Selenium en voeding

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Selenium (of Seleen) is een zeer belangrijk mineraal dat een zeer sterke anti-oxidant werking heeft. De meeste selenium vind je in de zogenaamde Paranoten (Brazil Nuts - de Bertholletia excelsa).

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Selenium is de bouwsteen van diverse enzymen in ons lichaam. Selenium is oa van belang voor een goede werking van de schildklierhormonen. Het bekendste enzym waarvan selenium deel uitmaakt is het gluthation peroxidase, dat als antioxidant zeer belangrijk is.

Dit enzym kan de schade die ontstaat aan de celwanden door de schadelijke werking van zuurstof tegengaan. Ook bindt de selenium zware (giftige) metalen die in het lichaam terecht zijn gekomen en is daarom van belang bij het ontgiftigen van het lichaam zoals bijvoorbeeld bij een kwikvergiftiging (amalgaan vullingen). 

De bewijzen dat een tekort aan het mineraal selenium de belangrijkste risicofactor voor het krijgen van prostaatkanker is stapelen zich op. In een Amerikaans onderzoek werd het seleniumgehalte van de teennagel van 181 mannen met gevorderde prostaatkanker vergeleken met 181 soortgelijke mensen zonder deze ziekte. De mensen werden voor wat betreft het seleniumgehalte in 5 groepen verdeeld. Gecorrigeerd naar andere bekende risicofactoren zoals het vóórkomen in de familie, bleek dat de groep met de hoogste seleniuminname (geschat circa 160m g per dag) 65% minder risico hadden gelopen op het krijgen van gevorderde prostaatkanker dan mensen met de laagste inname (circa 86m g per dag). (The Lancet, 1998)

Rol selenium bij kwikvergiftiging

Wat met mensen gebeurt die door kwik zijn vergiftigd: het weinige
selenium dat in het voedsel zit wordt bij hen bovendien zeer slecht
opgenomen door de inadequate darmwerking. Het meeste selenium gaat het
lichaam direkt weer uit. Er ontstaat een structureel tekort aan
selenium, typerend voor kwikvergiftiging, waardoor de ontgifting via
taurine en methylering niet kan plaatsvinden. Het gevolg is ophopng van
gifstoffen (w.o. kwik) in het lichaam zowel als in de hersenen. 
Maar het kan zijn dat inderdaad tijdelijk door de werking van methionine
de hoeveelheid methylkwik in de hersenen toeneemt. Maar toch is selenium
een essentieel sporen-element waar kwikbelaste mensen een tekort aan
hebben en door het tekort kunnen evengoed psychische klachten ontstaan,
maar dit proces gaat zo langzaam dat men er niets van merkt.

Volledige tekst hier


Video - Selenium in Cancer Prevention

The Stein Institute for Research on Aging presents recent findings by Dr. Gerhard N. Schrauzer, UCSD Department of Chemistry & Biochemistry on Selenium as a possible tool in cancer prevention.

Selenium research center

The Selenium Nutritional Research Center (SeNRC) is involved with all nutritional and biochemical aspects of selenium research. Selenium is a dietary-essential trace element (nutritional mineral) that is critical to the body’s defense against cancer and other illnesses involving free radicals and other reactive oxygen species (ROS).

At SeNRC, we direct much of our efforts to studying the relationship of various forms of selenium compounds for the prevention of cancer. Our scientists hold patents on the use of selenium compounds and the prevention and treatment of cancer, as well as composition patents on specific selenium compounds and combinations of selenium with other nutrients and/or antioxidants. (See US 6,090,414 and EP 0 750 911 B1) Prevention is the ultimate cure. Our patents issued and allowed also cover the composition of virtually all current multivitamins now made, distributed, or sold in the U. S., whether they contain selenium or not.

SeNRC’s Research Director, Dr. Richard A. Passwater, has been involved with selenium research since 1959, when selenium was postulated to be a component of an unidentified food factor called "factor 3" in livestock feed that was thought to spare the need for vitamin E and possibly be involved in liver and muscle health of poultry and livestock.

Cancer Prevention

Dr. Passwater’s research with selenium as a cancer preventative began more than four decades ago, and over the decades, thousands of mechanistic studies, hundreds of animal studies, dozens of epidemiological (population) studies, and at least three published clinical supplementation trials have verified his findings.

