ZINC
Micromineral Zinc is found in all organs, tissues, and body fluids. Here you can found zinc benefits and interesting health knowledge.
INTRODUCTION
The body of a human contains around 1.5 to 3.0 g of zinc. Zinc is found in all organs, tissues, and body fluids. Zinc, a member of the micro minerals, is a metal that can exist in several different valence states, but it is almost universally found as the divalent ion (Zn2+) in the human body. It is a very important mineral that supports a number of functions in the human body. It provides support to the immune system, enables the body to make proteins and DNA, contributes to wound healing, and plays a role in child growth and development. It additionally has antioxidant properties. Micromineral Zinc is found in all organs, tissues, and body fluids. Here you can found zinc benefits and interesting health knowledge.
ZINC OCCURRENCE
In food
Zinc is present in food in the form of complexes with nucleic acids and amino acids that are part of peptides and proteins. The content of zinc in food varies considerably. Excellent sources of zinc are seafood and red meat. Poultry, pork, and dairy products are other good animal sources of zinc. Whole grains along with legumes also provide moderate amounts of zinc. Cereals, some of which may be fortified, are thought to provide about 30% of the zinc in the US diet. Fruits contain little zinc. However, plant sources not only have lesser zinc contents but zinc from plants is also absorbed to a lower extent than zinc from animal sources (e.g., meat). The daily value for zinc used on food and supplement labels is 15mg.
ZINC AND FOOD PROCESSING
Processing of certain foods may affect zinc availability. Heat treatment can cause zinc in food to form complexes that resist hydrolysis, thereby making zinc less available for absorption. Maillard reaction products that are amino acid- carbohydrate complexes which are formed as a result of browning are particularly notable for inhibiting zinc's availability for absorption.
In medical products
Zinc supplements are available in several forms including oral tablets and lozenges, throat and nasal sprays, and nasal gels. Zinc is typically found in supplements as zinc oxide, zinc sulfate, zinc acetate, zinc chloride, and zinc gluconate. Topical products containing zinc, which usually contain zinc as zinc oxide or zinc chloride, which may be used in wound management and include paste bandages, occlusive adhesive dressings, alginates, stockings, and zinc saline dressings.
In Body
Endogenous zinc (up to about 4.5mg) from pancreatic, intestinal, and biliary secretions is released into the GI tract with food ingestion and augments the zinc present from dietary food source and any consumed supplements. Zinc secreted can be absorbed again and represents an important means for body zinc homeostasis.
ZINC DIGESTION
Zinc needs to be hydrolyzed from amino acids and nucleic acids before it can be absorbed. Zinc is believed to be liberated from these food constituents during the digestive process, most likely by the acidic environment of the stomach and upper duodenum and by proteases and nucleases in the stomach and small intestine.
TRANSPORT
Zinc which passes into the portal blood from the intestinal cell is predominantly transported loosely bound to albumin. Most of the zinc is then absorbed by the liver, where the mineral is initially concentrated. Zinc leaving the liver binds mainly (about 70–75%) to albumin, with the remaining zinc binding more tightly to a-2-macroglobulin. Two amino acids, histidine and cysteine, also loosely bind and transport (in a ternary complex as histidine-zinc-cysteine) less than 1% of zinc in the blood.
Plasma zinc concentration can range from approximately 70 to 120 μg/dL (10–18 µmol/L), with plasma containing approximately 3 mg of zinc. Plasma zinc concentrations decrease after eating, as well as during infection and trauma. Multiple transporters, including at least 14 ZIPs and 10 ZnTs, facilitate cellular zinc uptake and release, yet many of the mechanisms remain unclear. ZIP carriers 1, 2, 4, 5, 6, 7, 8, and 14 appear to be involved in cellular zinc uptake from extracellular locations and the release of zinc from intracellular stores, both to effect increased cytosolic zinc concentrations. ZIP14, for example, transports zinc into hepatocytes, and where its activity appears to be increased as part of the acute-phase response (as occurs with infections and trauma).
The ZIP5 transporter is expressed in the intestinal cell as well as in the pancreas, liver and kidneys. In the intestine, ZIP5 is found on the basolateral membrane, where it facilitates transport of serous zin to the mucosa. In other words, ZIP5 Moves zinc out of the circulation, that is, from the blood into the intestinal cell. The zinc transporter ZnT6, present on the enterocyte’s brush border membrane, is believed to mediate the exocytosis of zinc from the intestinal cell back into the lumen for final excretion in the faeces. However, not all ZIP carriers, however, exclusively transport zinc; many, like ZIP14 which transports both Fe21 and Zn21, carry other minerals as well.
