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Zinc: An Essential Trace Element
Zinc is a metal which plays a vital role in human health – it is an essential trace element and is sustained in healthy people within a normal range of concentrations (the exact values depend on age, ethnic origin and geographical factors), but within the UK, the reference range for healthy adults is between 12-25 µmol/l. In healthy people, trace metal levels are maintained by the body through a process called homeostasis; if the body is deficient in a given metal, more of it is absorbed from diet whereas if the body burden of the element becomes too high, it is excreted. Homeostasis maintains trace element status within the desired concentration ranges. Low levels of zinc (<6 µmol/l) have been associated with immune deficiency and also poor wound healing whereas excessive zinc levels (>40 µmol/l) are known to cause renal failure.
Zinc is found in the earth’s crust and consequently is present in soils and water bodies to an extent which will depend upon geological factors. Zinc is taken up from soils by plants. However, the zinc contained in certain plants may only be poorly absorbed in the human gut (notably plants that contain phytates) which can lead to zinc deficiency in certain regions. Human needs for zinc are usually satisfied by dietary sources (including meat from animals which graze on grass). In some individuals, the body is unable to absorb enough zinc from the diet (due to an “inborn error of metabolism") and a condition known as acrodermititis enteropathica develops. In this condition, sores and lesions appear on the skin; cuts and other injuries heal slowly, the patient may suffer hair loss, diarrhoea and late onset of puberty. Fortunately, these symptoms can be completely reversed by administration of zinc sulphate. Zinc is known to play a role in over 300 metallo enzymes.
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Ascorbic Acid: An Essential Vitamin
Vitamin C is found predominantly in citrus fruits (oranges, lemons, grapefruits), but is encountered in a wide range of fruits and vegetables. The chemical name for vitamin C is ascorbic acid. Vitamin C is an antioxidant (meaning that it stops things reacting with oxygen species or entities known as free radicals) within the body’s cells. It is believed that antioxidants may have a role in protecting the body from a range of conditions from cancer to heart disease and aging. Vitamin C is a water soluble vitamin that comes from diet (it cannot be synthesised by the body, nor does the body store it). Since the body does not store vitamin C, toxicity is very rare, but diets that do not contain enough vitamin C are problematic – this has been known for centuries since lack of vitamin C (due to the paucity of fresh food) afflicted sailors on long voyages of discovery. Symptoms of vitamin C deficiency may include: inflamed, or bleeding, gums; nose bleeds; easy bruising; dry, flaky skin; poor wound healing; reduced immunity and anaemia.
Some of the effects of insufficient dietary vitamin C are similar to those associated with zinc deficiency. So, are zinc absorption and vitamin C intake linked? Certainly, evidence has been shown that the uptake of non-heme iron (i.e. iron from non-animal sources; not associated with blood) in subjects that have fasted prior to the experiment is greatly enhanced (although the effect is not pronounced when dietary vitamin C is increased in non-fasting individuals). Vitamin C is an organic acid (ascorbic acid), so it will form complexes with metals. The bioavailability of zinc (or any other trace metal) is dependent upon its exact chemical form – its speciation – and it is this that dictates nutritive or toxic behaviour within the body. Zinc absorption and vitamin C consumption will probably cause the zinc to be present as zinc citrate which is likely to be a more bioavailable source than zinc associated with phytates from cereal crops. In these moieties, the zinc is so tightly bound that little of it is released in the gastrointestinal tract and most of the zinc consumed in this form is simply excreted.
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- Zinc: The biology and therapeutics of an ion, Editorial, Annals of Internal Medicine, July 15, 1996 vol. 125 no. 2 142-143
- US National Library of Medicine, Medline Plus, Vitamin C: http://www.nlm.nih.gov/medlineplus/ency/article/002404.htm