On December 25, 1996, Dr. Larry Clark and his colleagues published their large, prospective, randomized, placebo-controlled, double-blind Nutritional Prevention of Cancer (NPC) clinical study in the Journal of the American Medical Association (JAMA 1996; 276:1957-1963). This landmark research effort showed that daily supplementation of diets with 200 micrograms of selenium yeast cut the cancer death rate in half. That is cancer mortality was reduced 50 percent (p=0.002). Lung cancer deaths were reduced 53% (p=0.03)

Total cancer incidence was reduced 37 percent (p=0.001) and the total carcinoma incidence was reduced 45%. In addition, the three leading sites of cancer had significantly lower incidence; lung cancer incidence was reduced 46 percent (p=0.04), prostate cancer incidence was reduced 63 percent (p=0.002) and colon cancer incidence was reduced 58 percent (p=0.03). There was a 17% reduction in all cause mortality (p=0.14), which when adjusted for sex, current smoking and age yielded a 21% reduction in deaths from all causes (p=0.07).

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Bioactivity of selenium from Brazil nut for cancer prevention and selenoenzyme maintenance.

Ip C, Lisk DJ.

Department of Surgical Oncology, Roswell Park Cancer Institute, Buffalo, NY 14263.

Brazil nut (Bertholletia excelsa) is one of very few consumable products with exceptionally high levels of selenium. The mean selenium concentrations of two shipments of Brazil nut used in the present study were determined to be 16 and 30 micrograms/g. In contrast, most common foods contain much less selenium, from 0.01 to 1 micrograms/g. Previous research on selenium cancer chemoprevention invariably used a pure compound, whereas little information is available on the efficacy of selenium delivered naturally in a food form. This paper reports the results of two mammary cancer prevention experiments in the rat dimethylbenz[a]anthracene model by continuous feeding of selenium-rich Brazil nut (processed to a smooth-textured nut material for mixing in the diet). A dose-dependent inhibitory response was observed at dietary selenium concentrations of 1-3 micrograms/g. Interestingly, Brazil nut was found to be just as powerful as sodium selenite, if not more so, at similar levels of dietary selenium intake. Mammary cancer protection gland, and plasma.

The magnitude of tissue selenium accumulation was proportional to the amount of Brazil nut added to the diet. The nutritional biopotency of selenium in Brazil nut was also evaluated by the repletion of two selenoenzymes, glutathione peroxidase and type I 5'-deiodinase, in selenium-deficient rats. Supplementation with Brazil nut as the sole source of selenium produced an efficient gradient of enzyme restoration at 0.05-0.2 microgram/g of dietary selenium. A parallel comparison with sodium selenite indicated that the selenium in Brazil nut and selenite selenium were equally bioactive. Although at this point it can only be inferred that the above biologic effects are likely to be attributable to the high selenium content of Brazil nut, there is persuasive evidence to suggest that the models under investigation are responding to the selenium rather than to the other components of Brazil nut.

Fast response to oxidative stress

It has been demonstrated that, when taken as a supplement, Selenium modulates the cellular response to oxidative stress, inducing a faster restoration of the endogenous antioxidative defense system against the production of reactive oxygen species (Jozanov-Stankov O, Demajo M, Djujic I, Mandic M: Selenium intake as a modulator of responsiveness to oxidative stress. J Environ Pathol Toxicol Oncol 1998, 17:251-7.

Selenium and cancer

Selenium is a mineral with anti-cancer properties. Many studies in the last several years have shown that selenium is a potent protective nutrient for some forms of cancer. The Arizona Cancer Center posted a selenium fact sheet listing the major functions of selenium in the body [97]. These functions are as follows:

1. Selenium is present in the active site of many enzymes, including thioredoxin reductase, which catalyze oxidation-reduction reactions. These reactions may encourage cancerous cells to under apoptosis.

