Researchers reached this conclusion in a new review of studies involving more than 6,000 children of all ages.

“Our most important finding is that there is strong evidence that zinc supplementation benefits children suffering from diarrhea in developing countries, but only in infants over six months old,” said lead investigator Marzia Lazzerini, M.D. “Zinc reduces acute diarrhea duration in terms of mean duration and risk of diarrhea at given days. Zinc also reduces the duration of persistent diarrhea.”

Nearly 30 percent of children in the world are zinc deficient, Lazzerini said. She is a pediatrician with the World Health Organization (WHO) Collaborating Centre for Maternal and Child Health in Trieste, Italy.

Zinc deficiency is due primarily to inadequate dietary intake. Relatively expensive foods — such as meat and fish—contain high levels of zinc, the authors say. Although zinc also appears in cheap foods such as nuts, seeds, legumes and whole grain cereal, these foods can reduce absorption of zinc by the body. Zinc cannot be stored in the body.

“The addition of zinc to children in developing countries is rational and appropriate,” said William Cochran, M.D., who had no affiliation with the review. “Diarrhea is a contributing factor to the death of millions of children every year throughout the world. It contributes to death in children in the U.S. much less frequently, 100 to 500 [cases] per year,” said Cochran, a pediatric gastroenterologist and nutritionist and chair of the Department of Pediatrics at the Geisinger Clinic in Danville, PA.

The review appears in the current issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.

The reviewers identified 18 randomized controlled trials enrolling 6,165 children and comparing zinc treatment with placebo. Thirteen trials focused on treatment of acute diarrhea and the other five on treatment of persistent diarrhea.

The reviewers found zinc was effective for diarrhea in children over six months old. By the third day of zinc treatment, children were 31 percent less likely to suffer from acute diarrhea (in two studies) and were 45 percent less likely to do so after five days of treatment (also in two studies), Lazzerini said.

“This benefit withstood extensive subgroup analysis for nutritional status, geographic region, background zinc deficiency, zinc type and study setting,” the authors say.

Zinc also reduced diarrhea at by 29 percent after a week of treatment. Diarrhea at day seven can be a signal of persistent diarrhea, which can lead to severe dehydration and death.

Two of the studies included children younger than six months. The results showed no evidence of a therapeutic effect of zinc treatment in these children.

The authors said there were insufficient data to gauge the influence of zinc treatment on death, since the studies under review did not measure mortality. “The trials were not designed to look at hospitalization and mortality, but given these results, it’s expected that a policy of zinc supplementation to all children over six months with diarrhea in developing countries could also reduce hospitalization rate and mortality,” they concluded.

“Anything that can be done to limit diarrhea and, as a result, not limit the consumption of regular food may be of help in improving the high death rate associated with diarrhea in underdeveloped countries,” Cochran said.

Lazzerini M, Ronfani L. Oral zinc for treating diarrhoea in children (Review). Cochrane Database of Systematic Reviews 2008, Issue 3.

The Cochrane Collaboration is an international nonprofit, independent organization that produces and disseminates systematic reviews of health care interventions and promotes the search for evidence in the form of clinical trials and other studies of interventions. Visit http://www.cochrane.org for more information.