The importance of zinc has long been recognized; zinc ores being used in the making of brass as early as 1000 AD. Zinc was recognized as a distinct element in 1509, and evidence of its essentiality was demonstrated in plants in 1869 and in experimental animals in 1933. Zinc has been known as an essential trace element for plants, animals and humans, since 1930. The average adult has between 1.5 and 3 g of zinc. About 60% is placed in muscle, 30% in bones, and about 6% in the skin. The highest concentrations of zinc are found in the prostate gland and sperm of males, as well as in leukocytes and erythrocytes. High concentrations of zinc are also found in the retina of the eye, liver, kidney and hair.

Physiological role of zinc

Zinc participates in more than 200 enzymatic reactions in the body. It plays a key role in the synthesis and stabilization of genetic material. It is necessary for cell division and the synthesis and degradation of hydrocarbons, lipids and proteins. It is therefore considered as essential for tissue growth.

As a part of the enzyme copper-zinc superoxide dismutase (Cu/Zn SOD) it helps in protecting of cells and some compounds from the harmful effects of free radicals.

Zinc is necessary for the structure and normal function of cell membranes. It participates in the creation of connective tissue, teeth, bones, nails, hair and skin. Zinc plays an important role in the absorption of calcium in the bones and affects the action of growth hormone.

Zinc is considered as one of the most important nutrients for the immune system because it is essential for the formation of antibodies, white blood cells, thyroid gland and hormone function. Therefore, it is important to preserve the resistance to infection and for wound healing.

One of the important roles of zinc is secretion, synthesis and use of insulin. It also protects pancreatic β-cells (the cells that produce insulin) from falling apart. Zinc is involved in the metabolic processes of the thyroid and adrenal glands, ovaries and testes. It is important for the normal development of male sex hormones and the prostate. Normal functioning of the skin requires the presence of zinc. It is involved in the functioning of the sweat glands, activating local hormones, creating a protein that binds vitamin A, in control of inflammation and tissue regeneration.

Zinc is essential for normal growth and development of the fetus and for milk production during lactation. Zinc levels in pregnancy is associated with normal development of the palate, lip, brain, eyes, heart, bones, lungs and the urogenital system in infants. Adequate intake of zinc is essential for normal fetal growth and normal maintenance of pregnancy.

Zinc is necessary for the production of neurotransmitters in the brain. Normal function and releasing of vitamin A from the liver requires the presence of zinc. Zinc is necessary for the preprevention of sight, sense of taste and smell. It is the most abundant trace element in the eye. It is involved in the creation of hydrochloric acid in the stomach and in the conversion of fatty acid to prostaglandins, which regulate body processes such as heart rate and blood pressure. Zinc is necessary for muscle contraction and maintenance of acid-base balance in the body. It also helps in detoxification of alcohol.

There are over 20 known metalloenzyme in which structure enters zinc. Some of them are: Carbonic anhydrase; Lactate dehydrogenase; Glutamate dehydrogenase; Alkaline phosphatase; Thymidine kinase and other

It is not surprising that zinc deficiency is connected with multisystem dysfunction. The main zinc protein in saliva called cornstarch and plays an important role in the taste.

Metabolism

In the lumen of the small intestine, there is a factor (picolinic acid) that binds zinc, and is excreted by pancreas. To synthesize picolinic acid it is needed the presence of vitamin B6. Zinc picolinate complex is transported through the cell in the small intestine, and then to the liver where it is stored. Vitamin B6 deficiency can, particularly in the elderly lead to decreased absorption of zinc. Zinc, and iron in the mucosa cells can be binded to a protein. Then it is transmitted through the serous membrane of mucosa and binds to serum albumin. Copper can affect the absorption of zinc due to competition for the binding site on serum albumin. When the food has a lot of calcium phosphate and zinc deficiency can be difficult. Zinc is processed in pancreatic juice and to a lesser extent in bile. Therefore, the main route of zinc excretion is in the faeces. Significant amount of zinc may be lost through perspiration, especially in the tropics. When you increase the proportion of zinc (and copper) in the liver it is associated with metallothionein.

Zinc is not stored in the body enough so if food intake of zinc is poor it can relatively fast get to a zinc deficiency. The level of zinc in the body can best be determined by measuring it in erythrocytes and leukocytes.

The latest US study suggests that zinc plays a key role in the immune system by binding to specific proteins and thereby promotes individual reactions or stops the immune system. Studies in 1997, have shown that the absorption of zinc increases twice during lactation to provide greater synthesis of milk, but there is no evidence that this is happening during pregnancy.

Hormone therapy can often be the cause of poor absorption of zinc. The treatment with estrogen in postmenopausal women leads to decrease zinc absorption by 35% after only 3 months of treatment.

Food sources of zinc

An important source of zinc are animal proteins, while processed foods, citric fruits and vegetables without leaves contain it very small amount.

Foodstuffs rich in zinc are liver, oysters, meat, fish, hard cheese, hazelnuts, eggs and grains.

It is average absorbed about 20-40% zinc that was loaded with food. However, absorption depends of the amount of zinc in the body – less zinc it is easily absorbed. Other nutrients also influence on the digestibility of zinc. Zinc is easier to absorb from foods of animal origin (also from a fish), as these high-protein foods contain amino acids that bind zinc and make it more soluble. Zinc from plants, fruits and cereals hard to absorb because these foods contain compounds such as phytates and oxalates that bind zinc and reduce the amount of zinc that can be absorbed. Additives that are added to food may also reduce its absorption. The presence of EDTA and a large quantity of vegetable protein reduces the absorption of zinc. Absorption of zinc declines with age, so in people who are over 65 it can be absorbed just half of zinc than from the people between 25 and 30 years.

