At the start of the study, all the 250 children who participated were very low in vitamin D. The children drank locally produced milk with or without 300 IU of added vitamin D per day. Over the course of the winter, parents of children in the vitamin D group reported about half the number of respiratory infections as parents of kids in the placebo group.
In discussing their findings, doctors said the large benefit they saw was due to the very low levels of vitamin D at the start of the study, and suggest further study to determine the best level of vitamin D.