2. Selenium is a component of the antioxidant enzyme glutathione peroxidase.

3. Selenium improved the immune systems' ability to respond to infections.

4. Selenium causes the formation of natural killer cells.

5. P450 enzymes in the liver may be induced by selenium, leading to detoxification of some carcinogenic molecules.

6. Selenium inhibits prostaglandins that cause inflammation.

7. Selenium enhances male fertility by increased sperm motility.

8. Selenium can decrease the rate of tumor growth.

A serendipitous randomized, double-blind, controlled trial of a 200 µg/day selenium supplement in the southeastern region of the USA (where soil selenium levels are low) found that the primary endpoints of skin cancer were not improved by the selenium supplement, but that other cancer incidence rates were decreased by selenium [98,99]. There was a significant reduction in total cancer incidence (105 vs 137 cases, P = 0.03), prostate cancer (22 vs 42 cases, P = 0.005), a marginally significant reduction in colorectal cancer incidence (9 vs 19 cases, P = 0.057), and a reduction in cancer mortality, all cancer sites (40 vs 66 deaths, P = 0.008) (selenium versus control group cases reported, respectively) [98]. The selenium supplement was most effective in ex-smokers and for those who began the study with the lowest levels of serum selenium. Several prospective studies have also examined the role of selenium in cancer prevention, particularly for prostate cancer, summarized in Table 2.

Overall, it appears that poor selenium levels, especially for men, are a cancer risk. If a person has low selenium levels and other antioxidant defenses are also low the cancer risk is increased even further. Women do not appear to be as sensitive to selenium, as breast cancer has not been found to be influenced by selenium status in several studies [100-104], although both men and women were found to be protected by higher levels of selenium from colon cancer [100] and lung cancer [105,106]. Good vegetarian sources of selenium are whole grains and legumes grown in selenium-rich soil in the western United States, brazil nuts (by far the most dense source of selenium), nutritional yeast, brewers yeast, and sunflower seeds.

Reduction of Cancer Risk by Consumption of Selenium-Enriched Plants: Enrichment of Broccoli with Selenium Increases the Anticarcinogenic Properties of Broccoli

Mar 2003, Vol. 6, No. 1: 19-26

John W. Finley, PhD
States Department of Agriculture, Agricultural Research Service, Grand Forks Human Nutrition Research Center, Grand Forks, ND, USA

Plant-based diets and phytochemicals present in plants are associated with decreased risk of cancer. Brassica species, and broccoli in particular, are associated with reduced risk of several important cancers. Selenium (Se) is an essential nutrient that is covalently bound in a number of different chemical forms found in plants. Broccoli accumulates Se many-fold beyond the concentration of Se in the soil, and the chemical form of Se in broccoli is similar to the chemical form in high-Se garlic, a food with unique chemoprotective properties.

Se from broccoli grown to accumulate more than 500 µg Se/g did not accumulate in rat tissues or increase glutathione peroxidase enzyme activity to the same extent as Se salts or seleno-amino acids. Se from high-Se broccoli decreased the incidence of aberrant crypts in rats with chemically induced colon cancer by more than 50%, compared with controls. Se from high-Se broccoli also decreased the incidence of mammary tumors in rats treated with 7,12-dimethylbenz(a)anthracene (DMBA) and tumor number and volume in APCmin mice.

These results suggest that development of methods to increase the natural accumulation of Se in broccoli may greatly enhance its health-promoting properties.

The Impact of Iron and Selenium Deficiencies on Iodine and Thyroid Metabolism: Biochemistry and Relevance to Public Health

Oct 2002, Vol. 12, No. 10: 867-878

Michael B. Zimmermann
Laboratory for Human Nutrition, Swiss Federal Institute of Technology, Zürich, Switzerland.
Josef Köhrle
Institut für Experimentelle EndoKrinologie, Charité, Humboldt-Universitaet zu Berlin, Germany.

Several minerals and trace elements are essential for normal thyroid hormone metabolism, e.g., iodine, iron, selenium, and zinc. Coexisting deficiencies of these elements can impair thyroid function. Iron deficiency impairs thyroid hormone synthesis by reducing activity of heme-dependent thyroid peroxidase. Iron-deficiency anemia blunts and iron supplementation improves the efficacy of iodine supplementation. Combined selenium and iodine deficiency leads to myxedematous cretinism. The normal thyroid gland retains high selenium concentrations even under conditions of inadequate selenium supply and expresses many of the known selenocysteine-containing proteins. Among these selenoproteins are the glutathione peroxidase, deiodinase, and thioredoxine reductase families of enzymes.

