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Zinc

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Zinc has been widely used for some time now to treat cold, flu and other diseases because of its ability to boost the immune system. It is also used with other antioxidant compounds to treat age related vision loss. However, Zinc is also used in thousands of proteins synthesized by the body as well as participating in enzyme activation. Studies have also found that diets deficient in Zinc result in memory loss as well reduction in other bodily functions showing a strong connection to brain functions. Zinc can be found in seafood, meats, nuts and some grains.


[edit] Zinc and Human Health

Zinc is an essential mineral. It is the second most common trace metal, after iron, naturally found in the human body. Zinc is needed in small but critical concentrations and if the amount available is not adequate, plants and/or animals will suffer from physiological stress brought about by the dysfunction of several enzyme systems and other metabolic functions in which zinc plays a part. Zinc is involved in numerous aspects of cellular metabolism:

Zinc is naturally present in a variety of foods and added to others.

Phytates—which are present in whole-grain breads, cereals, legumes, and other foods—bind zinc and inhibit its absorption. Thus, the bioavailability of zinc from grains and plant foods is lower than that from animal foods, although many grain- and plant-based foods are still good sources of zinc. Since the bioavailability of zinc from vegetarian diets is lower than from non-vegetarian diets, vegetarians sometimes require as much as 50% more of the RDA for zinc than non-vegetarians. Fertilising with zinc reduces the phytate content. Techniques to increase zinc bioavailability include soaking beans, grains, and seeds in water for several hours before cooking them and allowing them to sit after soaking until sprouts form. Vegetarians can also increase their zinc intake by consuming more leavened grain products (such as bread) than unleavened products (such as crackers) because leavening partially breaks down the phytate. The effect of phytate is also modified by proteins in the diet and animal proteins can improve the absorption of zinc from the diet. Phytate, containing large amounts of phosphate, is present in higher concentrations in cereal grains that have been grown on soils with a relatively high phosphorus status.

Zinc is available also as a dietary supplement. Here, zinc takes several forms, including zinc gluconate, zinc sulfate, and zinc acetate. The elemental zinc content appears in the Supplement Facts panel/label on the supplement container. Research has not determined whether differences exist among forms of zinc in absorption, bioavailability, or tolerability. Zinc is also found in some over-the-counter drugs sold as cold remedies, typically in the form of lozenges and nasal sprays and gels. At this time, it is not clear whether zinc cold remedies can help treat the common cold. Moreover, there is a safety concern over use of nasal products as they have caused loss of smell.

Zinc supplements have the potential to interact with several types of medications and other nutrients.

A daily intake of zinc is required to maintain a steady state because the body has no specialized zinc storage system. The average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals, or Recommended Dietary Allowance (US RDA), for zinc is:

Age Male Female Pregnancy Lactation
Birth to 6 months 2 mg 2 mg
7 months to 3 years 3 mg 3 mg
4 to 8 years 5 mg 5 mg
9 to 13 years 8 mg 8 mg
14 to 18 years 11 mg 9 mg 13 mg 14 mg
19+ years 11 mg 8 mg 11 mg 12 mg
60 years or older 9.4 mg 6.8 mg

The adequate intake of zinc for 0 to 6 months of age infants is equivalent to the mean intake of zinc from breast milk. Infants aged 7–12 months should consume age-appropriate foods or formula containing zinc, in addition to breast milk. There is also a higher requirement for pregnant and lactating women because of high fetal requirements for zinc and the depletion of maternal zinc stores from lactation. Because of there high zinc requirements, infants, young children, and pregnant and lactating mothers are at the highest risk from the adverse outcomes of zinc deficiency.

Zinc deficiency occurs when there is inadequate zinc intake or absorption, increased losses of zinc from the body, or increased requirements for zinc. Gastrointestinal surgery and digestive disorders (such as ulcerative colitis, Crohn’s disease, and short bowel syndrome) can decrease zinc absorption and increase endogenous zinc losses primarily from the gastrointestinal tract and, to a lesser extent, from the kidney. Other diseases associated with zinc deficiency include malabsorption syndrome, chronic liver disease, chronic renal disease, sickle cell disease, diabetes, malignancy, and other chronic illnesses. Chronic diarrhea also leads to excessive loss of zinc. The deficiency also affects persons with sickle cell disease. This is possibly due to increased nutrient requirements and/or poor nutritional status. Also, approximately 30%–50% of alcoholics have low zinc status because ethanol consumption decreases intestinal absorption of zinc and increases urinary zinc excretion.

The deficiency affects a range of functions, chiefly,reproduction, growth, immunity and brain development. The deficiency is characterized by growth retardation, especially among infants and children, loss of appetite, abnormal neuro-behavioural developmentcitation needed,and impaired immune function and increased rates of infection. In more severe cases, zinc deficiency causes hair loss, diarrhea, delayed sexual maturation in adolescents, impotence in men, hypogonadism in males, eye and skin lesions, and even death, due to infections. Zinc deficiency is associated with decreased release of vitamin A from the liver, which may contribute to symptoms of night blindness. Weight loss, delayed healing of wounds, taste abnormalities, and mental lethargy, or lower alertness levels, can also occur. Many of these symptoms are non-specific and often associated with other health conditions; therefore, a medical examination is necessary to ascertain whether a zinc deficiency is present.

