The beneficial properties of zinc have been appreciated for many years, generations of mothers have applied zinc cream to cure nappy rash and it is common knowledge that zinc helps heal wounds. At least ten years ago the importance of dietary zinc supplements in the recovery from severe malnutrition was documented in Jamaica and more than 30 years ago it was recognized that severe zinc deficiency was responsible for dwarfism and failure to mature in young Iranian and Egyptian youths eating a diet consisting largely of whole meal bread. Now we have realized that lesser degrees of zinc deficiency are more common than was appreciated and the subclinical deficiency of zinc contributes to an increased incidence and severity of common but important infections such as diarrhea and pneumonia.

Diarrhea is one of the clinical manifestations of zinc deficiency in humans and animals. The syndrome Acrodermatitis enteropatica received its name because diarrhea was a cardinal feature. In this inherited congenital condition there is an abnormality in the intestinal absorption of zinc. This can be overcome by giving large oral doses of the mineral which result in the rapid disappearance of all symptoms including the diarrhea. In a clinical study Bangladesh, children who were hospitalized with persistent diarrhea had less diarrhea and showed improvements in mucosal permeability after supplements of zinc were administered. Diarrhea is associated with an increased loss of zinc in faeces so that one would expect that children with diarrhea would be at an increased risk of zinc deficiency.

 In recent years a number of other studies have corroborated these preliminary findings in children with both acute and persistent diarrhea. One of the first of these studies, in India, found that children with acute diarrhea got better faster when they received a zinc supplement. Other trials from different parts of the world have shown similar benefits and recently the data from these studies have been combined and demonstrate a consistent significant and clinically important benefit of supplements containing 20 mg elemental zinc in children aged under three years who present with acute diarrhea. Similar results have been found in studies of children with persistent diarrhea, including a study undertaken in Lima in which children with initial low plasma zinc levels who received zinc as opposed to placebo recovered more rapidly from their diarrhea. The large number of children included in the combined analysis was able to show that the beneficial effects of zinc were seen in children of both sexes and are evident whenever the supplement is administered during the episode. The positive response was not limited to malnourished children nor to any particular age group. These findings suggest that zinc supplements may represent an important therapeutic advance complementing the oral rehydration therapy which is currently recommended to prevent dehydration but which does not reduce the duration of the diarrhea.

Despite the widespread presence of zinc in common foods dietary deficiency is common because biovailability of zinc is reduced by the coexistence of inhibitory substances such a as fiber and phytates in food of vegetable origin. These substances inhibit the absorption of zinc. The diets of the poorest populations are often lack zinc being dependent on vegetable staples with little access to animal products, especially flesh products such as meat, fish and offal which are the richest sources of easily assimilated zinc. Children in these communities do not manifest obvious signs of zinc deficiency but have often been found to have low plasma zinc levels compatible with subclinical zinc deficiency. Thanks to recent investigations we now believe that this deficiency may be contributing to the high illness rates which are typical of these populations.

Studies in Vietnam, Mexico, Guatemala, India, Jamaica, Papua New Guinea and Peru have evaluated the impact of daily oral zinc supplements given to small children. These studies, in a combined analysis, demonstrate not only substantial benefits in diarrhea; a reduction in the severity and duration of episodes of diarrhea, and fewer episodes of persistent diarrhea and dysentery but also fewer episodes of respiratory illness notably less pneumonia the primary cause of infant deaths in most developing countries. The prevalence of days with fever and anorexia was also reduced by zinc supplementation. In the study in Lima the group of children receiving a supplement of 20mg of elemental zinc showed consistent reductions in illness rates and in the prevalence of symptoms of the same magnitude as other studies but in this case the differences did not reach statistical significance. In order to ensure that deficiencies of other micronutrients did not limit the beneficial effects of zinc this study included a group of children receiving zinc together with other vitamins and minerals including iron at levels which provided their daily requirements. In this group of children the improvements with zinc were not evident and in some cases there was a tendency for more illness. This serves as a warning that there is much still to learn about mixtures of micronutrients and we cannot assume that such mixtures will provide the same benefits as zinc alone or in tried and tested formulations.

Diarrhea and pneumonia are almost universal, responsible for much infant morbidity and mortality and of enormous public health importance but other diseases such as malaria cause millions of preventable adult and child deaths in many parts of the world. A study in a community in Papua New Guinea compared children receiving either zinc supplements or placebo and found that those receiving zinc had 40% less visits to the health centre for malaria attacks , especially those due to Plasmodium falciparum. If these findings are confirmed by other studies in different countries this could provide a much needed weapon in the fight against malaria. Zinc has also been reported to be of benefit in a wide variety of other conditions varying from prostatism, to acne and the common cold and these are the subject of current investigation.

How can these widespread and apparently dramatic benefits of zinc supplementation be explained? Zinc is essential for more than 300 enzymes, structural proteins and hormones. It is needed for diverse physiological processes and metabolic functions including many aspects of the immune system. Lack of zinc is associated with atrophy of the thymus, a gland which has a role in the maturation of lymphocytes, and it seems that the function of "T" lymphocytes are especially vulnerable to deficiency of this mineral. Zinc deficiency there is a reduction in delayed type hypersensitivity, and other immune responses mediated by T cells. Reduction in the production of antibodies and in the function of natural killer cells have also been reported in experimentally induced zinc deficiency in animals. In addition zinc is needed for cell replication and is thus essential for the regeneration of the intestinal mucosa, the healing of wounds and the turnover of epithelial cells necessary to maintain healthy skin. The combination of these essential functions explains why zinc plays such an important part in the protection against infections.