|Classification and external resources|
Zinc deficiency can occur in soil, plants, and animals. In animals, including humans, it is defined either qualitatively as insufficient zinc to meet the needs of the body and thereby causing clinical manifestations, or quantitatively as a serum zinc level below the normal range. Normal values for soil and plants, especially crops, have also been defined.
Zinc deficiency in humans results from reduced dietary intake, inadequate absorption, increased loss, or increased use. The most common cause is reduced dietary intake; as much as 25% of the world's population is at risk. Increasing the amount of zinc in the soil and thus in crops is an effective preventative measure.
Zinc plays an essential role in numerous biochemical pathways. It affects many organ systems, including the skin, gastrointestinal tract, central nervous system, and immune, skeletal, and reproductive systems. A lack of zinc thus has numerous manifestations, the most common of which are an increased rates of diarrhea, pneumonia, and malaria.
- 1 Classification
- 2 Signs and symptoms
- 3 Causes
- 4 Mechanism
- 5 Prevention
- 6 Epidemiology
- 7 History
- 8 Soils and crops
- 9 Research
- 10 References
- 11 Further reading
- 12 External links
Zinc deficiency affects about 2.2 billion people around the world. Zinc deficiency can be classified as acute, as may occur during prolonged inappropriate zinc-free total parenteral nutrition; or chronic, as may occur in dietary deficiency or inadequate absorption. Zinc deficiency can also be considered as mild, as typically accompanies dietary deficiency; or severe, as typically accompanies congenital defective absorption.
Signs and symptoms
Skin, nails and hair
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) and burning mouth syndrome.
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, however 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).
Zinc levels may increase or decrease hunger depending on the status of other nutrients, the developmental stage of the animal, and percentage body fat. There is evidence zinc deficiency decreases hunger, but there is no current evidence that zinc supplementation can decrease hunger, since no correlation between zinc supplementation and increasing leptin levels has been found.
Zinc deficiency may lead to anorexia and anorexia nervosa. Appetite disorders can, in turn, cause malnutrition and inadequate zinc intake, leading to a vicious cycle. The use of zinc in the treatment of anorexia has been advocated since 1979 by Bakan. At least 15 clinical trials have shown that zinc improved weight gain in anorexia. A 1994 trial showed that zinc doubled the rate of body mass increase in the treatment of anorexia nervosa. Deficiency of other nutrients such as tyrosine, tryptophan and thiamine could contribute to this phenomenon of "malnutrition-induced malnutrition".
The way zinc influences hunger may depend on the sodium/osmotic status of the organism, with low sodium/low zinc levels increasing hunger and conversely. An organism with a low level of zinc has an increased susceptibility to hypoosmotic stress and cell rupture. Zinc is known to affect osmolality by increasing sodium retention. In rats, the first visible sign of zinc deficiency is decreased food seeking behaviour.
Cognitive and motor function
Cognitive and motor function may be impaired in zinc deficient children. Zinc deficiency can interfere with many metabolic processes when it occurs 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.
Plasma zinc levels have been alleged to be associated with many psychological disorders. An increasing amount of evidence suggests that zinc deficiency could play a role in depression. Zinc may be an effective treatment.
Zinc deficiency during pregnancy can negatively affect both the mother and fetus. Animal studies indicate that maternal zinc deficiency can upset both 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. These effects may be mediated by the defective functioning of estrogen 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 on zinc supplementation trials during pregnancy did not report a significant effect of zinc supplementation on neonatal survival.
A diet which is high in phytate containing whole grains, high in foods grown in zinc deficient soil, or high in processed foods containing little or no zinc can result in zinc deficiency. 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. The following table summarizes most of the foods with significant quantities of zinc, listed in order of quantity per serving, unfortified. Note that all of the top 10 entries are meat, beans, or nuts. The recommended intake per day of zinc is 15 mg for adults and children over the age of four.