Tolerable Upper Intake Levels for Zinc

Age Male Female Pregnant Lactating
0 to 6 months 4 mg 4 mg
7 to 12 months 5 mg 5 mg
1 to 3 years 7 mg 7 mg
4 to 8 years 12 mg 12 mg
9 to 13 years 23 mg 23 mg
14 to 18 years 34 mg 34 mg 34 mg 34 mg
19+ years 40 mg 40 mg 40 mg 40 mg

Zinc toxicity can occur in both acute and chronic forms. Acute adverse effects of high zinc intake include nausea, vomiting, loss of appetite, abdominal cramps, diarrhea, and headaches. Intakes of 150–450 mg of zinc per day have been associated with such chronic effects as low copper status, altered iron function, reduced immune function, and reduced levels of high-density lipoproteins. Reductions in a copper-containing enzyme, a marker of copper status, have been reported with even moderately high zinc intakes of approximately 60 mg/day for up to 10 weeks. 80 mg per day of zinc in the form of zinc oxide for 6.3 years, on average, have been associated with a significant increase in hospitalizations for genitourinary causes, raising the possibility that chronically high intakes of zinc adversely affect some aspects of urinary physiology.

Zinc nutritional status is difficult to measure adequately using laboratory tests due to its distribution throughout the body as a component of various proteins and nucleic acids. Plasma or serum zinc levels are the most commonly used indices for evaluating zinc deficiency, but these levels do not necessarily reflect cellular zinc status due to tight homeostatic control mechanisms. Clinical effects of zinc deficiency can be present in the absence of abnormal laboratory indices. Clinicians consider risk factors (such as inadequate caloric intake, alcoholism, and digestive diseases) and symptoms of zinc deficiency (such as impaired growth in infants and children) when determining the need for zinc supplementation.

A great deal of work has established evidence that zinc deficiency is likely to be a public health problem both in terms of its magnitude and its health consequences. National food balance data indicate that as much as one half of the world’s population is at risk of inadequate zinc intake. This is due to the large size of population mainly on primarily plant-based diet. Compared to animal source foods, diets based exclusively or predominantly on plant products have relatively low zinc contents and provide poor amount of absorbable zinc. Moreover, national growth surveillance data indicate that 33% of preschool children in low-income countries have stunted growth, a condition that can be addressed with increased intake of zinc.

Though the magnitude of zinc deficiency is likely to be significant, zinc deficiency is not universal - it does not affect every country equally, or every population within those countries equally. Thus, there is a need for countries to assess the zinc status of their populations to develop suitable intervention programs. While provision of zinc supplements is the most rapid method of correcting zinc deficiency and its consequences, taking zinc supplements directly, or the addition of zinc to foods (‘food fortification’) are expensive, difficult to administer and, in many cases, they have not been sustainable and have failed to reach all the people at risk of zinc deficiency.


[edit] Zinc, Plants and the Soil

Many food products are derived directly from plants, including staples such as rice, wheat, maize and sorghum. The zinc content of animal food products can also be affected by the soil-plant relationships of zinc.

Apart from the major elements (carbon, hydrogen, oxygen, nitrogen, potassium, calcium, magnesium, phosphorus and sulphur), there are also eight essential trace elements for plants. These are zinc, copper, boron, manganese, iron, chlorine, molybdenum, and nickel. Off all these trace elements, plant zinc deficiency is the most commonly encountered and widespread deficiency problem. This deficiency can be observed by the some characteristic visible symptoms. Unfortunately, these symptoms often only occurs in cases of relatively severe deficiency. With marginally deficient soils, lowered plant yields is the only sign that can be determined as it may take several growing seasons before the visible symptoms appear.

Many plant species are affected by zinc deficiency on a wide range of soil types in most agricultural regions of the world. Plants' need for zinc varyies in quantity depending on the plant species and the zinc status of the soil on which they are grown. Zinc-deficient plants generally have low tissue zinc concentrations and therefore, in addition to reduced crop yields, the crop products from these deficient plants make a lower contribution to the zinc content of the human diet. This can be vitally important in subsistence rural economies where there is often insufficient diversity in the diet.

Rice, wheat and maize, the world’s three most important cereal crops both in terms of area harvested and in tonnages of grain produced, are affected by the deficiency. Maize is the crop species which is most susceptible to zinc deficiency and, in many countries, it receives the highest proportion of zinc fertiliser applications. This problem is highlighted by the increasing demand for maize as livestock feed and for ethanol production. Rice is also highly susceptible to zinc deficiency due to the flooding. Flooding the soil reduces zinc availability to rice and increases the concentrations of soluble phosphorus and bicarbonate ions which can exacerbate zinc deficiency problem. The new more water-efficient rice production systems are susceptible to the deficiency as well. Wheat, though less sensitive to the deficiency, is still severely affected in many parts of the world as it is grown on large areas of alkaline, calcareous soils with a relatively high phosphorus status. This relatively recent discovery of widespread zinc deficiency problems in rice and wheat is linked to the intensification of farming. And as zinc content of foods is of major importance and as varieties of all the cereal species, except rye, show a high degree of variability in zinc-efficiency, a field of research on the biofortification of plant foods with zinc is developing. This involves both the breeding of new varieties of crops with the genetic potential to accumulate a high density of zinc in cereal grains (genetic biofortifiaction) and the use of zinc fertilisers to increase zinc density (agronomic biofortification).

