Examples of the large group of trace elements are: zinc, iodine, selenium, copper, manganese, fluorine, chromium, and molybdenum. Trace elements are often co-factors of enzymes, and are therefore essential nutrients. The range between the dose necessary for good health and the toxic dose is relatively small for a number of trace elements (see Table 12.5.1). The trace elements that will be discussed here have an intermediate safety margin (10-33): zinc, copper, selenium, and fluorine.
Zinc is a co-factor of a variety of enzymes mediating metabolic pathways, such as alcohol dehydrogenation, lactic dehydrogenation, superoxide dismutation, and alkaline phosphorylation. It occurs especially in meat, (whole) grains and legumes. The RDA for zinc is 12 to 15 mg, depending on the age, while the zinc intake is about 10 mg/day.
Acute toxicity, including gastro-intestinal irritation and vomiting, has been observed following the ingestion of 2 g or more of zinc in the form of sulfate. Effects of relatively low intakes are of greater concern. After dietary intakes of 18.5 or 25 mg by volunteers, impairment of the copper state has been observed. Further, daily intake of 80 to 150 mg during several weeks caused a decrease in the high-density lipoprotein serum level.
Zinc intakes of 20 times the RDA for 6 weeks led to impairment of the immune system, and intakes of 10 to 30 times the RDA for several months led to hypercupremia, microcytosis, and neutropenia. For these reasons, chronic ingestion of zinc exceeding 15 mg/day is not recommended.
Copper is also incorporated in a number of enzymes including cytochrome oxidase and dopamine hydroxylase. It is found in green vegetables, fish, and liver. The copper intake varies from 1.5 to 3.0 mg/day for adults. This is also the RDA.
In general, toxicity from dietary sources is extremely rare. Liver cirrhosis and disturbances of brain functions (e.g., coarse tremor and personality change) have been reported. No adverse effects are to be expected from intakes of up to 35 mg/day for adults. Storing or processing acidic foods or beverages in copper vessels can add to the daily intake and cause toxicity from time to time.
Selenium can be of plant as well as of animal origin. It occurs in seafoods, kidneys, liver, and various types of seeds, e.g., grains. The level in plants depends on the selenium content of the soil in which the plants are growing. Selenium plays an important role in (lipid) peroxide detoxication. The detoxication is catalyzed by a selenium-containing enzyme, glutathione peroxidase.
The daily intake of selenium varies from 80 to 130 |ig. The RDA is set at 150 |ig. The toxic dose is about 30 times the RDA. Acute intoxication has been reported after ingestion of about 30 mg. Symptoms were nausea, abdominal pain, diarrhea, nail and hair changes, peripheral neuropathy, fatigue, and irritability. Chronic dietary intake of approximately 5 mg/day has been found to result in fingernail changes and hair loss (selenosis). In the seleniferous zone of China, a daily dietary intake of 1 mg of sodium selenite for more than 2 years resulted in thickened but fragile nails and garlic-like odor of dermal excretions.
Fluoride is present in low but varying concentrations in drinking water (1 mg/l), plants (e.g., tea), and animals (fish, 50 to 100 mg per 100 g). It accumulates in human bone tissue and dental enamel. Its beneficial effects on dental health have clearly been demonstrated.
Fluoride is toxic, if consumed in excessive amounts. The normal daily intake is 1 to 2 mg. Daily ingestion of 20 to 80 mg of fluoride leads to fluorosis. This is characterized by calcification resulting in effects on kidney function, and possibly muscle and nerve function. A single intake of 5 to 10 g of sodium fluoride by a 70 kg adult has been reported to cause death. Fluoride intakes above the level of 10 mg per day are not recommended for adults.
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