Importance of Zinc In Human Health

Zinc

- Zinc is  ubiquitous  component of animal and plant tissues.

- Zinc is ubiquitous in our bodies; about 70 percent of the mineral is in bone, 29 percent in muscle, and the remainder in the GI tract, skin, kidneys, and other major organs.

- Zinc is found in all body fluids, including gastro-intestinal tract .

The adult human body contains about 2–2.5 g of zinc with about 70% concentrated in the bone.

Zinc

Food Sources

- Good dietary sources of zinc are meat, poultry, eggs, and seafood.

- Oysters are the richest sources of zinc.

 - Cereals and legumes also contain significant amounts of zinc, but because of the presence of phytic acid in these foods, zinc is less available than that supplied by foods of animal origin.

Absorption and Metabolism

- About 40% of dietary zinc is absorbed, primarily in the small intestine, although the segment where the absorption is optimal is not known.

- Zinc absorption in the gut is facilitated by the presence of a low molecular weight ligand believed to be secreted by the pancreas.

Absorption and Metabolism

------  About 67% of the zinc in plasma is loosely bound to albumin and is considered to be the principal metabolically active fraction.

 - A portion is bound to the a2 macroglobulin,

transferrin, and the amino acids cysteine and histidine. The fraction bound to amino acids determines the amount that is filtered by the kidney.

 - The normal route of excretion is via the gastrointestinal tract originating from the pancreatic, biliary, and mucosal secretions .

A small fraction of zinc is normally excreted

daily in the urine.

Factors that may limit the net absorption of zinc

- High intake of dietary inhibitors of zinc absorption, such as cereal, corn and rice based diet high in phytate ,  fiber, and calcium

- High doses of supplemental iron

- Gastrointestinal diseases that limit zinc absorption, such as intestinal bypass, Crohn s disease, bacterial overload, viral or bacterial infections , steatorrhea ,and celiac disease

Functions

- Zinc is essential in the composition or function of over 70-100 enzymes involved in digestion and major metabolic pathways.

- Zinc plays diverse roles in carbohydrate, lipid, protein and nucleic acid metabolism and in cell growth.

 - Zinc is required for the development and activation of T-lymphocyte.

- It has a role in the synthesis, storage, and secretion of insulin by the pancreatic islet

   beta cells.

Functions

- Zinc is present in gustin, a salivary polypeptide that appears to be necessary for the normal development of taste buds.

- Zinc has a central role in male reproductive function, being involved in testicular development, spermatogenesis and sperm motility.

 - It has an important function in synthesis of DNA and RNA, protein synthesis, cell division and membrane stability.

Functions

- Vision

    Concurrent zinc deficiency can interfere with the mobilization of vitamin A from liver stores as well as the synthesis of rhodopsin. Thus, vitamin A deficiency is exacerbated by concurrent zinc deficiency.

- High concentrations of zinc are found in the retina with. age and low zinc in the retina, which appear to play a role in the development of age-related macular degeneration (AMD), which leads to partial or complete loss of vision. Zinc can protect against night blindness, and prevent the development of cataracts.

Nutritional Aspects of Pregnancy and Lactation

- During the course of normal pregnancy , there is a clear decrease in maternal serum zinc concentration to about 50 % of normal values specially in the third trimester , whereas concentration in fetal cord blood increase in  normally progressing pregnancies .

- Zinc deficiency in pregnancy is related to the decrease in serum albumin.

Although the fetal needs of this nutrient are highest in late pregnancy, it may also be critically important in very early pregnancy.

Nutritional Aspects of Pregnancy and Lactation

- An additional 3–5 mg of zinc is recommended during pregnancy.

- Deficiency during pregnancy  may cause fetal growth retardation, multiple congenital abnormalities, particularly of the skeletal and nervous systems, and low birth weight.

Zinc deficiency1

Zinc deficiency may be a result of :

1-dietary deficiency

(as in vegetarianism, slimming diets and starvation),

2-malabsorption (gastrointestinal disorders)

3-increased  requirement (as in chronic diseases, drug use)

4- increased loss (liver and kidney diseases , alcoholism, or chronic infection.)

Zinc deficiency

- Dietary deficiency can be due to a lack of zinc in foods consumed.

- It may also be caused by the presence of substances, that interfere with zinc absorption.

- Various disorders of the gastrointestinal tract may result in zinc depletion; for example enterocolitis, short bowel syndrome, celiac disease and cystic fibrosis. severe intestinal malabsorption syndrome,

Zinc deficiency

- Enterocolitis, short bowel syndrome, celiac disease and cystic fibrosis.

- Plasma zinc levels may be depressed in infectious hepatitis, alcoholism, protein-energy malnutrition, and sickle cell anemia.

