Humans contain roughly 20 mg of selenium distributed throughout the body (Combs, 2005). The element is usually incorporated into proteins. Due to the size similarities between selenium and sulfur atoms, selenium can be incorporated into amino acids, replacing the sulfur that naturally exists in sulfur-containing amino acids (Combs, 2005). These selenoamino acids can then be integrated into a variety of proteins (Combs, 2005). Two types of selenoamino acids exist. Selenocysteine and selenomethionine are both incorporated into proteins, but only selenocysteine amino acids have a physiological function (Combs, 2005). In the body selenium-containing proteins provide antioxidant protection, redox regulation, and thyroid hormone regulation (Combs, 2005). The antioxidant properties of selenium are extremely important in counteracting diseases involving oxidative stress (Rayman, 2008). Selenoproteins are also necessary to prevent neurological dysfunction and brainstem degeneration (Rayman, 2008).
Selenium can play an influential role in a variety of health issues and disease states. Studies have shown that addition of selenium in the diet can improve immune response even in healthy individuals (Rayman, 2008). Through the use of animal models, researchers have shown that selenium supplementation may play an important role in preventing HIV from mutating to a more powerful and dangerous form, and it may be possible that supplementation is necessary because retroviruses deplete their host’s stock of selenium so they can incorporate it into their own selenoproteins (Rayman, 2008). Selenium intake may also influence the reproductive health of men and women. Men’s fertility can be improved and women’s risk of miscarriage and pre-eclampsia can be reduced by increasing selenium intake (Rayman, 2008). Selenium may also have a role in the reduction of cancer risk. Current research has suggested that selenium metabolites may prevent cancer by inducing apoptosis in cancerous cells and inhibiting tumor cell invasion (Combs, 2005; Rayman, 2008).
The major species of selenium available in food determines what humans consume and accumulate within the body. The available species of selenium also govern the element’s final destination and the body’s use of the element (Rayman, 2008). For example, selenium obtained from broccoli (Se-methyl-selenocysteine) cannot accumulate in the tissues, nor does it have a significant physiological function (Rayman, 2008). However selenomethionine which originates from cereals or yeast can accumulate in proteins and be stored for long periods of time, allowing it to be used to maintain selenium levels throughout the body (Rayman, 2008).
Selenium in humans may be derived from meat, shellfish, nuts, whole grains, beans, peas, lentils, or fortified breakfast materials (Combs, 2005). Humans generally ingest selenium from plants. The uptake of selenium by plants depends upon the amount present in the soil (Rayman, 2008). The bioavailability of selenium to plants is controlled by concentration within the surrounding soil, pH and redox conditions, the amount of organic matter present, bacterial activity, and mineralogy among many other factors (Rayman, 2008). Within the soil, selenium levels vary because organic matter, iron hydroxides, and clay minerals can bind selenium removing it from the cycle of uptake by plants (Rayman, 2008). Additionally the particular species of selenium – determined by pH and redox conditions – present in soil affects the mobility of the element and bioavailability (Rayman, 2008). Therefore high levels of selenium in soil do not always result in selenium-rich crops, because the element may not be present in a form that can be used by the plant (Rayman, 2008).
Different plant species vary in the efficiency of their uptake of selenium. Some plants that accumulate selenium in significant amounts include Brazil nuts, broccoli, cabbage, garlic, and onions, all of which can contain up to 40,000 μg Se/g dry weight (Rayman, 2008). Cereals like wheat, oats, rye, and barley are non-accumulating crops; therefore they uptake relatively small amounts of selenium, no more than 100 μg Se/g dry weight (Rayman, 2008). The amount of selenium present in foods varies by country and region from excessively toxic amounts to inadequate quantities. Countries such as the United Kingdom, where selenium levels are low in plants, have begun supplementing animal feed with the element to increase human intake (Rayman, 2008).
Humans can suffer from both a deficiency in selenium and an excessive intake of selenium. Insufficient levels of selenium can cause cardiomyopathy, the deterioration of the heart muscle, possibly in conjunction with the mutated Coxsackie virus (Rayman, 2008). This condition, known as Keshan disease, is mainly prevalent in north-east China, however the cardiomyopathy of some Western patients has been associated with a lack of selenium in the intravenous nutrients provided (Rayman, 2009). Selenium toxicity is much less common than deficiency and symptoms of excessive selenium intake include hair loss, and brittle, thickened and stratified nails (Rayman, 2009). Chronic ingestion of high levels of selenium may lead to skin rash, weakness, tingling sensation, and diarrhea (Combs, 2005). Therefore selenium, like most nutrients must be consumed in the proper amounts or health problems could ensue. However the determination of a recommended daily intake of selenium is difficult due to the numerous factors that influence regulation of the element (Rayman, 2009). Advisory bodies must weigh both the benefits and the risks in order to determine an appropriate threshold level of intake.
References:
Combs, G.F. (2005) Geological Impacts on Nutrition. In Stone, D. Essentials of Medical Geology. Elsevier.
Rayman, M.P. (2008) Food-Chain Selenium and Human Health: Emphasis on Intake. British Journal of Nutrition. 100, 254-268.
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