Vitamin D in moderation protects against respiratory infections: Meta-analysis According to researchers, the study is the most comprehensive yet to focus on vitamin D and respiratory infections and could have significant implications for healthcare providers. A new study, published in The Lancet Diabetes & Endocrinology, has found that daily doses of 400-1000 IU are associated with protection against acute respiratory infections, but that higher doses are not. Four years ago, a meta-analysis of data from 25 randomized controlled trials of vitamin D supplementation for the prevention of acute respiratory infections showed a protective effect of this intervention. Now, the same researchers from, amongst other institutes, Karolinska Institutet, Harvard Medical School and Queen Mary University of London, have expanded the earlier research with an additional 18 studies and updated meta-analysis.