Adequate selenium nutrition supports efficient thyroid hormone synthesis and metabolism and protects the thyroid gland from damage by excessive iodide exposure. In regions of combined severe iodine and selenium deficiency, normalization of iodine supply is mandatory before initiation of selenium supplementation in order to prevent hypothyroidism. Selenium deficiency and disturbed thyroid hormone economy may develop under conditions of special dietary regimens such as long-term total parenteral nutrition, phenylketonuria diet, cystic fibrosis, or may be the result of imbalanced nutrition in children, elderly people, or sick patients.

Brazil nuts protect from breast cancer

The mineral selenium may help protect some women from developing breast cancer, research has suggested. The element, which can be found in brazil nuts, liver and kidneys, may help the body defend itself. Scientists from the University of Illinois believe they may have worked out how selenium interacts with a natural body chemical to offer protection.


Selenium - daily dose

Selenium is an essential component of at least 11 selenoenzymes or selenoproteins. There are two majorfamilies of selenoenzymes?glutathione peroxidases and deiodinases. The metabolic function of the glutathione peroxidases is to convert oxidized fat (lipid hydroperoxides), which is generated as the result of normal metabolism and contributes to heart disease and stroke, to less harmful compounds. This activity is similar to the antioxidant activity of vitamin E. The deiodinase enzymes regulate the metabolism of thyroid hormones. Interestingly, the recently discovered selenoenzyme thioredoxin reductase has been suggested to play a role in vitamin C metabolism. A human disease known to be caused by selenium deficiency and found in various regions of China is Keshan disease, a cardiomyopathy (disease of the heart muscle) in children.

Extensive data from studies in China have been used to establish the required and safe levels of selenium for humans. These studies revealed significant correlations between daily selenium intake and the selenium content of blood, breast milk, and urine. Significant correlations were also observed between urinary, plasma, hair, fingernail, and toenail selenium levels.

Fingernail brittleness and hair loss were used by the Chinese scientists, as well as the Panel, as the main criteria for chronic selenium toxicity, or selenosis, which occurs at an intake of about 5 mg (5,000 mcg) of selenium daily. Adverse effects were observed at daily dietary selenium intakes between about 600 and 1,600 mcg. The maximum safe dietary selenium intake was calculated to be about 800 mcg/day, but may be as low as 600 mcg in some individuals. The Panel set the UL for selenium at 400 mcg/day, which was selected to protect sensitive individuals. The Chinese scientists suggested a level of about 40 mcg daily as the minimum requirement, which is similar to the new RDA of 55 mcg/day. This RDA established by the Panel is based on the saturation of plasma glutathione peroxidase. An intake of less than 11 mcg daily of selenium will definitely put one at risk of deficiency.

Early epidemiological studies suggested a possible inverse relationship between selenium intake in humans and the incidence of certain cancers. More than 100 relevant experiments with animals exposed to various chemical and viral carcinogens have been carried out. The majority of these studies showed anticancer effects of selenium. Three human trials on selenium and cancer have been completed, and all of them showed positive results. In one trial, the addition of selenium to table salt significantly reduced the incidence of liver cancer in a Chinese population. After 5 years of supplementation with selenium, vitamin E, and beta-carotene, the incidence of stomach and esophageal cancer in another Chinese population was significantly reduced. However, it is not clear which supplement was mainly responsible for this effect. A study in the U.S. showed that 970 men supplemented with 200 mcg of selenium daily (as selenium-enriched yeast) for 4.5 years had a 63% reduction in the incidence of prostate cancer, as well as a significantly reduced incidence of colorectal, lung, and total cancers. These supplementation studies are consistent with a recent study showing one-half to two-thirds reduction in the risk of prostate cancer among men with the highest selenium status, as assessed by toenail levels of selenium that indicate long-term selenium intake. Overall, the evidence that selenium can lower the risk of prostate and possibly other human cancers was considered very promising by the Panel, but it concluded that there is currently no proof for an anticancer effect of selenium.

It is estimated that Americans consume about 100 mcg/day of dietary selenium. In the aforementioned prostate cancer study, subjects were given 200 mcg supplements daily, which boosted their estimated daily intake to about 300 mcg. To prevent selenium deficiency symptoms, a daily intake of 55 mcg is required. For maximal protection against certain cancers, a total daily intake of 200-300 mcg is probably necessary.

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