- One of the first symptoms of zinc deficiency is poor appetite and changes in the perception of taste and smell. Lethargy, slowing of activity, apathy and depression are also associated with deficiency.

Zinc and human milk

-          - Premature infants are especially at risk of   zinc deficiency because of their rapid growth.

-          - The high zinc content of colostrum helps satisfy this need. Infants fed human milk have higher levels of plasma zinc than do infants fed cow’s milk despite the fact that human and cow’s milk contain similar amounts of zinc. This suggests that zinc in human milk is more available than the zinc in cow’s milk.

  Zinc deficiency in infant

- Zinc deficiency in breast-fed only children is a rare disorder affecting mostly premature infants.

- However, some cases of the disease in term infants have also been reported.

- A more severe zinc deficiency occurs in infants with rare genetic disease, acrodermatitis enteropathica.

- This disease is transmitted by an autosomal recessive gene and the symptoms usually appear after the infants have been weaned from breast milk

- The disease may be treated by continuing breastfeeding or through supplemental zinc.

Zinc deficiency in infant

- Acrodermatitis enteropathica is a rare genetic disease characterized by alopecia, dermatitis, diarrhea, photophobia, psychological changes, failure to thrive, infections and death.

- The disease is believed to be due to a defect in zinc absorption, most likely due to insufficient synthesis of the low molecular weight zinc ligand.

DERMATITIS IN ZINC DEFICIENCY

 Zinc deficiency in infant

- Chronic zinc deficiency is associated

with growth retardation, delayed healing,

compromised immune function and a number

of effects on the male reproductive system.

Male reproductive system

Even relatively short-term (35 days) zinc       restriction (1.4–3.4 mg Zn/day) can affect andrological variables, causing :

- oligospermia

- impotence        

- hypogonadism

- impaired synthesis of testosterone      

Requirement

- Studies in human subjects have demonstrated that normal adults require about 12.5 mg of zinc daily when ingesting a mixed diet.  

- The turnover of body zinc is estimated to be

about 6 mg/day. Based on these and other data and the assumption that about 40% of

dietary zinc is absorbed, an RDA is established at 15 mg/day for males over 10 years of

age and 12 mg for females over 10 years of age (with an additional 3 mg during

pregnancy and 7 mg during lactation).

- The RDA for infants is about 3–5 mg and for the

preadolescent it is 10 mg.

Recommended Dietary Allowances for Zinc for
Infants over 7 months, Children, and Adults

Factors affecting requirements

- Requirements depend on, or are affected by,

            losses in intestinal secretions

             and through sweat or skin

             epithelial cell loss particularly.

 - Most zinc in the body is present as a component of

   lean body mass – muscle and bone , so Zinc requirements affected by  these factors.

Factors affecting requirements

- Since catch-up growth following periods of

   malnutrition or growth faltering (often secondary to diarrhea with loss of zinc) involves rapid deposition of new tissue, zinc requirements are greatly increased during  recovery from malnutrition.

- Further, zinc is an important component of lean body mass so requirements also increase during periods of accelerated growth.

Toxicity

- Zinc is relatively less toxic than other micro minerals. Doses up to 200 mg have produced no ill effects.

 - The ingestion of excess zinc resulting from the storage of food in Inorganic Elements (Minerals) 111 galvanized containers has resulted in fever, nausea, vomiting, and diarrhea.

-  Zinc and copper are mutually antagonistic and prolonged ingestion of excess zinc may produce evidence of copper deficiency if the copper intake is marginal.

- To avoid this complication, chronic use of zinc supplements should be limited to 40 mg/day.

Conclusion

- Zinc has its important physiological roles especially for:

-Vegetarians : because of the lower bioavailability.

-Pregnancy as it may slow fetal growth.

- Breast-fed older infants between the ages of 7 months and 12 months ,as human milk does not provide recommended amounts of zinc.

- lactation women.

 - Alcoholics : Alcohol decreases the absorption of zinc and increases loss of zinc in urine.

- Gastrointestinal surgery or who have digestive disorders that result in malabsorption,  Crohn’s disease and short bowel syndrome, are at greater risk.

Conclusion

- Everyone needs zinc. Children need zinc to grow, and adults need zinc for health. Growing infants, children, adolescents and pregnant and lactating women, athletes, vegetarians and the elderly often require more zinc.

References:

1-INTRODUCTION TO CLINICAL NUTRITION SECOND EDITION, REVISED AND EXPANDED VISHWANATH M. SARDESAI

2-PRACTICAL NUTRITIONAL TEQNIQUES Alan L. Buchman, MD, MSPH

1. 3-Pocket Atlas of Nutrition Hans Konrad Biesalski, M.D

4-Nutrition in Early LifeEdited by Jane B. Morg and John W.

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