In the past we have discussed a number of nutrients and how they contribute to the cow's dietary needs. This article will deviate from this format somewhat because it will not only address a nutritional issue related to the bovine but also to humans as well. Sometimes, as cattlemen we forget that we are actually in the food business. Ultimately the product we are producing is destined for the food market whether it is in the local grocery store, in a high dollar steakhouse or in a fast-food restaurant. As beef producers we have discussed at length our ability to deliver a product that is desirable and in demand. As a nutrient source for the consumer, beef is an excellent source of protein, B-Vitamins and minerals such as Zinc and Iron. Research that is currently emerging is also showing beef to be an excellent source of the trace mineral Selenium. We talked about selenium before and how important it is in the animal's diet and the effects it can have on health and reproduction. This research is showing us how important it is in our diets from these same perspectives. The data on health effects is truly staggering in its implications. If you have ever read one of these articles before I strongly recommend to take the time to read this one carefully. It will get a little technical and involved but it has information that could change all of our lives.
What is Selenium?
Selenium (Se) is a trace mineral that is essential to good health but required in very small amounts. Selenium is incorporated into proteins to make selenoproteins, which are important antioxidant enzymes. One of the most important of these is an enzyme know as glutathione peroxidase. The antioxidant properties of these Se inclusive enzyme systems help prevent cellular damage from free radicals. Free radicals are natural by-products of oxygen metabolism and a functional immune system that may contribute to the development of chronic diseases such as cancer and heart disease. Other selenoproteins help regulate thyroid function and play important roles in the immune system. As you can see, these are all very important effects as related to our day to day and long term health. The issue here is that when a person's Se status (the amount of circulating or stored Se in the body) is inadequate or even sub optimal, the body cannot manufacture the selenoproteins (enzymes) as needed to effectively function, as required to maintain day to day or long-term health. Thus, when a person's Se level is below required or simply low for a longer period of time we see an increase in the incidence of sickness and disease.
Dietary Sources of Selenium
Plant based foods are the major dietary sources of selenium in most countries throughout the world. The content of selenium in food depends on the selenium content of the soil where plants are grown or animals are raised. For example, researchers know that soils in the high plains of northern Nebraska, the Dakotas and other parts of the United States have high levels of selenium. People living in those regions generally have historically had the highest selenium intakes in the United States. This has not necessarily been the case in recent history when diets do not depend as much on locally grown foods as they do on foods transported in from other parts of the country. On a world-wide basis soils in some Scandinavian countries as well as in parts of China and Russia have very low amounts of selenium. Selenium deficiency is often reported in those regions because most food in those areas is grown and consumed locally.
Selenium also can be found in some meats and seafood. Animals that eat grains or plants that were grown in selenium-rich soil have higher levels of selenium in their muscle. Considerable research over the last few years has shown that the level of selenium in animal tissues can be enhanced by feeding specific types of Se supplements. Current research in the use of Selenium Yeast (a yeast product that has had the Se content enhanced through specific production and growth processes) has shown that the levels of Se in beef animal muscle tissue and in milk can be enhanced or increased through feeding of the product. This indicates that cattle fed or supplemented with Se yeast produce beef with a higher Se content in their meat than non-Se yeast supplemented animals. The National Cattlemen's Beef Association (NCBA) has been instrumental in promoting beef for it's nutritional value. Hopefully they will soon as Se as another nutrient of value that can be delivered through the consumption of beef.
Selenium content of foods can vary. For example, Brazil nuts may contain as much as 544 micrograms of selenium per ounce. They also may contain far less selenium. It is wise to eat Brazil nuts only occasionally because of their unusually high intake of selenium. Selected food sources of selenium are provided in Table 1.
Recommended Dietary Intake for Selenium
Recommendations for selenium are provided in the Dietary Reference Intakes developed by the Institute of Medicine. Dietary Reference Intakes (DRIs) is the general term for a set of reference values used for planning and assessing nutrient intake for healthy people.
There is insufficient information on selenium to establish a RDA for infants. An Adequate Intake (AI) has been established that is based on the amount of selenium consumed by healthy infants who are fed breast milk. Table 3 lists the AIs for selenium, in micrograms (ìg) per day, for infants.
Results of the National Health and Nutrition Examination Survey indicated that
diets of most Americans provide recommended amounts of selenium. The INTERMAP
study examined nutrient intakes of almost 5,000 middle-aged men and women in
four countries in the late 1990s, including the U.S. The primary aim of the
study was to evaluate the effect of dietary micronutrients on blood pressure.
Each study participant completed four, 24-hour dietary recalls, during which
they were asked to record everything consumed (food, beverages, and dietary
supplements) over the previous 24 hours. Selenium intake was lowest among residents
of China, the country with the highest known rate of selenium deficiency. Mean
dietary intake of selenium of U.S. participants was 153 µg for men and
109 µg for women. Both values exceed the recommended selenium intake
for adults and are further evidence of adequate selenium intakes in the U.S
based on what the RDA values. However, based on emerging data and what research
is seeing as the benefits for a higher Se intake, the RDA values probably
need to be re-evaluated.
Human selenium deficiency is rare in the U.S. but is seen in other countries, most notably China, where soil concentration of selenium is low. There is evidence that selenium deficiency may contribute to development of a form of heart disease, hypothyroidism, and a weakened immune system. There is also evidence that selenium deficiency does not usually cause illness by itself. Rather, it's absence and the subsequent inability for the body to manufacture Se-dependent enzymes related to the function of the immune system can make the body more susceptible to illnesses caused by other nutritional, biochemical or infectious stresses.
The Need for Supplemental Selenium?
In the U.S., most cases of selenium depletion or deficiency are associated with severe gastrointestinal problems, such as Crohn's disease, or with surgical removal of part of the stomach. These and other gastrointestinal disorders can impair selenium absorption. People with acute severe illness who develop inflammation and widespread infection often have decreased levels of selenium in their blood. Physicians will evaluate individuals who have gastrointestinal disease or severe infection for depleted blood levels of selenium to determine the need for supplementation.
Selenium occurs in staple foods such as corn, wheat, and soybean as selenomethionine, the organic selenium analogue of the amino acid methionine. Selenomethionine can be incorporated into body proteins in place of methionine, and serves as a vehicle for selenium storage in organs and tissues. Selenium supplements may also contain sodium selenite and sodium selenate, two inorganic forms of selenium. Selenomethionine is generally considered to be the best absorbed and utilized form of selenium with some limitiations.
As noted above Se is also available in 'high selenium yeasts', which may contain as much as 1,000 to 2,000 micrograms of selenium per gram. Most of the selenium in these yeasts is in the form of selenomethionine. This form of selenium was used in the large scale cancer prevention trial in 1983, which demonstrated that taking a daily supplement containing 200 micrograms of selenium per day could lower the risk of developing prostate, lung, and colorectal cancer.
A study conducted in 1995 suggested that the organic forms of selenium increased blood selenium concentration to a greater extent than inorganic forms. However, it did not significantly improve the activity of the selenium-dependent enzyme, glutathione peroxidase, which was more responsive to inorganic forms of Se such as Sodium selenite. Researchers are continuing to examine the effects of different chemical forms of selenium, but the organic form currently appears to be the best choice.
We have discussed much of the dietary needs for Se in our diets. In the next part of this series we will look at specific health effects, especially the emerging data on the effects improving Se status has on incidence of numerous types of cancer.
Dr. Steve Blezinger is a management and nutritional consultant with an office in Sulphur Springs, TX. He can be reached at 667 CR 4711 Sulphur Springs, TX 75482, by phone at (903) 885-7992 or by e-mail at [email protected]