Three different types of compounds are used as zinc fertilisers and these vary considerably in their zinc content, price and effectiveness for crops on different types of soils.

In addition to specific zinc fertilisers, some macronutrient fertilisers can contain sufficient zinc to act as a significant source of the micronutrient when used regularly at relatively high application rates. Perhaps the best known of these is single superphosphate which has been widely used as a phosphatic fertiliser in some parts of the world for more than one hundred years. Depending on the source of the phosphate rock used in its manufacture, single superphosphate can contain concentrations of up to 600 mg Zn kg-1. However, owing to concerns about over-fertilising with phosphorus after long-term use of superphosphate in some developed countries, smaller amounts of this, or other phosphatic fertilisers, are now being used. As a consequence, a more concentrated (high analysis) phosphate fertilisers, such as monoammonium phosphate (MAP) and diammonium phosphate (DAP), which have much lower zinc contents, are used in its place. Though phosphatic fertilisers, mono- or diammonium phosphate (MAP, DAP), and nitro-phosphorus fertilisers can be obtained with added zinc, there may be, in some cases, a need for specific zinc fertilisers to be used in association with these fertilizers. This is especially true where the zinc status of the soil is marginal or inadequate. Here, the increased phosphorus supply can induce zinc deficiency in the crop and, if not corrected, result in a reduced yield of grain with a lower total zinc content as well as an elevated phytate content and hence there will be proportionally less zinc available to the consumer.

Several zinc-containing materials which can also supply zinc to soils and crops include pig and poultry manures, biosolids (sewage sludge), composts made from urban solid wastes, and certain industrial waste products. Athough these materials can contain high concentrations of zinc, they frequently also have relatively high contents of other micronutrients, such as copper and/or nickel and non-essential, potentially toxic elements, such as cadmium and lead. Some biosolids can also contain significant concentrations of persistent organic pollutants. Use of these materials as zinc sources should be carefully controlled and based on broad-spectrum chemical analysis of the material. The zinc contained in pig or poultry manure results mainly from intentional additions to the animal feed as dietary supplements and hence their zinc contributions are more predictable and more straightforward to control.

Methods of zinc application include:

Note that zinc and copper compete for the same absorption sites on root surfaces. If the amounts applied of these two micronutrients are unbalanced, there is a danger that the element in excess will exacerbate or induce a deficiency of the other.

It is reported that the native soil zinc status is the dominant factor determining grain zinc concentrations followed by genotype and fertiliser. Unlike wheat, fertilization only increased total zinc content of rice by improved straw and grain yields, but did not significantly affect concentration in the grain. Developing cultivars that gave increases in zinc concentrations in grain in response to zinc fertiliser application is thought to be a challenge especially to rice breeders.

Apart from improving dietary intake, zinc-enriched grains generally result in seedlings with increased vigour and greater stress tolerance. Thus, with seedlings having a greater chance of survival and growing to maturity, it is possible to reduce seed rates and, consequently, reduce the cost of cereal production.

Continued use of zinc fertilisers for agronomic biofortification will have to be monitored carefully. This is because the soil ecosystem is likely to be adversely affected by zinc accumulation. Plants vary widely in their tolerance to zinc toxicity. In general, plants tend to have more advanced homeostatic mechanisms for enabling them to tolerate elevated levels of zinc than soil fauna and microorganisms. Threshold total zinc values in sensitive plant species range from 150 to 200 mg per kilogram while the range of 100 to 500 mg per kg can cause yield of many crops to be reduced by 25%. Toxicity treshold for earthworms is 200 mg per kg. In increasing toxicity for microoranisms, the order is: Manganese, lead, nickel, zinc, cadmium and copper. One of the most sensitive microorganisms to zinc toxicity is the nitrogen-fixing bacterium Rhizobium leguminosarum which forms nodules in the roots of clover and whose activity directly affects the growth of clover. The lowest observed effect concentration for R. leguminosarum was 90 mg/kg. It is also recommended that the maximum permissible zinc concentration in soils receiving sewage sludge in the UK should be reduced from 300 mg per kg, the European Commission limit (86/278/EEC), to 200 mg per kg. It is 100 mg/kg in Denmark, 200 mg/kg in Germany, 300 mg/kg in France and Italy and 150 mg/kg in Spain. The normal rates of application of zinc to deficient soils are in the range 5kg to 34 kg per hectare with several years between applications and these have been found from long experience to be safe.


[edit] References

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