The body contains 2 to 3 g of zinc (Zn), found mainly in bones, teeth, hair, skin, liver, muscle, leukocytes, and testes. One third of the 100 µg/dL (15.3 µmol/L) of zinc found in plasma is attached loosely to albumin, and about 2/3 is firmly bound to globulins. There are > 100 zinc metalloenzymes, including a large number of nicotinamide adenine dinucleotide (NADH) dehydrogenases, RNA and DNA polymerases, and DNA transcription factors as well as alkaline phosphatase, superoxide dismutase, and carbonic anhydrase. Dietary intake of zinc by healthy adults varies from 6 to 15 mg/day, and absorption is about 20%. Meat, liver, eggs, and seafood (especially oysters) are good sources. The RDA is 0.2 mg/kg/day for adults.

Deficiency: The signs and symptoms of zinc deficiency include anorexia, growth retardation, delayed sexual maturation, hypogonadism and hypospermia, alopecia, immune disorders, dermatitis, night blindness, impaired taste (hypogeusia), and impaired wound healing. The first signs of zinc deficiency in marginally nourished children are suboptimal growth, anorexia, and impaired taste. The most serious manifestations of zinc deficiency were reported in Iranian dwarfs. These adolescent boys, who consumed large amounts of clay, were retarded in growth and sexual development and had anemia, hypogonadism, hepatosplenomegaly, rough skin, and mental lethargy. After treatment with a well-balanced diet containing adequate amounts of zinc for 1 yr, pubic hair appeared, sexual organs increased in size, linear growth was resumed, and the skin became normal. The anemia responded to iron supplements. Zinc deficiency develops in some patients with cirrhosis because the ability to retain zinc is lost.

Biochemical signs associated with zinc deficiency include decreased levels of plasma zinc (< 70 µg/dL [< 10.7 µmol/L]), alkaline phosphatase, alcohol dehydrogenase in the retina (which accounts for night blindness), and plasma testosterone as well as impaired T-lymphocyte function, decreased collagen synthesis (resulting in poor wound healing), and decreased RNA polymerase activity in several tissues.

Clinical assessment of mild zinc deficiency is difficult because many of the signs and symptoms are nonspecific. Nonetheless, if a malnourished person has a borderline-low plasma zinc level, is subsisting on a high fiber and phytate diet containing whole-grain bread (which reduces zinc absorption), and has reduced taste sensitivity, an impaired lymphocyte response to mitogens, and reduced gonadal hormone function, then zinc deficiency should be suspected, and treatment with zinc supplements (15 to 25 mg/day) should be tried.

Maternal zinc deficiency may cause anencephaly in the fetus. Secondary deficiency occurs in liver disease, in malabsorption states, and during prolonged parenteral nutrition. Night blindness and mental lethargy may be features.

Acrodermatitis enteropathica--a rare autosomal recessive, once fatal disorder--results from malabsorption of zinc. The defect involves the failure to generate a transport protein that enables zinc to be absorbed in the intestine. Symptoms usually begin after an infant is weaned from breast milk. This disorder is characterized by psoriasiform dermatitis, hair loss, paronychia, growth retardation, and diarrhea. Zinc sulfate 30 to 150 mg/day orally results in complete remission.

Toxicity: Ingesting zinc in large amounts (200 to 800 mg/day), usually by consuming acidic food or drink from a galvanized container, can cause vomiting and diarrhea. Doses of zinc ranging from 100 to 150 mg/day interfere with copper metabolism and cause hypocupremia, RBC microcytosis, and neutropenia. Metal fume fever, also called brass-founders' ague or zinc shakes, is an industrial hazard caused by inhaling zinc oxide fumes; it results in neurologic damage.