This meta-analysis is fairly amazing and lends strong support to the idea that vitamin D metabolism affects more than bones alone. For years, we have known that vitamin D is closely linked to calcium absorption and metabolism, and that vitamin D deficiency can lead to osteoporosis and muscle weakness. More recently, epidemiologic studies have shown a relationship between low levels of vitamin D and illnesses such as colon, breast and prostate cancer, hypertension, multiple sclerosis, diabetes, dementia, pain syndromes, arthritis, and congestive heart failure. ¹ The challenge with interpreting data from these observational studies is determining whether a causal relationship exists between the low vitamin D levels and the outcome in question, and randomized trials have not supported a causal relation in many of these instances. The current study strongly supports correcting vitamin D deficiency to protect against URIs. Interestingly, a recent Cochrane review concluded that there is high quality evidence that vitamin D decreases asthma exacerbations by 40-60% in individuals with mild to moderate asthma.² The effect on asthma may be mediated by a decrease in URI with vitamin D supplementation. Given the small or non-existent risk with vitamin D supplementation, and the high benefit in individuals who are vitamin D deficient, we may consider using this knowledge tomorrow in our patients who have or are at risk for vitamin D deficiency and/or have respiratory illness that increases their risk if they get an upper respiratory infection. —Neil Skolnik, MD

1. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357:266-81.
2. Martineau AR, Cates CJ, Urashima M, et al. Vitamin D for the management of asthma. Cochrane Database of Systematic Reviews. 2016(9). doi:10.1002/14651858.CD011511.pub2.