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Zinc is a trace element considered a micronutrient, meaning a nutrient needed in very small amounts. It is found in almost every living cell. The significance of zinc in human nutrition and public health was recognized relatively recently (1961) and it is now considered to have a wide range of essential biological roles in maintaining life and health.
Zinc is considered essential to maintain health. It is required for the activity of numerous metalloen-zymes involved in metabolism, it maintains the immune system that protects the body against disease, and also supports normal growth and development during pregnancy, childhood, and adolescence. It plays three crucial roles:
- Catalytic role: Enzymes are proteins that are vitally important for speeding up the biochemical reactions (catalysis) of cells and organisms and nearly 200 different ones depend on zinc. Zinc-dependent enzymes can be found in all known classes of enzymes.
- Structural role: Zinc also maintains the structure of proteins and cell membranes. A finger-like structure, called a zinc finger motif, strengthens the structure of several important proteins and enzymes. For instance, that of the antioxidant copper-zinc superoxide dismutase enzyme. Copper is required for the catalytic activity of the enzyme, but zinc plays a critical structural role. Zinc also affects the structure and function of cell membranes, which become more likely to be damaged by harmful oxidative species (oxidative stress) with zinc loss.
- Regulatory role: Zinc finger proteins are also involved in the regulation of gene expression by binding to DNA and influencing the copying of specific genes. Zinc also plays a role in the regulation of cell signaling
(Illustration by GGS Information Services/Thomson Gale.)
and influences the release of hormones and the transmission of nerve impulses.
Additionally, zinc has the following functions:
- It is required for vision, taste, and smell.
- It maintains healthy a healthy connective tissue in skin.
- It helps tissue repair after burns and wound healing.
- It is needed for bone growth.
- It promotes the production of healthy white blood cells and antibodies, important components of the body's immune system.
- It is involved in the metabolism of carbohydrates, proteins and phosphorus.
- It is involved in the production of insulin in the pancreas.
Recent research reports indicate that zinc has been found to play a role in cell death (apoptosis) with implications for growth and development, as well as a number of chronic diseases. Zinc is also actively taken up by synaptic vesicles that store the neurotransmitters released by nerve cells, suggesting a new role in neuronal activity and memory.
Zinc is found in the body in a form bound to proteins within cells, especially in the nucleus, and cell membranes. The adult body contains about 1.5-2.5 g of zinc bound to various proteins. They occur in specialized areas of the brain that produce the chemical substances that can send messages from one nerve cell to another (neurotransmitters). Zinc is also found in the pancreas, adrenal gland, bones, liver, prostate and in the reproductive organs. Most of the zinc (75-88%) in blood is found in a red blood cell metalloenzyme called carbonic anhydrase. In the plasma, zinc is bound to proteins such as alpha-2-macroglobulin, albumin, transferrin and ceruloplasmin.
Zinc is found in a wide variety of foods. Oysters are the richest zinc source per serving, but since they are not consumed regularly in the American diet, red meat and poultry provide the majority of dietary zinc. Other good zinc sources include beans, nuts, certain seafood, whole grains, fortified breakfast cereals, and dairy products. Zinc absorption is more efficient from a diet high in animal protein than a diet rich in plant proteins. Phy-tates, which are found in whole grain breads, cereals, legumes and other products, are believed to decrease zinc absorption. Some good food sources of zinc include (per 1oz-serving or as indicated):
- oysters, 6 medium (16 mg)
- beef shank, lean (3 mg)
- beef chuck, lean (2.7 mg)
- beef tenderloin, lean (1.6 mg)
- pork shoulder, lean (1.4 mg)
- beef, eye of round, lean (1.3 mg)
- pork tenderloin, lean (0.8 mg)
- chicken leg, meat only (0.9 mg)
- chicken breast, meat only (0.9 mg)
- yogurt, plain, low fat (2.2 mg per cup)
- baked beans, canned with pork (0.6 mg)
- cashews, dry roasted, no salt (1.6 mg)
- pecans, dry roasted, no salt (1.4 mg)
- chickpeas, canned (2.6 mg per cup)
- mixed nuts, dry roasted, no salt (1.1 mg)
- walnuts, black, dried (1.0 mg)
- almonds, dry roasted, no salt (1.0 mg)
- milk (1.8 mg per cup)
- cheese, Swiss (1.1 mg)
- cheese, Cheddar (0.9 mg)
- cheese, Mozzarella, part skim (0.9 mg)
- beans, kidney, California red (1.6 mg per cup)
- peas, green, frozen (1.6 mg per cup)
- oatmeal, instant, low salt (0.8 mg per packet)
- flounder, sole (0.2 mg)
The Recommended Dietary Allowance (RDA) for zinc is:
- infants: (0-6 months): 3 mg
- infants: (7-12 months): 3 mg
- children (1-3 y): 3 mg
- children (4-8 y): 5 mg
- children (9-13 y): 8 mg
- adolescents (14-18): males, 11 mg, females, 9 mg
- adults: males, 11 mg, females, 8 mg
- pregnancy: 13 mg
- lactation: 14 mg
Zinc in nutritional supplements is available as zinc gluconate, zinc oxide, zinc aspartate, zinc picolinate, zinc citrate, zinc monomethionine and zinc histidine. They are distributed as stand-alone or combination products as tablets, capsules or liquids.
