Use of Dietary Supplements to Change Selenium Status

cot death in New Zealand was related to low selenium intake have not received wide support from experts in the field (Dolamore et al., 1992), there are indica-tions that the high incidence of asthma, particularly among children, may have a selenium connection (Flatt et al., 1990).

As a result of such findings, there has been continuing pressure from some members of the health professions, as well as from sections of the public, for a change in official attitudes towards selenium supplementation and for introduc-tion of legislaintroduc-tion similar to that in Finland requiring addiintroduc-tion of selenate to fer-tilizers. However, an unexpected improvement in the selenium status in recent years may have removed the need for official intervention. In 1993 blood sele-nium levels of New Zealanders were found to be 94 µg/l, compared to around the 61 µg/l recorded 2 decades previously (Thomson and Robinson, 1996). The improvement is believed to have been largely due to deregulation of importation of wheat from Australia and the USA as well as the availability of a variety of imported selenium-rich breakfast cereals (Winterbourne et al., 1992). Other dietary changes, such as an increase in consumption of fish, may also have played a part in the improvement (Public Health Commission, 1993).

10.5 Use of Dietary Supplements to Change Selenium Status

Many people today consume selenium supplements on a regular basis to increase their intake and improve their nutritional status. They do this in the belief either that selenium levels in the diet are inadequate or that the additional intake will pro-vide protection against a variety of health problems. It is widely believed also that selenium plays a protective role against oxidative damage caused by environmen-tal pollutants, ultraviolet radiation, and other hazards of modern living. Casey (1988), in a somewhat critical review, gave the name selenophilia (or love of sele-nium) to what she described as “the current strong interest in the use of selenium, over and above normal and apparently adequate levels of dietary intake, for the prevention, alleviation or cure of a variety of disorders that have not been shown to be directly associated with selenium.” However, while some of the expectations

of supplement users can be attributed to uncritical reporting of medical matters in the popular media, others are not without support from well recognized and seri-ous health professionals. Indeed much of today’s interest in selenium as a supple-ment was triggered by the report of Clark and his highly professional team, published in 1996 in very highly regarded Journal of the American Medical Association (Clark et al., 1996). The group’s finding that selenium supplementa-tion, at levels above normally recommended intakes, caused a reduction in risk of certain forms of cancer caused a flurry of interest, in the scientific as well as the general media. The report led many health professionals, as well as nonmedical members of the general public, to accept that selenium has antitumorigenic prop-erties (Combs, 2001). It also contributed to a widescale practice of self-medica-tion, sometimes with high doses of selenium, as a preventive against cancer (Reilly, 1997). According to one recent review (Veatch et al., 2005), the increased use of these products in the USA and elsewhere is the direct result of epidemio-logical studies that indicate that protection is best provided by a “supranutritional”

intake of selenium several times in excess of the nutritional requirement.

The use of dietary supplements in many countries is considerable and appears to be increasing. In the USA more than 62% of respondents to the National Health Survey in 2002 said that they had used a supplement in the previous year. In 1987 the figure had been 23.7% (National Center for Health Statistics, 2005). The NHANES III Survey of 1998–1994 found that 9% of all adults used supplements containing selenium (Institute of Medicine, 2000). Equivalent uses of selenium and other supplements are reported in other countries of the developed world.

Many different types of selenium supplements, with considerable variations in concentrations, are available both as “over-the-counter” (OTC) and medically prescribed products. They include inorganic forms, such as sodium selenite and selenate, defined organic forms, including selenoamino acids, principally seleno-methionine and selenocysteine, and more complex organic forms found in selenium-enriched yeast and other foods. The inorganic forms and the selenoamino acids are well-defined chemical entities with known physicochemi-cal characteristics. The selenoamino acids are available as racemates or as either

D- or ^L-isomeric forms (Nève, 1995).

These products are available, normally in tablet form, in quantities up to 200 µg, and sometimes more, per tablet. Though these amounts exceed the recom-mended daily intakes in the USA and many other countries, a dosage of 200 µg is generally considered safe and adequate for an adult of average weight subsist-ing on the typical American diet accordsubsist-ing to Schrauzer (2001).

10.5.1 Selenium-Enriched Yeast

There is some uncertainty about the composition of selenium-enriched yeast.

This is usually prepared by growing brewers’ or baker’s yeast (Saccharomyces cerevisiae) in a selenium-rich nutrient medium under conditions of sulfur limita-tion. This encourages the uptake of selenium to form seleno-analogs of organic compounds of sulfur. The yeast is usually prepared for pharmacological use by

isolation and washing, followed by lyophilization, and is then made into tablets (Power, 1995).

