Supplementation with vitamin D3 does not reduce the incidence or risk for upper respiratory tract infection (URTI) in adults, according to a new randomized controlled trial published onlineSeptember 6 and in the November 15 print issue of Clinical Infectious Diseases.

Judy R. Rees, MPH, PhD, from the Department of Community and Family Medicine, Section of Biostatistics and Epidemiology, Geisel School of Medicine at Dartmouth, Hanover, New Hampshire, and colleagues enrolled 2259 participants (age, 45 – 75 years) who were also taking part in a colorectal adenoma chemoprevention trial. Participants were randomly assigned to receive 1000 IU/day of vitamin D3, 1200 mg elemental calcium/day, both, or placebo. All participants were in good health and had serum 25-hydroxyvitamin D levels of 2 ng/mL or higher.

Among the 759 participants who completed the study, the researchers found no significant decrease in the rate ratio (RR) of URTI episodes between the treatment groups (RR, 0.93; 95% confidence interval [CI], 0.79 – 1.09) or winter days of illness (RR, 1.13; 95% CI, 0.90 – 1.43). There was also no decrease noted in composite syndromes of influenza-like illness (ILI; RR, 0.95; 95% CI, 0.62 – 1.46) or colds (RR, 0.93; 95% CI, 0.78 – 1.10).

“Vitamin D supplementation conferred no significant protection against colds, ILI, or any URTI overall, nor among those with the lowest baseline serum 25(OH) vitamin D, although participants whose baseline concentration was <12 ng/mL were specifically excluded from our trial,” note Dr. Rees and colleagues.

Participants were recruited from 11 clinical centers, and the study was conducted November 2009 through March 2011. URTI was defined as either ILI (fever and 2 or more of the following: sore throat, cough, muscle aches, or headache) or a cold (absence of ILI, 2 or more of the following on a single day: runny nose, nasal congestion, sneezing, sore throat, cough, and swollen or tender neck glands).

Throughout the study period, researchers administered semiannual telephone surveys to 2228 participants and found no decrease in the odds ratio (OR) of ILI (OR, 1.14; 95% CI, 0.84 – 1.54) or colds (OR, 1.03; 95% CI, 0.87 – 1.23) among patients receiving vitamin D3 supplementation. Baseline vitamin D status, body mass index, adherence, or influenza vaccination also provided no significant benefit.

The researchers acknowledge that self-selection of the 759 participants from the parent trial may have influenced results if participants dropped out early because of a lack of treatment effect. The study authors also note that self-reported adherence to study protocol and lack of laboratory conformation of URTI may also have affected the results.

Michael Gleeson, PhD, from the School of Sport, Exercise and Health Sciences, Loughborough University, Leucestershire, United Kingdom, told Medscape Medical News, “Although participant numbers were large, I suspect that this dose of vitamin D3 is insufficient to affect respiratory illness incidence in individuals who are not vitamin D deficient,” and that “an effect might be seen in a more illness-prone population such as athletes.” Dr. Gleeson was not affiliated with the study.

“The effects on URTI of supplementation in adults with vitamin D deficiency (<12 ng/mL) should be addressed in future trials,” conclude the study authors. Studies should also address at what dose of vitamin D3 affects “markers of immune function that are important in defense against respiratory infections,” added Dr. Gleeson.