Zinc
Description
A component of over 80 enzymes in our bodies, zinc functions in more enzymatic reactions than any other trace mineral. Zinc is necessary for normal cell division and function. In rapidly growing tissues, zinc deficiency will retard the synthesis of DNA, RNA and protein.
Zinc works synergistically with vitamin A and essential fatty acids in many bodily functions. Optimal zinc levels must be attained if optimal health is desired.
Human zinc deficiency was first characterized in 1963 in adolescent boys from the Nile Delta. The principal features of the deficiency were growth failure (short stature), delayed sexual maturation, anemia, enlarged internal organs and a history of geophagia (clay eating). After treatment with zinc, patients matured sexually and grew taller. Similar reports have appeared from Iran, Turkey, Morocco, Portugal, China and the United States.
Absorption
Zinc absorption in humans is believed to proceed as follows.
Following ingestion of zinc, the pancreas secretes picolinic acid into the intestine. The picolinic acid then forms a complex with zinc that enhances absorption of zinc from the intestine. In humans and animals, the quantity of zinc transported across the absorptive cells of the intestine appears to be directly related to the availability of picolinic acid.
Picolinic acid is synthesized in our bodies from the amino acid tryptophan. Vitamin B-6 is needed for this reaction to occur. Inborn errors in metabolism that affect the conversion of tryptophan to picolinic acid, or a vitamin B-6 deficiency, will result in impaired zinc absorption.
In addition, vitamin B-6 supplementation may also enhance zinc absorption. Providing zinc as zinc picolinate bypasses the need for picolinic acid secretion by the pancreas. However, other agents are also known to influence zinc absorption, including: methionine, cysteine, histidine, reduced glutathione, citrate, and prostaglandin E2.
A decrease in pancreatic secretions would result in impaired zinc absorption. This is probably the major factor responsible for the decreased zinc levels in the elderly. Pancreatic insufficiency is a common condition with increasing age. This means even though an individual may be consuming high levels of zinc, a deficiency may result due decreased absorption. Supplementation with zinc picolinate appears to be particularly indicated in individuals with even mild pancreatic insufficiency.
Inhibitors of zinc absorption
The absorption of zinc is inhibited by several dietary factors. Organic inhibitors of zinc include phytates and components of dietary fiber. These compounds bind zinc in the gut and make it unavailable for absorption. Inorganic inhibitors of zinc absorption include calcium (at high levels), copper, iron and cadmium. Clay and possibly other substances consumed by persons who eat dirt (pica) may also inhibit the availability of zinc for intestinal absorption.
Method of Action
Zinc is an important metallic constituent of the enzyme carboxypeptidase A, a pancreatic enzyme active in protein degradation.
Zinc is found in highest concentration in the liver, with lesser amounts found in the pancreas, kidney, and pituitary gland. Zinc absorption occurs primarily in the small intestine. Zinc-binding ligand molecules act to transport zinc across the mucosal cells of the intestine, where it is picked up by albumin molecules for transport to the liver and other organs.
Zinc is a constituent of the enzyme carbonic anhydrase. This enzyme is, in turn, a constituent of red blood cells and gastric juices, and plays an important role in the deposition of calcium salts in teeth and bones.
The enzyme alcohol dehydrogenase contains zinc and is essential for the conversion of alcohol to an aldehyde, thereby facilitating alcohol metabolism in the liver. The function of this enzyme and its relationship to the development of liver cirrhosis is conspicuously tied to characteristic zinc deficiencies in alcoholic patients.
Zinc is found in other enzymes as well, including lactic dehydrogenase (active in glycolysis), alkaline phosphatase (active in maintaining phosphate levels near bone), and glutamic dehydrogenase (found in blood platelets).
Zinc is important as a structural constituent for RNA proteins and is essential for the proper activity of the RNA synthesizing enzyme, RNA polymerase. Zinc also plays a role in metabolism and three dimensional structure of DNA.
Zinc is found in alpha-macroglobulin, an important protein in the body's immune system. This globulin firmly binds about 30% of plasma albumin, which functions primarily as a transport protein.
Zinc appears to be a cofactor in the synthesis and degradation of collagen, and has effects in reproduction and healing.
Factors which can impair zinc absorption include calcium, fiber, copper, iron, and various diseases, such as Crohn's disease.
Properties & Uses
Zinc plays a vital role in immunity, the special senses, wound healing, prostatic function, rheumatoid arthritis and acne.
Zinc and Immunity
Adequate zinc levels in the tissue are necessary for proper function of the immune system. Zinc deficiency results in an increased susceptibility to infection.
