Trace mineral deficiency

Selenium Selenium, in the form of selenocysteine, is a component of the enzyme glutathione peroxidase, which serves to protect proteins, cell membranes, lipids, and nucleic acids from oxidant molecules. As such, selenium is being actively studied as a chemopreventive agent against certain cancers, such as prostate. Selenocysteine is also found in the deiodinase enzymes, which mediate the deiodination of thyroxine to triiodothyronine (Chap. 335).

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Zinc Zinc is an integral component of many metalloenzymes in the body; it is involved in the synthesis and stabilization of proteins, DNA, and RNA and plays a structural role in ribosomes and membranes. Zinc is necessary for the binding of steroid hormone receptors and several other transcription factors to DNA. Zinc is absolutely required for normal spermatogenesis, fetal growth, and embryonic development.

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Vitamin E Deficiency: Treatment Symptomatic vitamin E deficiency should be treated with 800–1200 mg of αtocopherol per day. Patients with abetalipoproteinemia may need as much as 5000–7000 mg/d. Children with symptomatic vitamin E deficiency should be treated with 400 mg/d orally of water-miscible esters; alternatively, 2 mg/kg per d may be administered intramuscularly. Vitamin E in high doses may protect against oxygen-induced retrolental fibroplasia and bronchopulmonary dysplasia, as well as intraventricular hemorrhage of prematurity.

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Toxicity Acute toxicity of vitamin A was first noted in Arctic explorers who ate polar bear liver and has also been seen after administration of 150 mg in adults or 100 mg in children. Acute toxicity is manifested by increased intracranial pressure, vertigo, diplopia, bulging fontanels in children, seizures, and exfoliative dermatitis; it may result in death. In children being treated for vitamin A deficiency according to the protocols outlined above, transient bulging of fontanels occurs in 2% of infants, and transient nausea, vomiting, and headache occur in 5% of preschoolers.

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Dietary Sources The retinol activity equivalent (RAE) is used to express the vitamin A value of food. One RAE is defined as 1 µg of retinol (0.003491 mmol), 12 µg of βcarotene, and 24 µg of other provitamin A carotenoids. In older literature, vitamin A was often expressed in international units (IU), with 1 RAE being equal to 3.33 IU of retinol and 20 IU of β-carotene, but these units are no longer in current scientific use. Liver, fish, and eggs are excellent food sources for preformed vitamin A; vegetable sources of provitamin A carotenoids include dark green and deeply colored...

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Flavonoids Flavonoids constitute a large family of polyphenols that contribute to the aroma, taste, and color of fruits and vegetables. Major groups of dietary flavonoids include anthocyanidins in berries; catechins in green tea and chocolate; flavonols (e.g., quercitin) in broccoli, kale, leeks, onion, and the skins of grapes and apples; and isoflavones (e.g., genistein) in legumes. Isoflavones have a low bioavailability and are partially metabolized by the intestinal flora.

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Biotin Biotin is a water-soluble vitamin that plays a role in gene expression, gluconeogenesis, and fatty acid synthesis and serves as a CO 2 carrier on the surface of both cytosolic and mitochondrial carboxylase enzymes. The vitamin also functions in the catabolism of specific amino acids (e.g., leucine). Excellent food sources of biotin include organ meat such as liver or kidney, soy, beans, yeast, and egg yolks; however, egg white contains the protein avidin, which strongly binds the vitamin and reduces its bioavailability.

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Toxicity The safe upper limit for vitamin B6 has been set at 100 mg/d, although no adverse effects have been associated with high intakes of vitamin B 6 from food sources only. When toxicity occurs, it causes a severe sensory neuropathy, leaving patients unable to walk. Some cases of photosensitivity and dermatitis have also been reported. Folate, Vitamin B12 See Chap. 90. Vitamin C Both ascorbic acid and its oxidized product dehydroascorbic acid are biologically active.

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Pellagra: Treatment Treatment of pellagra consists of oral supplementation of 100–200 mg of nicotinamide or nicotinic acid three times daily for 5 days. High doses of nicotinic acid (2 g/d in a time-release form) are used for the treatment of elevated cholesterol and triglyceride levels and/or low high-density lipoprotein (HDL) cholesterol level (Chap. 350). Toxicity Prostaglandin-mediated flushing due to binding of the vitamin to a G protein–coupled receptor has been observed at daily doses as low as 50 mg of niacin when taken as a supplement or as therapy for dyslipidemia.

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Thiamine (Vitamin B1) Thiamine was the first B vitamin to be identified and is therefore also referred to as vitamin B1. Thiamine functions in the decarboxylation of αketoacids, such as pyruvate α-ketoglutarate, and branched-chain amino acids and thus is a source of energy generation. In addition, thiamine pyrophosphate acts as a coenzyme for a transketolase reaction that mediates the conversion of hexose and pentose phosphates. It has also been postulated that thiamine plays a role in peripheral nerve conduction, although the exact chemical reactions underlying this function are unknown.

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Thiamine Deficiency: Treatment In acute thiamine deficiency with either cardiovascular or neurologic signs, 100 mg/d of thiamine should be given parenterally for 7 days, followed by 10 mg/d orally until there is complete recovery. Cardiovascular improvement occurs within 24 h, and ophthalmoplegic improvement occurs within 24 h. Other manifestations gradually clear, although psychosis in Wernicke-Korsakoff syndrome may be permanent or persist for several months.

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Table 71-2 Deficiencies and Toxicities of Metals Element Deficiency Toxicity Tolerable Upper (Dietary) Intake Level Boron No biologic Developmental defects, male 20 mg/d function determined sterility, (extrapolated from data) animal testicular atrophy Calcium Reduced bone Renal insufficiency (milk-alkalai syndrome), mg/d 2500 (milk- Element Deficiency Toxicity Tolerable Upper (Dietary) Intake Level mass, osteoporosis nephrolithiasis, iron absorption impaired alkalai) Copper Anemia, growth retardation, keratinization Nausea, vomiting, 10 mg/d defective dia...

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Harrison's Internal Medicine Chapter 71. Vitamin and Trace Mineral Deficiency and Excess Vitamin and Trace Mineral Deficiency and Excess: Introduction Vitamins and trace minerals are required constituents of the human diet since they are either inadequately synthesized or not synthesized in the human body. Only small amounts of these substances are needed for carrying out essential biochemical reactions (e.g., acting as coenzymes or prosthetic groups).

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