Many different commercial preparations of selenium yeast are available. Their chemical composition varies, depending on the different culture conditions used.

The preparations can contain different amounts of different selenium compounds, including selenoamino acids, selenoproteins, selenosulfides, and inorganic forms of the element (Korhola et al., 1986). Levels of organically bound selenium have been reported to range, in some commercial preparations of selenium yeast, from 0 to 97% of the total selenium content (Uden et al., 2003). Rayman (2004) found that selenomethione accounted for between 60 and 84% of selenium species in ten different kinds of commercial selenium yeasts. Other species in these yeasts included Se-cystine, Se-cysteine, Se-methyl-Se-cysteine, and Se-cystathione, all at less than 1%. Though a few selenium yeasts contain predominantly selenite or selenate (Schrauzer and McGinness, 1979), the majority of commercial prepara-tions contain mainly selenomethionine (Rayman, 2004).

In 2002 the European Parliament and Council of the EU issued a directive on permitted food supplements. This specified a “positive list” of approved supple-ments that included inorganic forms of selenium, but not selenium yeast. This means that once the directive is put into effect, the sale of selenium yeast will no longer be permitted in the EU (Rayman 2004). The decision was based on the opinion of the Scientific Committee on Food of the European Commission (EC) that selenium yeast supplements were poorly characterized and that there was danger that selenium from the selenomethionine in the yeast could build up to toxic levels in body tissues.

10.5.2 Selenomethionine in Selenium Yeast

The reasoning behind the directive has been challenged by Rayman in a well-reasoned review. She points to the wide use of selenium yeast, both in carefully monitored clinical trials and as a dietary supplement. She produces evidence to show that, when manufactured by reputable manufacturers, selenium yeast is of reproducible quality and defined selenomethionine content. Moreover there is no evidence of toxicity from selenomethionine even at levels far above the EC tolerable upper limit (UL) of 300 µg/day. This is also the view of Schrauzer (2000) who believes that concern at the possibility that incorporation of selenomethionine into body proteins could increase selenium to toxic levels, is not warranted because a steady state is established, which prevents uncontrolled accumulation of the element. Moreover the release of selenomethionine from body proteins could not result in selenium toxicity since no mechanism for the selective release of selenomethionine during catabolism exists. Indeed, he argues that selenomethionine, or enriched food sources of it, are appropriate forms of selenium for human nutritional supplementation. He believes that, since higher animals cannot synthesize the amino acid, yet from it all needed forms of sele-nium are produced, selenomethionine meets the criteria for an essential amino acid (Schrauzer, 2003).

10.5.3 Variations in Levels of Selenium in Supplements

The concern expressed by the EC Scientific Committee at the absence of infor-mation about the composition of selenium yeast and its poor characterization, has been echoed by others. Combs (2001) noted that published composition data, especially relating to different selenium species in yeast, were very limited. At the time he wrote he could only find information on a single commercial selenium yeast product. He commented that in the absence of compositional information, and with no published standards of product identity for selenium-enriched yeast, it was not clear whether the data he had found described general characteristics of selenium-enriched yeasts or merely specific traits of that particular product.

Consumer acceptance, he noted, called for inclusion on labels of information about the selenium content of the product, as well as for the establishment of quality control procedures. These were necessary to minimize risk of selenium overexposure and to ensure delivery of known forms of the element.

The absence of regulations on the purity and potency of supplements, and, in the case of selenium supplements, the hazard associated with the absence of good manufacturing practices has been noted by Veatch et al. (2005). They believe that their neglect accounts for the unacceptable wide variations in levels of selenium sometimes observed even in the same batch of some commercial products.

Among the examples they cite is a value of 27.3 mg selenium per tablet in a sin-gle manufacturing lot, a value 182 times higher than stated on the label. They believe that this is not an isolated case, and could account for the 13 cases of sele-nium toxicity from consumption of OTC selesele-nium supplements reported in the USA in 1 year. Their own investigation, which examined levels in 15 commercial products, found differences between their analytical data and that stated on the labels ranging from −13.4 to + 19.5%, with individual tablets ranging from 0.78 to 1.6 times the stated dose. While these figures, according to the authors, do indi-cate that, compared to earlier reports, the accuracy of selenium supplements has improved over recent years, they still give cause for concern. They point out that one popular multivitamin, labeled at 200 µg/tablet, contained selenium in excess of 300 µg and that many of its users could well exceed the 400 µg/day tolerable UL of intake.