According to a new study from New Zealand, vitamin D supplementation among adults who already have sufficient blood levels does not prevent colds or ease their severity. However, this might not apply to people with low vitamin D levels who may benefit from additional intake.
In the randomized controlled trial, 322 healthy adults who received an initial dose of 200,000 IU of oral vitamin D3 and one month later started receiving regular monthly doses of 100,000 IU or a placebo for 18 months, thus including two winter seasons, did self-reports on the occurrence of common cold symptoms, such as the sudden onset of a runny nose, nasal stuffiness, sore throat, or a cough not attributed to allergies (1). The participants had an average blood vitamin D concentration of 29 ng/mL (a level above 20 ng/mL is generally considered adequate for bone and overall health in adults). The supplementation increased the levels to over 48 ng/mL throughout the period of the study. The study results showed that the number of reported upper respiratory tract infections did not significantly differ between the groups over the course of the study, nor were there any secondary benefits, such as easing the severity or reducing the duration of symptoms.
The researchers concluded that healthy adults in developed countries who were already keeping their vita-min D levels in the range recommended for bone health did not seem to need any extra intake to prevent upper respiratory tract infections. It would be very important, however, to keep in mind that these findings might not apply to people with lower levels who have an actual vitamin D deficiency. For example, a daily dose of the vitamin has been shown to have a dramatic impact in vitamin D-deficient children, halving the incidence of upper respiratory infections (2). Another trial in adults with chronic obstructive pulmonary disease only showed a benefit of supplementation in reducing exacerbations in vitamin D-deficient individuals with levels under 10 ng/mL (3).
In addition, the study did not determine whether daily dosing would have been any more effective than the monthly doses used in the trial. It was uncertain whether some conditions (e.g. infections) would require a smaller steady dose of vitamin D supplementation for benefit, the scientists commented. Vitamin D is appea-ling as a preventive treatment because of its role in immunity and inducing antimicrobial compounds in the body.