ZINC ABSORPTION
Zinc absorption occurs primarily in the proximal small intestine, that is, the duodenum and upper jejunum. Two mechanisms are responsible for intestinal zinc absorption.1. Carrier mediated transport
2. Diffusion.
The primary means of absorption with usual intakes, which is up to about 7-9mg of zinc, is saturated and carrier mediated. The protein carrier Zrt- and Irt- like protein 4 (ZIP4) is the major transporter of zinc across the enterocytes brush border membrane and is expressed throughout the gastrointestinal tract. Zinc intake influences ZIP4. In addition to carrier mediated transport, para cellular diffusion of zinc through the tight junction of enterocytes enables the absorption. This process is thought to contribute to absorption of zinc when zinc intakes typically 20mg or more exceed the capacity of ZIP4 carriers. Although other carrier proteins such as DMT1, present in gastrointestinal tract bind zinc, they are not thought to contribute except in minor capacity to zinc absorption.
Mechanism Of Zinc Absorption
Bound zinc is released from food where they are present primarily with proteins and nucleic acid. Most of the zinc is taken up by Zrt and Irt like protein 4 (ZIP4) across the brush border membrane of the intestinal cell. Divalent mineral transporter 1 (DMT1) and amino acid play a minor role in zinc absorption across the brush border membrane. Some zinc may be directed into faces if bound to inhibitors, or absorption may be enhanced by organic acid, decreasing pH and chelators. With high zinc intake, zinc may be absorbed between the cells. In the cell, zinc can either be used functionally or it may be stored in vessels, in trans Golgi network, or as a part of metallothionein. Zinc can be transported across the basolateral membrane by transporter ZnT1. Zinc binds to protein for transport in the blood.
FACTORS AFFECTING ZINC ABSORPTION
Zinc binds with iron, chelators or ligands. Whether these substances act either as enhancers or inhibitors depends on the digestibility and absorbability of chelates/ligands of zinc formed.ENHANCERS OF ZINC ABSORPTION
- Ligands or chelators including organic acid and prostaglandin may bind and promote zinc absorption.
- Pancreatic secretion is thought to contain unidentified constituents that enhance zinc absorption.
- Improved absorption of zinc from an acidic environment.
- Amino acids like histidine and methionine are known to enhance zinc absorption.
INHIBITORS OF ZINC ABSORPTION
- Phytic acid found in plant food, particularly legumes, nuts, seeds and whole grain cereals. It binds to zinc and forms a zinc phytic acid complex that is large, insoluble and poorly absorbed.
- Polyphenol found in tea, coffee, fruits and vegetables. The impact of these substances on an individual’s zinc status varies with amount consumed and other sources of zinc in diet.
- Minerals such as iron and calcium also inhibit zinc absorption.
Use Of Zinc in Intestinal Cell
- Zinc entering the enterocyte may have the following fates:
- Used functionally within the enterocytes.
- Preserved or sequestered in the enterocyte
- Transported through the cytosol and across the basolateral membrane for entry into the blood and thus use by other tissues.
ZINC REGULATION AND HOMEOSTASIS
The GIT is the major site where regulation of homeostasis of zinc occurs. This regulation involves adjustments in both, uptake of zinc and endogenous excretion in the feces. Maintaining a constant level of zinc in the cell and its homeostasis is vital for survival of the organism. The adjustments made in gastrointestinal absorption and endogenous excretion are synergistic. It is seen that variations in excretion appear quickly with changes in intake of zinc just above or under the optimal intake while, zinc absorption responds more slowly, but has the ability to cope with large fluctuations in intake. Extremely low or with prolonged borderline intakes, secondary homeostatic adjustments may increase changes in the GIT. These secondary adjustments include alterations in urinary zinc excretion, a shift in plasma zinc turnover rates and possibly, retention of zinc released from specific tissues, such as bone, in other tissues to maintain function.
ZINC EXCRETION
Zinc is lost from the body primarily via the gastrointestinal tract, kidney and skin. Most zinc is excreted through the gastrointestinal tract in the feces, the amount lost increases or decreases depending on body zinc concentrations. Faecal zinc losses range from 3mg/day among adults ingesting very low dietary zinc up to about 4.6mg with dietary zinc intakes of 15mg; higher dietary zinc intakes promote greater faecal losses of zinc. Small amount of zinc is also lost via kidneys and skin as well as semen. Most zinc is filtered by kidneys and reabsorbed by tubules. ZnT1 is thought to control renal zinc resorption.
Around 0.3-0.7mg of zinc/day is excreted in the urine. Zinc losses 0.4-0.7mg/day occur with exfoliation of skin and with sweating. Loss of zinc in men consisted of 0.63mg in urine, 0.54mg by integumental and sweat, 0.1mg in semen and 2.57mg through intestine. Loss of zinc in women consisted of 0.44mg in urine, in integumental and sweat 0.46mg, in menses 0.1mg and endogenous intestinal zinc losses were 2.3mg.
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