Micronutrient Mineral Deficiencies

Chapter 51 Micronutrient Mineral Deficiencies

Micronutrients include vitamins ( Chapters 45–50) and trace elements. By definition, a trace element is <0.01% of the body weight. Trace elements have a variety of essential functions (see Table 51-1 on the Nelson Textbook of Pediatrics website at www.expertconsult.com). With the exception of iron deficiency, trace element deficiency (see Table 51-1) is uncommon in developed countries, but some deficiencies (iodine, zinc, selenium) are important public health problems in a number of developing countries. Because of low nutritional requirements and plentiful supply, deficiencies of some of the trace elements are extremely rare in humans and typically occur in patients receiving unusual diets or prolonged total parenteral nutrition without adequate delivery of a specific trace element. They can also occur in children with short bowel or malabsorption. Excess intake of trace elements (see Table 51-1) is uncommon, but it can result from environmental exposure or overuse of supplements.

For a number of reasons, children are especially susceptible to trace element deficiency. First, growth creates an increased demand for most trace elements. Second, some organs are more likely to sustain permanent damage due to trace element deficiency during childhood. The developing brain is particularly vulnerable to the consequences of certain deficiency states (iron, iodide). Similarly, adequate fluoride is most critical for dental health during childhood. Third, children, especially in the developing world, are more prone to gastrointestinal disorders that can cause trace element deficiencies due to malabsorption.

A normal diet provides adequate intake of most trace elements. However, the intake of certain trace elements varies significantly in different geographic locations. Iodide-containing food is plentiful near the ocean, but inland areas often have inadequate sources, leading to goiter and hypothyroidism. Iodine deficiency is not a problem in the USA because of the widespread use of iodized salt; however, symptomatic iodine deficiency (goiter and hypothyroidism) is common in many developing countries. Selenium content of the soil and consequently of food is also quite variable. Dietary selenium deficiency (associated with cardiomyopathy) occurs in certain locations, such as some parts of China.

The consequences of severe isolated trace mineral deficiency are illustrated in certain genetic disorders. The manifestations of Menkes disease (Chapter 592) are due to a mutation in the gene coding for a protein that facilitates intestinal copper absorption. This mutation results in severe copper deficiency; subcutaneous copper is an effective treatment. The recessive disorder acrodermatitis enteropathica (Chapter 663) is secondary to malabsorption of zinc. These patients respond dramatically to zinc supplementation.

Children can have apparently asymptomatic deficiencies of certain trace elements but still benefit from supplementation. As an example, zinc is highly effective in treating children before or during diarrheal illnesses in the developing world.

Zinc deficiency is quite common in the developing world and is often associated with malnutrition or other micronutrient deficiencies (iron). Chronic zinc deficiency is associated with dwarfism, hypogonadism, dermatitis, and T-cell immunodeficiency. Diets rich in phytates bind zinc, impairing its absorption. Zinc supplementation in at-risk children reduces the incidence and severity of diarrhea, pneumonia, and possibly malaria. In developing countries, children who have diarrhea and who have been treated with zinc (20 mg/day orally for 14 days) have improved morbidity and mortality rates.

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