A crucial component of the immune system, zinc has also been credited with speeding up recovery from colds. Dr Wynnie Chan is not convinced

Zinc is a major mineral, needed for the functioning and formation of many enzymes within the body. It has a role to play in the metabolising or breaking down of proteins, carbohydrates and fats, but it is not produced or stored in the body and so must be provided by diet alone.

Zinc is needed for the formation of the building blocks of genetic material. It helps with the healing of wounds and is needed to form the structures of hormones such as insulin and growth hormone and for the functioning of the immune system.

Zinc in the diet
Zinc is found in a wide variety of foods but especially oysters, meat and dairy products. In general, red meat contains a higher amount of zinc than white meat. Whole grains and legumes provide moderate amounts of zinc. As a rule, the more refined the cereal the less zinc it contains, since zinc is mainly found in the outer layer of the grain.

Generally, around a third of the zinc from diet is absorbed but if a high fibre diet containing plenty of wholegrain cereals, legumes and excess amounts of bran are eaten, absorption decreases. This is because wholegrain cereals and legumes contain compounds known as phytates that can bind with zinc and inhibit the mineral from being absorbed by the body. Animal protein improves absorption.

Deficiency of zinc affects the mouth, skin and gut lining. Reduced taste sensation due to the loss of taste buds is commonly associated with zinc deficiency, as is impaired immune function and wound healing and growth retardation in children. The risk of zinc deficiency is higher among people who do not eat meat and animal products and who also consume high amounts of wholegrain cereals and legumes. The recommended daily intake for zinc is 7.0mg a day for adult women and 9.5mg a day for adult men.

Taking over 30mg a day of zinc is not recommended. Large amounts have been shown to cause nausea, vomiting, diarrhoea and fever. Long-term high intake of zinc may interfere with the absorption of iron, copper and manganese as well as impair immune function. Caution should be exercised in taking large amounts of zinc from dietary supplements.

Zinc and the common cold
There have been a number of studies suggesting that zinc can be effective in the prevention of the common cold by, for example, inhibiting the viruses from replicating or by boosting the immune system. In a recent study published in the Annals of Internal Medicine in 2000, volunteers who sucked a zinc lozenge every two to three hours for several days reduced the duration of their cold symptoms by four days. A 2006 study involving 200 children found that taking zinc sulphate syrup once daily significantly reduced the number of colds and cold-related school absences compared with those taking placebo, and that those who did catch a cold had a shorter duration and reduced severity of symptoms. However, a number of other studies did not show any positive effects, so at present, there is not enough scientific evidence to recommend the use of zinc as a prophylactic treatment for the common cold.