Zinc deficiency is defined either as insufficient body levels of zinc to meet the needs of the body, or as a zinc blood level below the normal range. However, since a decrease in blood concentration is detectable only after long-term or severe depletion, blood levels of zinc are not a reliable biomarker for zinc status. Common symptoms include increased rates of diarrhea. Zinc deficiency affects the skin and gastrointestinal tract; brain and central nervous system, immune, skeletal, and reproductive systems. Zinc deficiency in humans is caused by reduced dietary intake, inadequate absorption, increased loss, or increased body system use. The most common cause is reduced dietary intake. In the U.S., the Recommended Dietary Allowance (RDA) is 8 mg/day for women and 11 mg/day for men. The highest concentration of dietary zinc is found in oysters, meat, beans, and nuts. Increasing the amount of zinc in the soil and thus in crops and animals is an effective preventive measure. Zinc deficiency may affect up to 17% or 2 billion people worldwide.
Signs and symptoms
Skin, nails, and hair Zinc deficiency may manifest as acne, eczema, xerosis (dry, scaling skin), seborrheic dermatitis, or alopecia (thin and sparse hair). It may also impair or possibly prevent wound healing.
Mouth Zinc deficiency can manifest as non-specific oral ulceration, stomatitis, or white tongue coating. Rarely it can cause angular cheilitis (sores at the corners of the mouth).
Vision, smell, and taste Severe zinc deficiency may disturb the sense of smell and taste. Night-blindness may be a feature of severe zinc deficiency, although most reports of night-blindness and abnormal dark adaptation in humans with zinc deficiency have occurred in combination with other nutritional deficiencies (e.g., vitamin A).
Immune system Impaired immune function in people with zinc deficiency can lead to the development of respiratory, gastrointestinal, or other infections, e.g., pneumonia. The levels of inflammatory cytokines (e.g., IL-1β, IL-2, IL-6, and TNF-α) in blood plasma are affected by zinc deficiency and zinc supplementation produces a dose-dependent response in the level of these cytokines. During inflammation, there is an increased cellular demand for zinc, and impaired zinc homeostasis from zinc deficiency is associated with chronic inflammation.
Diarrhea Zinc deficiency contributes to an increased incidence and severity of diarrhea.
Appetite Zinc deficiency may lead to loss of appetite.
Cognitive function and hedonic tone Cognitive functions, such as learning and hedonic tone, are impaired with zinc deficiency. Moderate and more severe zinc deficiencies are associated with behavioral abnormalities, such as irritability, lethargy, and depression (e.g., involving anhedonia). Zinc supplementation produces a rapid and dramatic improvement in hedonic tone (i.e., general level of happiness or pleasure) under these circumstances. Zinc supplementation has been reported to improve symptoms of ADHD and depression.
Psychological disorders Low plasma zinc levels have been alleged to be associated with many psychological disorders. Schizophrenia has been linked to decreased brain zinc levels. Evidence suggests that zinc deficiency could play a role in depression. Zinc supplementation may be an effective treatment in major depression.
Growth Zinc deficiency in children can cause delayed growth and has been claimed to cause stunted growth in one-third of the world's population.
During pregnancy Zinc deficiency during pregnancy can negatively affect both the mother and fetus. Animal studies indicate that maternal zinc deficiency can upset the sequencing and efficiency of the birth process. An increased incidence of difficult and prolonged labor, hemorrhage, uterine dystocia, and placental abruption has been documented in zinc-deficient animals. The defective functioning of estrogen may mediate these effects via the estrogen receptor, which contains a zinc finger protein. A review of pregnancy outcomes in women with acrodermatitis enteropathica, reported that out of every seven pregnancies, there was one abortion and two malfunctions, suggesting the human fetus is also susceptible to the teratogenic effects of severe zinc deficiency. However, a review of zinc supplementation trials during pregnancy did not report a significant effect of zinc supplementation on neonatal survival. Zinc deficiency can interfere with many metabolic processes during infancy and childhood, a time of rapid growth and development when nutritional needs are high. Low maternal zinc status has been associated with less attention during the neonatal period and worse motor functioning. In some studies, supplementation has been associated with motor development in very low birth weight infants and more vigorous and functional activity in infants and toddlers.
Testosterone production Zinc is required to produce testosterone. Thus, zinc deficiency can lead to reduced circulating testosterone, which could lead to sexual immaturity, hypogonadism, and delayed puberty.
Causes
Dietary deficiency Zinc deficiency can be caused by a diet high in phytate-containing whole grains, foods grown in zinc-deficient soil, or processed foods containing little or no zinc. Conservative estimates suggest that 25% of the world's population is at risk of zinc deficiency. In the U.S., the Recommended Dietary Allowance (RDA) is 8 mg/day for women and 11 mg/day for men. RDA for pregnancy is 11 mg/day. RDA for lactation is 12 mg/day. For infants up to 12 months, the RDA is 3 mg/day. For children ages 1–13 years the RDA increases with age from 3 to 8 mg/day. The following table summarizes most of the foods with significant quantities of zinc, listed in order of quantity per serving, unfortified. Note that the top 10 entries are meat, beans, or nuts.
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