Vitamin D supplementation did not prevent influenza-like illness as diagnosed retrospectively by questionnaires in subjects participating in randomized clinical trials.
BACKGROUND
Vitamin D deficiency has been associated with a number of diseases, including influenza. Whether or not this reflects a causal relationship is unknown. We therefore wanted to examine if supplementation with vitamin D would affect the incidence and severity of influenza-like disease.


METHODS
Questionnaires on influenza were sent to subjects participating in ongoing placebo-controlled intervention studies with vitamin D supplementation, up until the end of April 2010.


RESULTS
Five hundred and sixty-nine subjects from 10 different clinical trials were included in the study, of whom 289 were randomized to receive vitamin D (1111-6800 IU/day) and 280 to receive placebo. Influenza-like disease during the previous fall/winter was reported in 38 subjects in the vitamin D group and 42 in the placebo group (non-significant), of whom 25 and 26 subjects, respectively, fulfilled our clinical criteria for influenza. In these latter subjects, the duration of illness was significantly longer among those in the vitamin D group than among those in the placebo group (median 7 (range 2-60) days vs median 4 (range 2-18) days; p = 0.007). However, this difference was not statistically significant if all 38 (vitamin D) and 42 (placebo) subjects who reported symptoms were included.


CONCLUSIONS
Our results do not support the hypothesis that high doses of vitamin D supplementation will have a pronounced effect on influenza-like disease in populations not targeted for high influenza risk.
Endocrinology Research Group, Institute of Clinical Medicine, University of Tromsø, and Medical Clinic, University Hospital of North Norway, Tromsø, Norway. rolf.jorde@unn.no
Rolf
R
Jorde
Miles
M
Witham
Wim
W
Janssens
Lars
L
Rolighed
Kyra
K
Borchhardt
Ian H
IH
de Boer
Guri
G
Grimnes
Moira S
MS
Hutchinson
D003625:Data Collection; D019587:Dietary Supplements; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000517:prevention & control*; D012189:Retrospective Studies; D011795:Surveys and Questionnaires; D014807:Vitamin D / Q000494:pharmacology*
D014807:Vitamin D