Zinc deficiency most often occurs when zinc intake is inadequate or poorly absorbed and it can have serious health consequences. Moderate to severe zinc deficiency is rare in the United States. However, it is highly prevalent in developing countries. The symptoms of severe deficiency include the slowing or cessation of growth and development, delayed sexual maturation, skin rashes, chronic and severe diarrhea, immune system deficiencies, poor wound healing, decreased appetite, impaired taste sensation, night blindness, swelling and clouding of the corneas, and behavioral disorders. These symptoms were first accurately described when a genetic disorder called acrodermatitis enter-opathica was linked to zinc deficiency. Although mild dietary zinc deficiency is unlikely to cause such severe symptoms, it is known to contribute to several health problems, especially in young children. Zinc deficiency leads to impaired physical and neuropsychological development, and to an increased risk of life-threatening infections in young children. Individuals at risk of zinc deficiency include:
- infants and children
- pregnant and breastfeeding women, especially teenagers
- patients receiving intravenous feeding
- malnourished individuals, including those with anorexia nervosa
- people with severe or persistent diarrhea
- people with malabsorption syndromes, including celiac disease and short bowel syndrome
- people with inflammatory bowel disease, including Crohn's disease and ulcerative colitis
- people with alcoholic liver disease
- people with sickle cell anemia
- elderly people
- strict vegetarians whose major food staples are grains and legumes because the high levels of phytic acid in these foods lower the absorption of zinc
Fortified foods include many types of breakfast cereals that make it easier to consume the RDA for zinc. However, they also make it easier to consume too much zinc, especially if zinc supplements are also taken. Anyone considering zinc supplementation should accordingly first consider whether their needs could be met by dietary zinc sources and from fortified foods. Intakes between 150 and 450 mg of zinc per day lead to copper deficiency, impaired iron function, reduced immune function, and reduced levels of high-density lipoproteins, the “good cholesterol”. A few isolated cases of acute zinc toxicity have been reported for food or beverages contaminated with zinc present in galvanized containers. Single doses of 225-450 mg of zinc are known to induce vomiting. Milder gastrointestinal distress has been reported at doses of 50-150 mg/day of supplemental zinc.
The simultaneous administration of zinc supplements and certain antibiotics, such as tetracyclines and quinolones, may decrease absorption of the antibiotic with potential reduction of their action. To prevent this interaction, it is recommended to take the zinc supplements and antibiotics at least two hours apart. Metal chelating agents like penicillamine, used to treat copper overload in Wilson's disease, and diethylene-triamine pentaacetate (DTPA), used to treat iron overload, can lead to severe zinc deficiency. Anticonvulsant drugs, such as sodium valproate, may also cause zinc deficiency. The prolonged use of diuretics may increase urinary zinc excretion, resulting in increased zinc losses. A medication used to treat tuberculosis, ethambutol, has been shown to increase zinc loss in rats.
Interactions of zinc taken with other supplements are as follows:
- Calcium: May lower zinc absorption in postmenopausal women.
- Iron: May reduce the absorption of both iron and zinc.
- Phosphate salts: May lower the absorption of zinc.
- L-cysteine: May increase the absorption of zinc.
- L-histidine: May also enhance the absorption of zinc.
In the case of zinc deficiency, oral zinc therapy usually results in the complete disappearance of symptoms, but it must be maintained indefinitely in individuals with the acrodermatitis enteropathica.
Excessive intake can be corrected by bringing levels back to the RDA values.
It has been estimated that 82% of pregnant women worldwide are likely to have inadequate zinc intakes. Zinc deficiency has been associated with a number of pregnancy complications, including low birth weight, premature delivery, and labor and delivery complications.
The adverse effects of zinc deficiency on immune system function are also likely to increase complications in children that have infectious diarrhea. Persistent diarrhea contributes to zinc deficiency and malnutrition. Recent research has shown that zinc deficiency may also increase the harmful effects of toxins produced by diarrhea-causing bacteria like E. coli. Zinc supplementation in combination with drinking plenty of liquids has also been shown to significantly reduce the duration and severity of childhood diarrhea.
Significant delays in growth and weight gain, known as growth retardation or failure to thrive, are common symptoms of mild zinc deficiency in children. But since many of the symptoms associated with zinc deficiency are general and also observed with other medical conditions, parents should not assume that they are due to a zinc deficiency. It is important to consult with a health care professional concerning medical symptoms so that appropriate care can be given.
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Monique Laberge, Ph.D.