The results of a meta-analysis of 31 studies, published in the October, 2006 issue of the American Journal of Clinical Nutrition, found that individuals with higher selenium levels or a greater intake of the mineral had a lower risk of coronary heart disease.
For their review, researchers at Johns Hopkins in Baltimore selected 25 observational studies that measured blood or toenail selenium and 6 randomized trials that evaluated the effects of selenium supplementation on heart disease risk. Fourteen of the observational studies were cohort studies (which examine a particular group of individuals), and 11 were case-control studies (which compare individuals with a disease or condition to matched controls who do not have the condition).
Among the observational studies, a 50 percent increase in selenium concentrations was associated with a reduction in heart disease risk of 24 percent. In the cohort studies, those with highest selenium concentrations had a 15 percent lower risk of heart disease than those whose selenium concentrations were lowest, and in the case-control studies, the risk was 57 percent lower for those who had the greatest selenium levels. Analysis of randomized trials found an 11 percent reduction in risk among those who took supplements compared to participants who received a placebo.
In their discussion of the findings, the authors observe that selenoproteins have antioxidant properties which reduce hydrogen peroxide and lipid hydroperoxides, regenerate antioxidant systems, or may protect endothelial cells from peroxynitrite and lipid peroxidation. Another factor involved in the development of heart disease associated with reduced levels of selenium is increased platelet aggregability and vasoconstriction. Additionally, having adequate selenium levels may protect the cardiovascular system from toxic metals, including arsenic, cadmium and mercury, which cause oxidative damage.
Although the findings of the meta-analysis were positive for selenium, the authors write that the validity of observational studies can be misleading, and recommend large ongoing clinical trials to confirm that having low selenium is a risk factor for cardiovascular disease.
"Heavy metals" are chemical elements with a specific gravity that is at least 5 times the specific gravity of water. The specific gravity of water is 1 at 4°C (39°F). Simply stated, specific gravity is a measure of density of a given amount of a solid substance when it is compared to an equal amount of water. Some well-known toxic metallic elements with a specific gravity that is 5 or more times that of water are arsenic, 5.7; cadmium, 8.65; iron, 7.9; lead, 11.34; and mercury, 13.546 (Lide 1992).
For some heavy metals, toxic levels can be just above the background concentrations naturally found in nature. Therefore, it is important for us to inform ourselves about the heavy metals and to take protective measures against excessive exposure. In most parts of the United States, heavy metal toxicity is an uncommon medical condition; however, it is a clinically significant condition when it does occur. If unrecognized or inappropriately treated, toxicity can result in significant illness and reduced quality of life (Ferner 2001). For persons who suspect that they or someone in their household might have heavy metal toxicity, testing is essential. Appropriate conventional and natural medical procedures may need to be pursued (Dupler 2001).
The association of symptoms indicative of acute toxicity is not difficult to recognize because the symptoms are usually severe, rapid in onset, and associated with a known exposure or ingestion (Ferner 2001): cramping, nausea, and vomiting; pain; sweating; headaches; difficulty breathing; impaired cognitive, motor, and language skills; mania; and convulsions. The symptoms of toxicity resulting from chronic exposure (impaired cognitive, motor, and language skills; learning difficulties; nervousness and emotional instability; and insomnia, nausea, lethargy, and feeling ill) are also easily recognized; however, they are much more difficult to associate with their cause. Symptoms of chronic exposure are very similar to symptoms of other health conditions and often develop slowly over months or even years. Sometimes the symptoms of chronic exposure actually abate from time to time, leading the person to postpone seeking treatment, thinking the symptoms are related to something else.
As an essential cofactor of glutathione peroxidase, selenium is an important antioxidant. It is also involved with iodine metabolism, DNA repair, immune function, and the detoxification of heavy metals. High doses of vitamin C (over 1 gram) may reduce the absorption of selenium. This mineral is best taken one hour before or 20 minutes after taking vitamin C supplements.
Se-Methylselenocysteine (SeMC) is a naturally occurring seleno-amino acid that is synthesized by plants such as garlic and broccoli. Unlike selenomethionine, which is incorporated into proteins in place of methionine, SeMC is not incorporated into any proteins, thereby being fully available for the synthesis of selenium-containing enzymes such as glutathione peroxidase.
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