The human body absorbs approximately 400kg zinc over the average 70-year lifespan and at any one time there should be 2-4 gm zinc in the body. It is the second most abundant mineral ion ( Magnesium is the first) in the body and is the only metal that appears in all enzyme classes The body absorbs 20-40% of zinc in food, zinc from animal foods being more readily absorbed (twice as much ) than zinc from plant foods. Zinc is also more readily absorbed with a protein meal and although the body cannot store zinc and it is needed every day in small amounts (50mg or less), it may be held in metallothionine reserves and transferred in metal transporter proteins. Metallothionines in the intestinal cells are capable of adjusting the absorption of zinc by 15-40%. Thus control of cellular zinc homeostasis is maintained by zinc proteins and zinc binding metallothioneines. Zinc is needed for over 300 enzymes in the body and makes up part of 3000 different proteins in the body. Muscles (60%) and bones (30%) contain 90% of the body’s zinc. High concentrations of zinc are found in the prostate gland and semen and the choroid of the eye.
If bone is reabsorbed or muscle is broken down then some zinc can be reutilised and in cases of zinc depletion changes in immune status alter before any decrease in levels of plasma zinc. There is a small exchangeable pool of zinc (100-200mg ) that depends on recently absorbed zinc and the intestinal excretion of zinc. As with Magnesium, the efficiency of absorption of zinc is inversely related to the amount of zinc present in the body. The greater the level of body zinc, the less absorption occurs. Zinc, Magnesium Calcium and Iron all compete for transporters in the intestine for uptake above a threshold of approximately 800mg so consuming these minerals together below this level should not interfere with uptake Zinc is found in all cells in the body and the daily requirement is dependent on age and activity.
Zinc deficiency is due to
- Soil deficiency.
- Some drugs deplete zinc.
- Vegetarian and vegan diets may be deficient.
- High cereal based diets, containing high phytate foods which can bind with zinc and impair absorption.
- Cooking with water can result in leaching of up to 50% of zinc levels of the food.
- Refined processing of wheat and baked goods can result in up to 75% zinc loss
Tetracycline and quinolone antibiotics react with zinc in the intestines inhibiting the absorption of both the antibiotic and zinc. The antibiotic should be taken 2 hours or more before or at least 4-6 hours after the zinc supplement to avoid this.
In short over 300 enzymes are zinc dependent, including enzymes involved in the synthesis of certain proteins such as collagen and wound healing. Also needed for thymic hormone activation and maintaining a normal immune system, testosterone and oestrogen, fertility and reproduction including cell division. It is involved in gene regulation, maintaining acid/base balance in the body and normal carbohydrate, fat and protein metabolism. It is needed for normal bones, skin, hair and nails and normal brain function including maintenance of normal vision. It can also act as an antioxidant, protecting DNA, lipids and proteins in the body.
Zinc Contributes to
Normal DNA synthesis. Although the exact role of zinc in DNA synthesis is not fully understood but it does play a structural role in zinc fingers, which are finger shaped proteins. Due to their shape, these proteins can bind to DNA and RNA allowing them to function in Gene expression. These proteins are the most common transcription factors in living organisms, transcription factors are proteins that bind to DNA and control the transfer of genetic information to RNA Put simply Zinc is needed for reading genetic instructions and lack of zinc may mean that instructions get misread or not read at all.
Normal acid/base metabolism. Acid/Base balance is the balance between acid and alkaline to keep body fluids as close to a neutral pH (pH7) as possible. Carbon dioxide and water are rapidly converted to bicarbonate and water (and back again) to maintain acid base balance in the blood and other tissues. The enzyme responsible for this is the zinc dependent enzyme Carbonic Anhydrase. Studies have shown that dietary deficiency of zinc reduces red blood cell carbonic anhydrase activity
Normal carbohydrate metabolism. Deficiency of zinc results in a drop of metabolic rate. Zinc dependent messenger RNA is needed to synthesise the enzymes required for carbohydrate metabolism so zinc deficiency may result in lack of these enzymes. Zinc may also interact with insulin by controlling the uptake of glucose by adipocytes (fat cells). Zinc deficiency results in impaired carbohydrate metabolism.
Normal cognitive function Zinc is highly concentrated in the cerebral cortex, pineal gland and hippocampus and zinc deficiency is associated with impaired memory formation and mood disorders. In the hippocampus zinc can reach concentrations of 8% of the total brain zinc. Zinc ions are also NDMA (N-methyl-D –aspartate) antagonists (NDMAs control memory function and excessive NDMA activation results in cell death due to excess calcium influx into neuronal cells ) so zinc becomes important for normal neuronal function and memory and delaying brain cell death . Normal fertility and reproduction. Steroid hormones such as testosterone and oestrogen are derived from cholesterol and zinc plays an important role in cholesterol metabolism. Low dietary zinc is associated with low concentrations of several hormones including testosterone.
