Monday 23 March 2009
A lack of zinc is unusual because it is reasonably abundant in meats, fish, shellfish, whole-grain cereals, and legumes. Most cases of zinc deficiency have been related to either total parenteral nutrition unsupplemented by zinc or the aforementioned rare genetic syndrome that interferes with absorption.
Zinc deficiency should be suspected in any case of obscure growth retardation or infertility associated with a distinctive rash (acrodermatitis enteropathica). Oral zinc supplementation is promptly curative.
Zinc deficiency is usually due to insufficient dietary intake, but can be associated with malabsorption, acrodermatitis enteropathica, chronic liver disease, chronic renal disease, sickle cell disease, diabetes, malignancy, and other chronic illnesses.
The essential features of zinc deficiency are:
(1) a distinctive rash, most often around the eyes, nose, mouth, anus, and distal parts, called acrodermatitis enteropathica;
(2) anorexia, often accompanied by diarrhea;
(3)growth retardation in children;
(4) impaired wound healing;
(5) hypogonadism with diminished reproductive capacity;
(6) altered immune function;
(7) impaired night vision related to altered vitamin A metabolism;
(8) depressed mental function;
(9) an increased incidence of congenital malformations in infants of zinc-deficient mothers.
Symptoms of mild zinc deficiency are diverse. Clinical outcomes include depressed growth, diarrhea, impotence and delayed sexual maturation, alopecia, eye and skin lesions, impaired appetite, altered cognition, host defense properties, defects in carbohydrate utilization, and reproductive teratogenesis.
Mild zinc deficiency depresses immunity, although so does excessive zinc
Animals with a diet deficient in zinc require twice as much food to attain the same weight gain as animals given sufficient zinc.
Groups at risk for zinc deficiency include the elderly, vegetarians, and those with renal insufficiency. There is a paucity of adequate zinc biomarkers, and the most widely used indicator, plasma zinc, has poor sensitivity and specificity. Diagnosing zinc deficiency is a persistent challenge.
Nearly 2 billion people in the developing world are deficient in zinc. In children it causes an increase in infection and diarrhea, contributing to the death of about 800,000 children worldwide per year.
The World Health Organization advocates zinc supplementation for severe malnutrition and diarrhea. Zinc supplements help prevent disease and reduce mortality, especially among children with low birth weight or stunted growth.
However, zinc supplements should not be administered alone, since many in the developing world have several deficiencies, and zinc interacts with other micronutrients.
Zinc deficiency is plants’ most common crop micronutrient deficiency; it is particularly common in high-pH soils. Zinc-deficient soil is cultivated in the cropland of about half of Turkey and India, a third of China, and most of Western Australia, and substantial responses to zinc fertilization have been reported in these areas.
Plants that grow in soils that are zinc-deficient are more susceptible to disease. Zinc is primarily added to the soil through the weathering of rocks, but humans have added zinc through fossil fuel combustion, mine waste, phosphate fertilizers, limestone, manure, sewage sludge, and particles from galvanized surfaces. Excess zinc is toxic to plants, although zinc toxicity is far less widespread.