|Food||mg in one serving||Percentage of recommended daily intake|
|Oysters, cooked, breaded and fried, 3 ounces (about 5 average sized oysters)||74.0||493%|
|Beef chuck roast, braised, 3 ounces||7.0||47%|
|Crab, Alaska king, cooked, 3 ounces||6.5||43%|
|Beef patty, broiled, 3 ounces||5.3||35%|
|Cashews, dry roasted, 3 ounces||4.8||33%|
|Lobster, cooked, 3 ounces||3.4||23%|
|Pork chop, loin, cooked, 3 ounces||2.9||19%|
|Baked beans, canned, plain or vegetarian, ½ cup||2.9||19%|
|Almonds, dry roasted, 3 ounces||2.7||18%|
|Chicken, dark meat, cooked, 3 ounces||2.4||16%|
|Yogurt, fruit, low fat, 8 ounces||1.7||11%|
|Chickpeas, cooked, ½ cup||1.3||9%|
|Cheese, Swiss, 1 ounce||1.2||8%|
|Oatmeal, instant, plain, prepared with water, 1 packet||1.1||7%|
|Milk, low-fat or non fat, 1 cup||1.0||7%|
|Kidney beans, cooked, ½ cup||0.9||6%|
|Chicken breast, roasted, skin removed, ½ breast||0.9||6%|
|Cheese, cheddar or mozzarella, 1 ounce||0.9||6%|
|Peas, green, frozen, cooked, ½ cup||0.5||3%|
|Flounder or sole, cooked, 3 ounces||0.3||2%|
Acrodermatitis enteropathica is an inherited deficiency of the zinc carrier protein ZIP4 resulting in inadequate zinc absorption. It presents as growth retardation, severe diarrhea, hair loss, skin rash (most often around the genitalia and mouth) and opportunistic candidiasis and bacterial infections.
Numerous small bowel diseases which cause destruction or malfunction of the gut mucosa enterocytes and generalized malabsorption are associated with zinc deficiency.
Exercising, high alcohol intake, and diarrhea all increase loss of zinc from the body. Changes in intestinal tract absorbability and permeability due, in part, to viral, protozoal, or bacteria pathogens may also encourage fecal losses of zinc.
The mechanism of zinc deficiency in some diseases has not been well defined; it may be multifactorial.
Wilson's disease, sickle cell disease, chronic kidney disease, chronic liver disease have all been associated with zinc deficiency. It can also occur after bariatric surgery, mercury exposure and tartrazine.
Although marginal zinc deficiency is often found in depression, low zinc levels could either be a cause or a consequence of mental disorders and their symptoms.
As biosystems are unable to store zinc, regular intake is necessary. Excessively low zinc intake can lead to zinc deficiency, which can negatively impact an individual's health. The mechanisms for the clinical manifestations of zinc deficiency are best appreciated by recognizing that zinc functions in the body in three areas: catalytic, structural, and regulatory. Zinc (Zn) is only common in its +2 oxidative state, where it typically coordinates with tetrahedral geometry. It is important in maintaining basic cellular functions such as DNA replication, RNA transcription, cell division and cell activations. However, having too much or too little zinc can cause these functions to be compromised.
In its catalytic role, zinc is a critical component of the catalytic site of hundreds of metalloenzymes. In its structural role, zinc coordinates with certain protein domains, facilitating protein folding and producing structures such as ‘zinc fingers’. In its regulatory role, zinc is involved in the regulation of nucleoproteins and the activity of various inflammatory cells. For example, zinc regulates the expression of metallothionein, which has multiple functions, such as intracellular zinc compartmentalization and antioxidant function. Thus zinc deficiency results in disruption of hundreds of metabolic pathways, causing numerous clinical manifestations, including impaired growth and development, and disruption of reproductive and immune function.
Five interventional strategies can be used:
- Adding zinc to soil, called agronomic biofortification, which both increases crop yields and provides more dietary zinc.
- Adding zinc to food, called fortification.
- Adding zinc rich foods to diet. The foods with the highest concentration of zinc are proteins, especially animal meats, the highest being oysters. Per ounce, beef, pork, and lamb contain more zinc than fish. The dark meat of a chicken has more zinc than the light meat. Other good sources of zinc are nuts, whole grains, legumes, and yeast. Although whole grains and cereals are high in zinc, they also contain chelating phytates which bind zinc and reduce its bioavailability.
- Oral repletion via tablets (e.g. zinc gluconate) or liquid (e.g. zinc acetate). Oral zinc supplementation in healthy infants more than six months old has been shown to reduce the duration of any subsequent diarrheal episodes by about 10 hours.
- Oral repletion via multivitamin/mineral supplements containing zinc gluconate, sulfate, or acetate. It is not clear whether one form is better than another. Zinc is also found in some cold lozenges, nasal sprays, and nasal gels.
