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Most studies linking vitamin D deficiency to various maladies, including respiratory infections, are observational. Because wintertime vitamin D deficiency is common in Finland, researchers there were able construct a rigorous blinded interventional study, repleting vitamin D in otherwise healthy adults and examining effects on days of work lost because of respiratory disease.
The researchers enrolled 164 healthy male army conscripts (age range, 18–28) and gave them vitamin D (400 IU) or placebo daily from October through March. At the start of the study, mean serum 25-hydroxyvitamin D [25(OH)D] concentrations in both groups were slightly less than 32 ng/mL (80 nmol/L) — considered in the “insufficient” range. During the study, levels remain…