The vitamin is thought to aid the immune system in fighting certain infections and inflammations, such as measles and infections caused by some food-poisoning organisms.
Indeed, the impact of vitamin A supplements on diarrhoea in children is reported to be due to an effect on the immune response in the intestine. The body's response to a gastrointestinal infection by organisms such as E. coli is inflammation, which reduces the colon's ability to absorb water and results in diarrhoea. In the developing world, diarrhoea is said to b the most common cause of death of young children, with over 1.5 million deaths annually.
The new science, published in the current issue of the Journal of Nutrition (Vol. 136, pp. 2600-2605), looked at the effect of vitamin A supplements on levels of the molecule, monocyte chemoattractant protein-1 (MCP-1), which is associated with a state of increased inflammation and is also involved in the pathogen-specific mucosal immune response.
The researchers, from ten different universities and hospitals in the US and Mexico including Harvard, the University of Texas, and the Universidad Autonoma de Queretero, recruited 127 Mexican children between the ages of 5 and 15 months and randomly assigned them to receive either a vitamin A supplement or a placebo at two month intervals.
The age of the child determined the dose of the supplement, with children under one year of age receiving bi-monthly doses of 20,000 international units (IU) of retinol, and children over one receiving 45,000 IU.
Stool samples were collected during the summer months were analysed for MCP-1 concentrations as well as gastrointestinal pathogens and symptoms of diarrhoea.
"Overall, children who received the vitamin A supplement had reduced fecal concentrations of MCP-1 compared with children in the placebo group (median pg/mg protein: 284.88 versus 403.39, [respectively])," reported lead author Kurt Long from Harvard School of Public Health.
Reduce levels of MCP-1 is associated with less inflammation, which in term suggests less diarrhoea.
The supplementation also impacted on MCP-1 levels in children with infections, like the bacteria Escherichia coli or the human roundworm Ascaris lumbricoides. For children infected with E. coli and given the vitamin A supplement, MCP-1 levels were 62 per cent lower than the placebo group, while children infected with A. lumbricoides had MCP-1 levels 38 per cent lower after vitamin A supplementation than placebo.
"These findings suggest that vitamin A has an anti-inflammatory effect in the gastrointestinal tract by reducing MCP-1 concentrations," concluded the researchers.
Further research is needed, particularly in other populations and age groups, as well as detailed mechanistic studies to find how the vitamin A impacts on MCP-1 levels.
Vitamin A deficiency (VAD) is a public health problem in more than 50 per cent of all countries, especially in Africa and South-East Asia, according to the World Health Organisation, and causes blindness in up to 500,000 children each year. The human body converts beta-carotene in the diet into vitamin A.