Testosterone. Circulating testosterone and free testosterone appears to increase with oral zinc intake. In one study supplementing with 250 mg zinc sulphate for 6 weeks increased testosterone by 85% in people on hemodialysis.
Free Testosterone is converted to DHT (dehydrotestosterone) by the enzyme 5alpha-reductase ) primarily in the prostate gland, testes , adrenal glands and hair follicles. DHT is increased in infertile men and as it has an affinity for the hair follicles can result in male pattern baldness. Zinc has been shown to inhibit ( up to 98%)the enzyme 5 alpha reductase.
Semen: Semen is very rich in zinc. Sperm count, motility and physical characteristics of sperm increase and improve with some groups of infertile men.
Zinc deficiency has also been associated with increased expression of oestrogen receptors. The enzyme aromatase converts testosterone to oestrogen and zinc decreases aromatase activity so preventing excessive conversion of testosterone to oestrogen. Zinc deficiency can cause testicular cell death, increase protein oxidation in the testes, dysregulating other enzymes and proteins resulting in degeneration of testicular structures and impaired testosterone secretion.
Why should I Take A Zinc Supplement?
Normal macronutrient metabolism. Macronutrients are carbohydrates, fats and proteins. Zinc is needed for the enzymes that metabolise carbohydrates, fats and proteins
Normal metabolism of fatty acids- zinc is needed for the conversion of linoleic acid to Gamma Linolenic acid (GLA) and for the synthesis of prostaglandins series 1 ( Anti inflammatory prostaglandins) Zinc also plays an essential role in maintaining a balance between to different forms of prostaglandins.
Maintenance of normal serum testosterone concentrations, so involved in fertility and reproduction. Zinc plays a role in cell signalling, influencing hormone release and nerve function.
Normal metabolism of vitamin A. Zinc is necessary to maintain normal concentrations of vitamin A in the plasma, being essential for normal mobilization of Vitamin A from the liver. Zinc deficiency decreases the synthesis of Retinol Binding protein (RBP) in the liver leading to lower levels of RBP in the plasma.It influences the absorption, transport and utilisation of Vitamin A. . Zinc is also required for the enzyme Alcohol dehydrogenase , responsible for converting retinol to retinal, essential for eye function.
Normal protein synthesis. One of the important zinc dependent proteins is Gustin which is involved in taste and smell. Poor or absent gustin levels results in impaired taste and smell. Other important zinc containing enzymes are carboxopeptidase which helps break down protein. Zinc deficiency also impairs the synthesis of the protein Opsin, the precursor of Rhodopsin, which if decreased, results in abnormal dark adaptation of the eye. Zinc is also required for the enzyme alcohol dehydrogenase , responsible for converting retinol to retinal, essential for eye function. Haemoglobin is a protein and zinc s important in haemoglobin synthesis.
Maintenance of normal bones. Zinc regulates the secretion of calcitonin from the thyroid gland and therefore influences bone turnover. Zinc appears to regulate the bone matrix calcification in osteoblasts. Zinc deficiency decreases the activity of matrix proteins, type 1 collagen and alkaline phosphatase decreasing Calcium and Phosphorus accumulation. Therefore zinc deficiency may become a risk factor for poor extra cellular matrix calcification.
Maintenance of normal hair and nails Zinc is needed for building keratin and formation of collagen and for facilitating cell division that makes hair growth possible.
Maintainance of normal skin. Collagen in skin is produced by zinc dependent enzymes , the collagenases. Type 1 collagen is produced in the skin and is a structural long lived protein produced by fibroblasts. Collagen constitutes 70% skin mass and give the skin its structure and resistance to traction and strains. Total collagen decreases 1% a year resulting in decreased elasticity and aging skin. Zinc is essential not only for the enzymes producing collagen but also the cross linking that give collagen its stability. Human studies have shown that decreased zinc resulted in decreased total collagen.
Maintenance of normal vision Zinc supplementation alone significantly reduced the risks of developing AMD in subjects at higher risk. Zinc deficiency also impairs the synthesis of the protein Opsin, the precursor of Rhodopsin, which if decreased, results in abnormal dark adaptation of the eye. Zinc is also required for the enzyme alcohol dehydrogenase , responsible for converting retinol to retinal, essential for eye function.
Contributes to normal function of the immune system. Plays a central role in the immune system affecting cellular and humoral immunity. It is essential for thymic dependent T cells . Zinc deficiency results in decreased levels of all types of white blood cells. It is also required for the production of Thymulin (thymic hormone) Zinc ions also exhibit direct anti microbial activity.
Contributes to protecting the cells from oxidative damage, protecting the DNA, lipids and proteins . Loss of zinc from biological membranes increases their susceptibility to oxidative damage. Zinc is also necessary for the antioxidant enzyme Super Oxide Dismutase (SOD)and low levels of zinc supplementation resulted in increased levels of glutathione peroxidase , SOD and decreased lipid peroxidation.