Zinc appears to be vital for normal function of the thymus gland, the synthesis and secretion of thymic hormones, and the protection of the thymus from cellular damage.
Several defects in immune function related to aging are reversible upon zinc supplementation, again highlighting the importance of this nutrient to the elderly.
As well as stimulation of the immune system, zinc has displayed virus inhibiting activity. In one double-blind clinical study, zinc supplementation significantly reduced the average duration of colds by seven days.
Zinc and the Special Senses (Sight, Taste, Smell)
Zinc is essential for the maintenance of vision, taste and smell. A zinc deficiency would result in the impaired function of these special senses.
Night blindness is often due to a zinc deficiency.
The loss of the sense of taste and/or smell is a common complaint in the elderly.
Zinc supplementation has been shown to improve some individuals with decreased taste and/or smell acuity.
Zinc and Wound Healing
Zinc is required for protein synthesis and cell growth, and therefore for wound healing. Zinc supplementation has been shown to decrease wound-healing time, while a zinc deficiency leads to prolonged healing of the wound. Zinc supplementation is indicated to aid protein synthesis and cell growth following any sort of trauma, such as burns, surgery, and wounds.
Zinc and the Prostate
Zinc concentrations in the human prostate are very high. A zinc deficiency may be a contributing factor in the high rate of prostate enlargement in this country. It is estimated 50-60% of the men between the ages of 40 and 59 years of age have prostatic enlargement.
Zinc supplementation has been shown to reduce the size of the prostate and symptomatology in the majority of the patients. Male infertility as a result of decreased sperm count may be caused by a zinc deficiency.
Zinc in Rheumatoid Arthritis
Zinc levels in patients with rheumatoid arthritis are typically low. Zinc supplementation was found to reduce joint swelling, joint tenderness and morning stiffness in of these patients.
Zinc in Acne
The importance of zinc in normal skin function is well known. Typically, serum zinc levels are lower in 13 to 14 year old males than in any other age group. This group is also the most susceptible to acne. During puberty, there is an increased requirement for zinc due to increased hormonal production. One group of researchers believes low levels of zinc are responsible for acne.
Zinc supplementation has been shown to be as effective as tetracycline but without the side effects. It appears that zinc is able to normalize some of the hormonal factors responsible for acne. It will often take up to 12 weeks of supplementation before a dramatic effect is noted. In one study, the mean acne score decreased from 100% to 15% after 12 weeks of treatment with zinc.
The exact mechanism responsible for zinc's positive effect on acne has not been determined. However, zinc is intricately involved in local hormone activation, retinol-binding protein formation, wound healing and tissue regeneration.
Zinc also is essential in the maintenance of normal levels of vitamin A.
Zinc in Pregnancy
Low zinc levels during pregnancy are associated with intrauterine growth retardation, congenital malformations (particularly neural tube defects), fetal alcohol syndrome and low birth weight.
The most severe manifestation of zinc deficiency occurs in infants with the genetic disease acrodermatitis enteropathica. Signs and symptoms of this disease include a severe rash covering the entire body, diarrhea, failure to thrive, frequent infections and death, unless the infant is administered very large doses of zinc. Although the exact metabolic defect has not yet been identified, evidence exists that it involves a defect in picolinic acid metabolism (discussed below under Absorption).
Infants with acrodermatitis enteropathica have responded therapeutically to picolinic acid supplementation.
Consequence of Deficiency
Although severe zinc deficiency is very rare in developed countries, many individuals in the United States have marginal zinc deficiency. This is particularly true in the elderly population. This may be reflected by an increased susceptibility to infection, poor wound healing, or a decreased sense of taste or smell.
The high occurrence of zinc deficiency in the elderly can be due to either a decreased dietary intake and/or a decreased ability to absorb zinc.
Signs and Symptoms of Zinc Deficiency
There are many signs and symptoms indicating mild to moderate zinc deficiency, including the following:
Frequent and/or severe infections
Sleep disturbances
Behavioral disturbances
Delayed wound healing
Impaired glucose tolerance
Night blindness
Reduced appetite, anorexia
Decreased sense of taste and/or smell
Growth retardation
Delayed sexual maturation
Impotence, infertility
Dandruff and/or hair loss
Skin disorders
Lack of menstruation
It has been reported that white spots on the fingernails reflect zinc status. This may have some significance, as poor wound healing due to a zinc deficiency secondary to trauma to the nail bed may be responsible for the white spots in some subjects.
