NEW YORK (Reuters Health) - Heavy supplementation of vitamin D did not reduce diarrheal illnesses among children aged three and younger in a large trial conducted in Kabul, Afghanistan.
The results are disappointing in light of a body of research that has consistently linked high vitamin D levels to improved immune system function and deficiency to a higher risk of infectious diseases, the researchers write in the journal Pediatrics.
"It was thought that vitamin D supplementation would help in the prevention of diarrheal illnesses," lead author Dr. Adam Aluisio of the Department of Disease Control at the London School of Hygiene and Tropical Medicine said.
"Vitamin D plays a role in the body's defenses against microbes such as viruses and bacteria - we know this from biochemical studies," he told Reuters Health.
But previous studies have relied on laboratory experiments and observations from the field, both of which fall short of medicine's gold standard - the randomized clinical trial. Aluisio and colleagues found evidence of only one other vitamin D trial for diarrhea that was conducted with fewer than 50 children in Bangladesh.
Around the world, and especially in poorer countries, diarrheal diseases kill 1.8 million children under the age of five every year, according to the World Health Organization.
Supplemental vitamin A and zinc have been shown to reduce diarrheal illnesses in children, and researchers have hoped that vitamin D might be another tool to protect young children.
For their study, Aluisio's team extracted data from a trial conducted between 2007 and 2009 that looked at vitamin D's possible effects on childhood pneumonia. During that trial, the researchers, who include some co-authors of the current study, recorded diarrhea incidents as well.
More than 3,000 babies under one year were randomly assigned to one of two groups. The first group received 100,000 international units of vitamin D3 in liquid form once every three months for almost two years. The second group received a placebo for the same time period.
Field workers collected data from participating families twice each week, asking parents whether their child had experienced diarrhea in the previous 24 hours.
Both groups of families averaged roughly three diarrhea incidents per child, per year. There were no differences between the babies who got the vitamin D and those who didn't in the number of diarrhea bouts, the average time until the first bout, or in other factors such as malnutrition and how often mothers washed their hands.
The researchers confirmed that the children who received the supplements did have higher blood levels of vitamin D than the comparison group. But what constitutes a healthy level of vitamin D for optimal immune function may be different than current standards, which are for bone health, the authors point out in their report.
In global public health, which seeks cheap and simple solutions to common problems like childhood diarrhea and malnutrition, there is more work to be done, Aluisio said, adding: "We need to know more about how vitamin D supplementation works in the body in relation to disease."
In poor regions, even those with sunny climates like Afghanistan, vitamin D deficiency is a common problem, said Dr. Robert Black, director of the Institute for International Programs of Johns Hopkins Bloomberg School of Public Health in Baltimore.
"It's a study well done," said Black, who was not involved with the research. "Even though it doesn't show a benefit, it's a step forward."
"Now we can move on and focus on other minerals and vitamins that may prevent these infections," he said, adding that zinc and vitamin A are two strong contenders for diarrhea prevention.
Vitamin D remains "important for bone health, calcium metabolism and possibly immune function," Black told Reuters Health. He stated that this study answers the question regarding any links between vitamin D and diarrhea for this age group living in poor, urban regions.
Aluisio said more studies should be done on different age groups. He pointed to a study from Bogotá, which found that among children with an average age of nine, the risk for gastrointestinal illnesses doubled when a vitamin D deficiency was present.
"I don't think this proves that there is no effect on diarrheal diseases," Aluisio said. "It does highlight that we don't know enough about the immune function and potential protective effect of vitamin D especially in relation to different causes of diarrheal illnesses in children."
SOURCE: bit.ly/180HCr3 Pediatrics, online September 9, 2013.