Recommended daily allowance

Some diseases, such as liver suffering from alcoholism, burns, postoperative conditions, stress, weight loss, chronic infections, viral hepatitis, diabetes and certain kidney diseases require increased zinc intake. Also, people who are extensively involved in sports must be supplied by the large amounts of zinc in the body.

Zinc deficiency

The zinc deficiency occurs when zinc absorption is impaired as is the case of Acrodermatitis enteropathica - a rare autosomal recessive disease in which there are dermatological, ophthalmic, gastrointestinal and neuropsychiatric symptoms with growth retardation and hypogonadism. Secondary zinc deficiency is caused by malabsorption due to many reasons or due to too large excretion in the urine. The latter appears to be a relatively common cause of zinc deficiency in patients with sickle cell anemia. Zinc from food in the intestines can bind phytate which is abundant in some types of bread. Zinc-phytate complex is not absorbed, and can occur the form of zinc deficiency syndrome that includes growth retardation, hypogonadism, alopecia and decreased appetite. Acute alcohol consumption appears to increase the excretion of zinc in urine.

Zinc deficiency leads to skin lesions such as acne and eczema on face, psoriasis, hair loss; it slows down the growth and development and leads to the apathy. It delays menstrual cycle in women. Also there are sleep problems, frequent colds, intestinal hypersensitivity, and weakened sense of taste, sight and smell. It also increases the possibility of illness from cancer. In men, there is less production of sperm and defects in reproductive organs. In youth causes slow sexual development, and irregular periods in women. It also causes poor development of mental abilities. Leads to poor development of the newborn. Loss of taste and smell, anemia, loss of appetite, white spots on nails, poor transmission of nerve impulses, mental disorders, susceptibility to infection, delayed wound healing are effects of zinc deficiency.

There may be complications in childbirth in pregnant women with zinc deficiency.

In patients with zinc deficiecy serum ribonuclease activity is high, while the activity of carbonic anhydrase in red blood cells is low. In patients with zinc deficiency it is difficult to heal wounds.

Diseases of the gastrointestinal system, such as various inflammatory processes in the stomach and celiac disease reduce the absorption of zinc in the body and can easily get to the symptoms of zinc deficiency. Low levels of zinc can also be found in patients with sickle-cell anemia and in people who have a disorder in the metabolism of fats.

If zinc deficiency occurs early in pregnancy it increases the risk of congenital defects in the foetus, low weight newborns, miscarriage, premature delivery, mental disorders and behavioral problems in the newborn. It also increases the risk of high blood pressure in pregnant women. Mothers with low levels of zinc during pregnancy run the risk that their babies are more prone to infections.

In the elderly absorption of zinc is significantly reduced and they often have the symptoms of zinc deficiency. In addition, older people often take medications that interfere with the absorption of zinc. Symptoms of zinc deficiency include loss of appetite, slow wound healing, loss of taste and a weakened immune system.

Alcoholism also contributes to the lack of zinc in the body, especially if the liver is diseased. Zinc deficiency in alcoholics is associated with changes in the metabolism of vitamin A, reduced immunity, vision problems and problems with sexual function.

Changes in zinc metabolism may be occurred in persons with diabetes (type I and type II). In this case, zinc is excreted in the urine. Also the lack of zinc in the diet increases the risk of developing diabetes.

Low levels of zinc in the body are associated with cardiovascular disease. There is evidence that zinc protects the inner layer of blood vessels from damage and thus helps in preventing of atherosclerosis. This can be explained by its anti-inflammatory and anti-oxidative properties.

People with HIV (and AIDS) often suffer from a lack of zinc in the body which affects the immune system.

Zinc deficiency leads to a decrease or loss of vision because of several enzymes that play an important role in the functioning of the eye, contain zinc. The level of these enzymes decreases with age. Zinc deficiency leads to degeneration of the central part of the retina.

Premenstrual syndrome could be disturbed due to the lack of zinc and copper.

It has been shown that zinc levels in infertile men are lower, leading to low testosterone levels and the creation of a smaller quantity of sperm. Zinc deficiency in adolescence can lead to delayed puberty because zinc controls testosterone levels in men.

Zinc deficiency affects the immune system because it leads to reduced amounts of several types of T cells, natural killer cells and other components of the immune system. This leads to increased susceptibility to infections and slow healing of wounds.

Lack of zinc in the diet can slow bone growth in adolescents and increase the chance of developing osteoporosis in later years.

Zinc levels can be reduced in asthmatics.

Overdose

Zinc toxicity due to acute or chronic ingestion of high quantities of zinc supplements can also occur and lead to impaired immune response, hypocupraemia, microcytosis, neutropaenia, inhibition of copper and iron absorption, respiratory and gastrointestinal toxicity, inhibition of neurological development and a decline in HDL levels.

Excess zinc in the body is rare, but if there are more than 200 mg of zinc the following symptoms occur: abdominal pain, nausea and vomiting.

Other symptoms include dehydration of the body, lethargy, anemia and dizziness.

Prolonged intake of large amounts of zinc in the body can cause a copper deficiency in the body. This can occur if the daily intake is more than 25 mg of zinc. Prolonged intake of zinc in amounts greater of 150 mg per day leads to decreased function of the immune system and decreased levels of HDL cholesterol that leads to heart disease.

Increased zinc intake during pregnancy can be harmful to the foetus.

Zinc in medicine

Zinc is one of the elements, which is very essential for the healthy functioning of the body. Although the entries in small quantities, its role is great. Zinc protects us from many diseases and participates in building collagen. It also helps in better functioning of the enzyme.

Some of the diseases which could be treated with zinc are: Reduced intelligence in children; Coldness; Contact dermatitis; Down's syndrome; Infertility in men; Night blindness; Weight loss

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