Severe zinc deficiency is rare, and is mainly seen in persons with acrodermatitis enteropathica, a severe defect in zinc absorption due to a congenital deficiency in the zinc carrier protein ZIP4 in the enterocyte. Mild zinc deficiency due to reduced dietary intake is common. Conservative estimates suggest that 25% of the world's population is at risk of zinc deficiency. Zinc deficiency is thought to be a leading cause of infant mortality.
Providing micronutrients, including zinc, to humans is one of the four solutions to major global problems identified in the Copenhagen Consensus from an international panel of economists.
Significant historical events related to zinc deficiency began in 1869 when zinc was first discovered to be essential to the growth of an organism (Aspergillus Niger). In 1929 Lutz measured zinc in numerous human tissues using the dithizone technique and estimated total body zinc in a 70 kg man to be 2.2 grams. Zinc was found to be essential to the growth of rats in 1933. In 1939 beriberi patients in China were noted to have decreased zinc levels in skin and nails. In 1940 zinc levels in a series of autopsies found it to be present in all tissues examined. In 1942 a study showed most zinc excretion was via the feces. In 1950 a normal serum zinc level was first defined, and found to be 17.3 - 22.1 micromoles/liter. In 1956 cirrhotic patients were found to have low serum zinc levels. In 1963 zinc was determined to be essential to human growth, three enzymes requiring zinc as a cofactor were described, and a report was published of a 21 year old Iranian man with stunted growth, infantile genitalia, and anemia which were all reversed by zinc supplementation. In 1972 fifteen Iranian rejected army inductees with symptoms of zinc deficiency were reported: all responded to zinc. In 1973 the first case of acrodermatitis enteropathica due to severe zinc deficiency was described. In 1974 the National Academy of Sciences declared zinc to be an essential element for humans and established a recommended daily allowance. In 1978 the Food and Drug Administration required zinc to be in total parenteral nutrition fluids. In the 1990s there was increasing attention on the role of zinc deficiency in childhood morbidity and mortality in developing countries. In 2002 the zinc transporter protein ZIP4 was first identified as the mechanism for absorption of zinc in the gut across the basolateral membrane of the enterocyte. By 2014 over 300 zinc containing enzymes have been identified, as well as over 1000 zinc containing transcription factors.
Soils and crops
Soil zinc is an essential micronutrient for crops. Almost half of the world’s cereal crops are deficient in zinc, leading to poor crop yields. Many agricultural countries around the world are affected by zinc deficiency. In China, zinc deficiency occurs on around half of the agricultural soils, affecting mainly rice and maize. Areas with zinc deficient soils are often regions with widespread zinc deficiency in humans. A basic knowledge of the dynamics of in soils, understanding of the uptake and transport of zinc in crops and characterizing the response of crops to zinc deficiency are essential steps in achieving sustainable solutions to the problem of zinc deficiency in crops and humans.
Soil and foliar application of zinc fertilizer can effectively increase grain zinc and reduce the phytate:zinc ratio in grain. People who eat bread prepared from zinc enriched wheat have a significant increase in serum zinc.
Zinc fertilization not only increases zinc content in zinc deficient crops, it also increases crop yields. Balanced crop nutrition supplying all essential nutrients, including zinc, is a cost effective management strategy. Even with zinc-efficient varieties, zinc fertilizers are needed when the available zinc in the topsoil becomes depleted.
Plant breeding can improve zinc uptake capacity of plants under soil conditions with low chemical availability of zinc. Breeding can also improve zinc translocation which elevates zinc content in edible crop parts as opposed to the rest of the plant.
Central Anatolia, in Turkey, was a region with zinc-deficient soils and widespread zinc deficiency in humans. In 1993, a research project found that yields could be increased by 6 to 8-fold and child nutrition dramatically increased through zinc fertilization. Zinc was added to fertilizers. While the product was initially made available at the same cost, the results were so convincing that Turkish farmers significantly increased the use of the zinc-fortified fertilizer (1 per cent of zinc) within a few years, despite the repricing of the products to reflect the added value of the content. Nearly ten years after the identification of the zinc deficiency problem, the total amount of zinc-containing compound fertilizers produced and applied in Turkey reached a record level of 300,000 tonnes per annum. It is estimated that the economic benefits associated with the application of zinc fertilizers on zinc deficient soils in Turkey is around US$100 million per year. Zinc deficiency in children has been dramatically reduced.
There is some evidence that zinc may have an effect on cancer and further study is recommended.
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