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Nutrient - Zinc - Essential trace element

By Nutri People

Zinc is an essential nutrient and the second most abundant trace mineral in the body after iron. The importance of zinc for the normal growth and survival of animals and plants was recognised a long time ago. Despite this, it was not until the early 1960s that zinc was recognised as essential for humans. Since then, zinc has been shown to play an important role in an enormous number of bodily processes. Zinc is widely distributed in the body – in bones, teeth, skin, hair, liver, muscle, immune cells, and testes – with some tissues, such as the prostate gland in men, having very high concentrations. Such a wide distribution suggests that zinc plays an important role within us.
 
The importance of zinc
 
Zinc is a key mineral for cell and tissue renewal. As such, it plays an important role in supporting normal growth and repair of body tissue. Zinc is also important for blood clotting, sperm production and the formation of the male sex hormone called testosterone. It also acts as an antioxidant, and helps to maintain proper vision and the normal sense of taste and smell.
 
Zinc is necessary for the functioning of more than 200 enzymes. Some of these enzymes have antioxidant functions, protecting us against damage by free radicals, and others are important for the breakdown and use of carbohydrates, fats and proteins. Zinc is also required for the enzyme that converts vitamin A to retinal, a form of vitamin A necessary for night vision.
 
Proteins within us play very important roles in ensuring our health and well-being, and many require the presence of zinc to perform these important roles. Examples of such proteins include vitamin D receptors, sites within our bodies where vitamin D is able to attach itself and exert its many positive health effects. Our cell membranes, the ‘skins’ of the body cells that hold the cell together, also contain zinc, which provides both structural support and antioxidant protection. Zinc also plays a role in supporting some of the cells of our immune system (called T-cells), which function to fight off infections, and within the hormonal system, zinc has been shown to support insulin activity. Recently, research has also shown that areas of the brain that play important roles in learning, memory and our emotions contain high levels of this mineral.
 
Zinc absorption
 
Because zinc is an essential trace mineral and our bodies do not produce it, it is important to make sure that our daily intake is high enough. Zinc is found in both animal and plant foods. However, zinc in plant foods is not as available for use by the body as the zinc from animal foods. Rich sources of highly absorbable zinc include meats, organ meats, poultry (especially dark meat), fish and crustaceans. Eggs and dairy foods are also good sources of absorbable zinc.
 
Plant foods (wholegrain breads, unrefined cereals and grains, legumes, nuts and seeds) have modest levels of zinc, but also contain substances called phytates (also known as phytic acid). Phytates bind to zinc in the intestine, forming a zinc-phytate complex, which is large, insoluble and poorly absorbed. Because the high levels of phytates in plant foods reduce zinc absorption, strict vegetarians/vegans, whose main food sources of zinc include grains and legumes, may have greater zinc requirements than non-vegetarians. Fermented foods, including leavened grain products (such as bread) and fermented cereal porridges, have a lower phytate content because the fermenting organism produces enzymes that are able to break down the phytate. As a result, the zinc present in these foods is likely to be absorbed more effectively.
 
Zinc also has the potential to interact with a number of other nutrients and this interaction may influence its absorption. For example, high intakes of calcium and iron may reduce zinc absorption, whereas the amino acids histidine, cysteine and methionine may increase it.
 
Food sources of zinc
 
There are a wide variety of zinc food sources available. The following table lists some of these dietary sources and shows the average amount of zinc present in milligrams (mg) per 100g portion.
 

 

Food
Average Zinc Content
(mg/100g)
Organ meats (liver, kidney)
5.2
Meat (beef, pork)
3.8
Poultry (chicken, duck)
2.4
Chicken/turkey (dark meat, no skin, cooked)
4.4
Chicken/turkey (light meat, no skin, cooked)
1.2
Seafood (without oysters)
2.9
Cod (cooked)
0.6
Halibut (cooked)
0.6
Tuna (can/oil/drained)
0.9
Tuna (can/water/low or no sodium)
0.8
Oysters
11.3
Crab
5.4
Dairy products
1.8
Milk
0.4
Cheese (cheddar)
3.2
Cheese (cottage, uncreamed)
0.5
Yoghurt
0.8
Eggs (chicken, duck)
1.3
Seeds and nuts
5.4
Almonds
3.5
Cashews
5.6
Peanuts
3.2
Bread (white flour and yeast)
0.9
Wholegrain cereal
1.9
Beans, lentils
1.5
Refined cereal grains (i.e. white flour, white rice)
0.6
Vegetables
0.5
Fruits
0.1
Potatoes
0.3
 
Zinc deficiency
 
The general causes of zinc deficiency include a low intake, increased requirements and increased losses from the body.
 
Zinc deficiency may also develop in those with malabsorption syndromes, bowel inflammation diseases, diabetes mellitus, sickle cell disease and injuries (e.g., severe burns or other trauma). Groups at risk of zinc inadequacy include alcoholics, vegetarians, pregnant and lactating women, older infants who are exclusively breastfed and the elderly. Additionally, certain prescribed drugs have the potential to reduce zinc absorption, and may predispose to zinc deficiency, including quinolone and tetracycline antibiotics, certain metal chelating agents, anticonvulsant drugs and diuretics.
 
While severe zinc deficiency is uncommon, signs of a marginal zinc deficiency may include:
  • Poor appetite
  • Fatigue
  • White spots on the fingernails
  • Increased susceptibility to infections
  • Decreased sense of taste or smell
  • Delayed wound healing
  • Mouth ulcers
  • Decreased ability to see at night
  • Skin problems
  • Loss of hair
 
Are there risks associated with too much zinc?
 
Despite the many health-supporting effects of zinc, it is important not to consume too much of it. Very high levels of zinc can actually interfere with the absorption of copper, which can lead to a copper deficiency. Excess intakes can also cause anaemia and impair the immune system.
 
Recommended intakes
 
The RDA (recommended daily allowance) for zinc is set at 15 mg/day and is the amount needed to prevent deficiency, not necessarily the level required to promote optimum health, which may actually be higher. 

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