The process of cell division. Zinc contributes to normal DNA synthesis and cell division. Zinc appears to be essential for Insulin like growth factor (IGF) which induces cell proliferation. Reduced zinc availability appears to affect membrane signalling and secondary messengers that coordinate cell proliferation. Ref : The Role of Zinc in Growth and Cell Proliferation by Ruth MacDonald published In The American Society for Nutritional Sciences Reference
What Are The Symptoms Of A Mild Zinc Deficiency?
- Loss of appetite.
- Poor growth.
- Weight loss.
- Diminished taste or smell.
- Poor wound healing.
- Skin problems, acne, psoriasis atopic dermatitis.
- Poor vision, night blindness.
- White spots on finger nails.
- Depression, apathy.
What Are The Symptoms Of A Severe Zinc Deficiency?
- Delayed sexual and bone maturation
- Skin lesions
- Loss of appetite
- Hair loss
- Increased susceptibility to infections
- Behavioural changes
The passage of zinc into the body
Studies involving direct comparison of bioavailability of different forms of zinc in humans are few. The important fact is that the form of zinc needs to become dissociated into zinc ions which then bind to ligands ( proteins ) that transport the zinc into the cells of the small intestine. There are specific transport proteins that carry zinc across the cell membrane into the portal circulation where it is transported directly to the liver before being released into the circulation for delivery to all tissues. Approximately 70% of zinc is bound to serum albumin ( a plasma protein ) and factors altering serum albumin in turn affect serum zinc levels. Serum zinc has a rapid turnover to meet tissue demands.
Zinc is lost through the skin and kidneys (combined loss of 0.5-0.8mg/day) , more zinc being lost when the body sweats more, as in hot climates and during strenuous exercise. Approximately half of all zinc eliminated from the body is lost through the shedding of epithelial cells in the gastro intestinal tract (0.5- 3mg/day) and although a considerable amount is secreted through both biliary and intestinal secretions, most of the secretions are reabsorbed regulating the zinc balance. Starvation and muscle breakdown also increase zinc loss through the urine.
As already mentioned, protein enhances the absorption of zinc and a phytate rich diet (from cereals, grains, corn and rice) inhibit the absorption of zinc.
There is a very fine balance between zinc and copper. Zinc reduces the amount of copper your body absorbs because copper competes with zinc to bind with metallothionein, the binding protein that brings zinc into the intestinal cells. The ratio of zinc : copper is arguably more important than the concentration of either copper or zinc, a common problem being excessive copper in water from copper pipes or copper cookware.
Zinc also competes with iron to bind with blood transferring, illustrating the importance of a balance of these minerals. The ECRDA for zinc is 10 mg less is required for babies, children and teenagers and more for pregnant and breastfeeding ladies.
Recommended Daily Allowance For Zinc Supplements
Bioavailability Of Different Forms Of Zinc Supplements
There are many forms of zinc compounds.
- Zinc Picolinate 20%
- Zinc Ascorbate 15%
- Zinc Chloride 48%
- Zinc Sulphate 22%
- Zinc Carbonate 52%
- Zinc Citrate 31%
- Zinc Bisglycinate 25%
There is not much substantial evidence of greater effectivity of one form of zinc over another as absorption of zinc in the body is subject to so many variables.
However, a small research study (15 healthy young adults in a randomised, double blind three way cross over study, receiving 10mg of elemental zinc as a supplement without food just published (20 November 2013) found that the bioavailability of zinc citrate was 61.3% , of zinc gluconate was 60.9% and of zinc oxide was 49.9 % Previous zinc intake may affect zinc bioavailability studies. Variables include;
- Existing zinc status of the individual. The lower the zinc status of the individual, the greater the absorption of zinc.
- People that sweat a lot are subject to more zinc loss, for example athletes, those in hot climate, menopausal ladies experiencing night sweats.
- Dosage of zinc- as zinc intake in dosages is increased , percentage absorption decreases probably due to the saturation of the transport mechanisms.
- Zinc absorption appears to be decreased in the elderly.
- Zinc absorption is increased with dietary protein intake.
- The type of protein in a meal affects zinc bioavailability. Animal protein enhances absorption.
- Phytates in cereals and soy inhibit absorption of zinc by binding with it ( except zinc bisglycinate found in Metabolics zinc formula).
- Caesin in milk and calcium inhibit absorption by binding with zinc ions.
- Iron inhibits absorption of zinc.
- Copper ( in high amounts ) inhibits Zinc absorption. In studies using 15mg zinc combined with 2mg copper no inhibition of absorption was found.
- Cadmium- toxic levels of cadmium can inhibit zinc absorption
Types of zinc supplements may remain a personal preference, although generally zinc should not be taken on an empty stomach (as it can result in nausea) should be taken with an animal protein meal , away from cereals and taken in conservative doses to increase absorption. Long term zinc intake is recommended with copper (see zinc formula) as this is zinc bisglycinate, the only form not affected by phytates and balanced with a small amount of copper.