The study followed nearly 2,800 children, ages 5-12 over the course of a year in the US.
Researchers at the University of Michigan School of Public Health and Michigan State University found that children with less vitamin A in their blood got sick more often with diarrhoea with vomiting and cough with fever.
"No study had estimated the potential role of vitamin A alone in this age group" of children between 5 and 12, said Dr Eduardo Villamor, senior author of the study.
The researchers also studied other micro-nutrients that are important to the immune system, including zinc, iron, folate and vitamin B12. But only vitamin A was related to illness.
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The study found that for every 10 micrograms per decilitre of vitamin A present in the blood, children experienced 18 per cent fewer days with diarrhoea and vomiting, 10 per cent fewer days of cough and fever, and 6 per cent fewer visits to the doctor.
Villamor said the effects of vitamin A could vary in different countries due to differences in diet and the microorganisms that cause illness.
He said that it's uncertain whether giving children vitamin A pills will be an easy solution.
The study was published in the Journal of Nutrition.