as a result of dietary and nutritional practices, lifestyle habits, such as smoking and alcohol consumption, chemical exposure and other environmental factors.

Certain forms of cancer are because of free radical oxidation that causes DNA damage in human cells. Because DNA regulates cell multiplication, these damaged cells can begin to multiply abnormally and further damage surrounding healthy tissue, until the whole body is invaded by these wildly proliferating cells. The first suggestion that selenium might be anti-carcinogenic occurred in the late 1960s.

Researchers noted an association among people with adequate selenium levels and reduced risk of some kinds of cancers. In addition, selenium was shown to reduce tumor formation and growth in rat studies. These observations further were 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.

There are at least 24 selenium-containing proteins. But only eight to 10 of these proteins have been well known by scientists.

Most selenium-containing proteins are antioxidant enzymes. These enzymes are involved in maintaining antioxidant status, and they work in conjunction with vitamins E and C. For example, one selenium-containing protein called gluthathione peroxidase protects against the formation of free radicals - those loose molecular cannons that can damage DNA. In this situation, selenium may work interchangeably (and in synergy) with vitamin E.

In test tube studies, selenium inhibited tumor growth and regulated the cell life cycle, ensuring that they died when they were supposed to rather than turning into tumor cells. Thus, selenium-containing proteins reduce DNA damage in human cells and help to maintain the healthy status of our bodies. Selenium also is essential for the enzymes that activate thyroid hormones and for proper motility of sperm mitochondria.

In addition, selenium protects against free radicals, mutagens, toxic heavy metals and certain bacterial, fungal and viral pathogens.

A recent study at the Grand Forks Human Nutrition Research Center has demonstrated that methylselenol, a selenium metabolite, prevents tumor cell migration and invasion. We then designed an experiment in which normal human cells and cancer cells were exposed to methyselenol. Our data indicate that methyselenol greatly inhibits the proliferation of cancer cells but not normal cells. Thus, selenium cannot only protect you from free radical oxidation and enhance your immunity but also help prevent the growth and progression of tumors.

The selenium requirement increases under stress, just as the requirement for certain vitamins increases during infections. Although too much selenium actually can be toxic to the system, research indicates that the majority of folks in the U.S. are not getting enough dietary selenium to help their bodies fight cancer.

So, what are good dietary sources of selenium? Seafood, mushroom, egg yolks, poultry and kidney, liver and muscle meats contain selenium. In addition, whole grains, seeds and vegetables such as garlic, onions and broccoli also can be good sources of selenium.

But the content of selenium in both whole grains and vegetables is directly related to the content of selenium in the soil on which these crops were grown. In general, the amount of selenium is correlated with the volcanic ash. Thus, the selenium content in soil is higher in the western U.S. than that in the eastern part.

Many areas of North Dakota have soils enriched in selenium. Crops grown on these soils contain high concentrations of selenium. It is possible that eating crops grown on high-selenium soils in North Dakota will reduce the risk of cancer. This is important for North Dakota agriculture as well as our health.

Last Modified: 2/13/2008
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