Some judgment is required to ascertain the significance of these lesions in a particular individual, i.e., do the white spots correspond to the level of trauma to the nail beds?
Since loss of taste acuity is a well-documented sign of zinc deficiency, measurement of taste acuity is a useful test of zinc status. The taste solution is made by dissolving 1 gram zinc sulphate heptahydrate (Znso4.7H20) in 1 liter of distilled water. The response to tasting 5 ml of this solution normally falls within one of the following four categories.
1. No specific taste or other sensation is noticed after the solution has been kept in the mouth for ten seconds.
2. No immediate taste is noticed, but after a few seconds a slight taste variously described as "dry," "mineral," "furry," or "sweet" develops.
3. A definite, though not strongly unpleasant, taste is noted almost immediately and tends to intensify with time.
4. A strong and unpleasant taste is noted almost immediately.
A reaction of category 1 or 2 suggests a zinc deficiency and a need for zinc supplementation.
Conditions Predisposing to Zinc Deficiency
Conditions predisposing to zinc deficiency can be divided into four categories:
Decreased intake:
| Anorexia nervosa | Alcoholism |
| Fad diets | Old age |
| Protein deficiency | Low socioeconomic status |
| Vegetarianism |
Increased body losses:
| Acute infections/inflammation | Burns |
| Starvation | Post trauma |
Decreased absorption:
| Diabetes mellitus | Hepatic disease |
| High phytate diet | Celiac disease |
| Alcoholism | Inflammatory bowel disease |
| High dietary iron:zinc ratio | Diarrhea |
| Chelating agents | Intestinal resection |
| Acrodermatitis enteropathica | Chronic blood loss |
| Dialysis | Short bowel syndrome |
| Achlorhydria/hypochlorhydria | Pancreatic insufficiency |
Increased Requirement:
| Old age | Oral contraceptive use |
| Pregnancy and lactation | Growth spurts and puberty |
Toxicity Factors
Zinc can be toxic if taken in excess of two grams per day (per kilogram of body weight). Symptoms of this toxicity include gastrointestinal irritation and vomiting. High zinc consumption may promote copper deficiency due to the antagonistic relationship of these two minerals.
Acute zinc toxicity is quite rare, as the ingestion of amounts large enough to cause toxicity symptoms (2 grams per kilogram body weight) will usually provoke vomiting.
The area between severe deficiency and toxicity is termed the gray area of nutrition. Chronic intake of zinc (usually at a level greater than 45 mg elemental zinc per day) may significantly reduce copper levels and promote anemia.
Zinc poisoning can also occur upon ingestion of food that has been stored in galvanized containers.
Recommended Dietary Allowance
| age | RDA (mg) | RNI (mg) | ||
| infants/children | ||||
| 0-6 months | 5 | 4.0 | ||
| 6-12 months | 5 | 5.0 | ||
| 1-3 years | 10 | 5.0 | ||
| 4-6 years | 10 | 6.5 | ||
| 7-10 years | 10 | 7.0 | ||
| males | ||||
| 11-14 years | 15 | 9.0 | ||
| 15+ years | 15 | 9.5 | ||
| females | ||||
| 11-14 years | 12 | 9.0 | ||
| 15+ years | 12 | 7.0 | ||
| pregnancy | 15 | - | ||
| lactation | ||||
| (1st 6 months) | 19 | 13.0 | ||
| (2nd 6 months) | 16 | 9.5 |
For over thirty years, Recommended Daily Amounts has existed in the United Kingdom. It has been used to measure the adequacy of an individual's diet. However, in 1991 the Committee on Medical Aspects of Food Policy (COMA) gave forth a whole new set of figures upon the request of the Department of Health's Chief Medical Officer. Reference Nutrient Intake (RNI) is one of these sets collectively known as "Dietary Reference Values." RNI is an amount of a nutrient that is enough for almost every individuals, even someone who has high needs for the nutrient. This level of intake is, therefore, considerably higher than what most people would need. If individuals are consuming the RNI of a nutrient they are most unlikely to be deficient in that nutrient.
Food Sources
| Beans | Beef |
| Cheese | Chicken |
| Clams | Corn - Raw |
| Cowpea | Crab |
| Egg | Green pea |
| Hot cocoa | Lamb |
| Lentils | Milk |
| Oysters | Peanut butter |
| Peanut | Pork |
| Rye bread | Shrimp |
| Spinach | Tea - black |
| Tuna | Turkey |
| Veal | Whole wheat |
| Whole wheat bread |
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