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MINERALS

low selenium where, in fact, humans have been diagnosed with selenium deficiency—Keshan disease (cardiomyopa- thy) and Keshan–Beck disease (degenerative joint disease). Although other factors may be involved, selenium defi- ciency is clearly a predisposing factor. Selenium functions as part of several important enzymes. The most prominent is a soluble enzyme, glutathione peroxidase, whose func- tion is to reduce hydrogen peroxide and organic (lipid) per- oxides, thus preventing the oxidative destruction of cell membranes. Selenium is incorporated into the enzyme as the amino acid selenocysteine by reactions that are unique to selenium. Together with vitamin E, selenium, as a struc- tural component of glutathione peroxidase, forms an an- tioxidant defense against oxidative stress. The requirement for selenium has been estimated by various methods. On the basis of intakes in regions of China with and without deficiency disease, approximately 20 g/day is considered an adequate amount to prevent deficiency. The estimated safe and adequate selenium intake suggested by the U.S. National Research Council ranged from 50 to 200 g/day in 1980. An amount to maintain the highest serum glu- tathione peroxidase activity appears to be 70 and 55 g/day for an average man or woman, respectively, which became the Recommended Dietary Allowance (RDA) in 1989. In 1996, the World Health Organization recommended 40 and 30 g/day for men and women, respectively. Intakes greater that 400 g/day are considered to be the maximum safe level. Selenium is thus an example of a nutrient that possesses a relatively narrow range of intakes that are safe and that meet requirements.

Manganese

Normal body content of manganese is very low—ap- proximately 15 milligrams or very similar to iodine. In contrast to iodine, manganese deficiency has not been ob- served in humans but has occurred naturally in chickens and experimentally in many other species. Manganese is required by several enzymes, which may or may not be inolved in the symptoms of a manganese deficiency. Symptoms include impaired growth, skeletal abnormali- ties, and defects in lipid and carbohydrate metabolism. The role of manganese in the synthesis of the mu- copolysaccharide component of bone and cartilage is the most crucial whereas mineralization of bone appears to be independent of manganese. Excessive manganese will interfere with iron absorption. Under conditions of iron deficiency, manganese absorption is increased. Both iron and manganese appear to share a common site for ab- sorption. The recommendations for manganese intake are based on estimates of normal dietary intakes of 2 to 5 mg/day. This amount is thought to be sufficient to re- place the 50 percent of body manganese that is lost every 3 to 10 weeks.

Chromium

Chromium is one of the most intriguing and potentially important trace elements because it appears to influence

ENCYCLOPEDIA OF FOOD AND CULTURE

the action of a critical hormone, insulin. Unfortunately, the definitive role of chromium in this regard awaits fur- ther study. Decreased sensitivity of peripheral tissues to insulin appears to be the primary biochemical lesion in experimental chromium deficiency. Impaired glucose tolerance has been attributed to chromium deficiency in several experimental models. Also, several patients re- ceiving total parenteral nutrition have responded to chromium supplementation in the predicted manner, that is, improved glucose tolerance. These findings have established chromium as an essential nutrient for humans but the specific deficiency symptoms in those who re- ceive enteral feeding have not emerged. Overt chromium deficiency is very unlikely under normal conditions due to the small amounts of chromium needed. Moreover, a marginal deficiency is very difficult to identify due to the lack of reliable markers for diagnoses concerning chromium. Currently, there is little or no evidence that chromium supplements are either warranted or effective. Even the recommended intakes for adults (50 to 200 g/day) are uncertain due to the lack of reliable meth- ods for assessment.

Fluoride

Fluoride is not generally considered to be an essential el- ement for humans. It is, however, considered beneficial in that normal intakes appear to reduce the incidence of dental caries. The mechanism of this benefit is thought to be due to incorporation of fluoride into the mineral matrix of tooth enamel, thus producing a more resistant mineral apatite crystal. Over 99 percent of the fluoride found in the body is found in bones and teeth as a com- ponent of this mineral apatite crystal. An unusually high intake of fluoride causes permanently discolored or mot- tled teeth, a condition identified in children drinking wa- ter with 2 to 3 parts of fluoride per million. The level of fluoride commonly maintained in municipal water sup- plies is 1 part per million.

Silicon and Nickel

Silicon is the most abundant mineral in the Earth’s crust. It is thus surprising that a need for silicon in biological systems has not been more prominent. Limited research conducted since 1974 has indicated a role for silicon in the development of mature bones in chickens and rats. A human requirement has not been established but esti- mates in the range of 10 to 20 mg/day have been sug- gested. Most likely intakes of this magnitude occur under normal conditions. Nickel deficiency has been experi- mentally produced in several species. Growth depression and changes in iron metabolism have been described. Nickel has been discovered in the enzyme urease from bacteria, fungi, yeasts, algae, plants, and invertebrates. Many other enzymes exist for which nickel is apparently a component. Thus, it is likely that nickel plays an es- sential functional role in higher organisms, including hu- mans.

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