Copper functions along with iron in many metabolic processes. Copper is involved in the cytochrome oxidation system in tissue cells that produces energy. It is also a component of other oxidative systems for amino acid production. In addition, copper and iron are essential for the production of hemoglobin.
Like zinc, copper is a component of many important enzymes. Tyrosinase is a copper-containing enzyme that is used in the formation of melanin, the substance responsible for the color of hair, skin, and eyes. Lysyl oxidase is an enzyme used to crosslink collagen and elastin in the formation of connective tissue, including bone, blood vessels, skin, lungs, and teeth. Most plasma copper is bound to ceruloplasmin. Copper also binds to superoxide dismutase, which scavenges for superoxide free radicals and protects against oxidative damage.
Copper is absorbed in the small intestine and carried by the copper-binding transport protein metallothione through the plasma. In the plasma it is bound to amino acids, primarily histidine, and to serum albumin for transport to the liver. In the liver, copper is bound to an alpha-globulin to form ceruloplasmin. It is then delivered to cells that have ceruloplasmin receptors on their surface.
Copper is primarily excreted by bile into the gastrointestinal tract, although some unabsorbed copper is excreted through the feces. Biliary excretion is responsible for the body's balance of copper.
Copper deficiency is very rare. It has been documented in premature, low-birth- weight infants, in malnourished infants, and in patients who receive intravenous total parental nutrition for long periods of time.
There are, however, genetic disorders that affect copper metabolism. Wilson's disease causes a defect in the ability of copper to form ceruloplasmin. Impaired ability of the liver to excrete copper into the bile results in an overload of copper, which can damage the liver, brain, and kidney and lead to dementia and liver failure. Menkes' disease results in decreased copper absorption into the intestine and can lead to mental retardation, abnormal bone formation, and susceptibility to infection.
The estimated safe and adequate intake for copper in adults is 1.5-3 mg/day and ranges from 0.4-0.7 mg/day for infants and 0.7-2.5 mg/day for children.
The richest dietary sources of copper, containing greater than 0.2 mg per portion, includeshellfish, nuts, seeds, legumes, whole grain, liver, and other organ meats. Moderate sources, containing 0.1-0.2 mg per portion, are grain products, chocolate, dried fruits, mushrooms, tomatoes, bananas, and potatoes. Eggs, dairy products, white grains, green beans, broccoli, and cabbage contain lower amounts of copper.
Dosage recommendations are usually based on zinc intake. The recommended ratio of zinc to copper is 10:1. It has been suggested that supplementation should not exceed 3 mg of copper.
Excessive copper intake results in gastrointestinal pain, nausea, vomiting, and diarrhea. More serious effects include coma, oliguria, hepatic necrosis, and death. There is no well-established level of copper intake that causes toxicity, although gastrointestinal effects have been seen with intakes as low as 0.07 mg/kg of body weight.