The media has picked up on the hype: vitamin D may actually help prevent the common cold. Or influenza. Or swine flu.

They’ve also been shooting down vitamin D as an immune booster. You can find writers waving away vitamin D’s effects on the common cold or flu at CNN or Medscape.

So which is it?

Typically, when people report on the positive effects of vitamin D on the immune system, they’re reporting the results of population studies. Many studies have shown a correlation between vitamin D levels and incidents of infection, especially of the common cold or flu.1 Not that you need a study to recognize that there is some sort of relationship there — the flu season always comes during winter, whichever hemisphere you’re in. Flu incidence is highest when vitamin D levels are lowest (due to lack of sun).

The question has always been whether that relationship is causal or coincidental.

We know that Vitamin D plays some role in our immune system. We know that, in vitro, the active form of vitamin D enhances various capabilities of important immune system cells.2 But does the effect of vitamin D on the molecular and cellular level actually lead to a stronger immune system and lower incidence of infection?

That question requires more than a population study. The only way we can really show that a supplement aids the immune system is through rigorous intervention studies — ideally, large-scale randomized controlled trials.

The negative media articles I listed above were actually reports of negative results in vitamin D intervention studies already undertaken. Both studies looked at relative incidence of respiratory infection between a control group and a group supplemented with vitamin D. Both studies showed no significant difference.

The thing is, both studies also excluded subjects with “low” vitamin D levels.

The study reported on by CNN used subjects with a mean baseline vitamin D level of 29ng/mL (approximately 70 nmol/L) — essentially a “sufficient” vitamin D level.3 According to data from the National Health and Nutrition Examination Surveys, at least 50% of the United States population have vitamin D levels lower than that!4

The study reported by Medscape similarly excluded patients with not-unusually-low vitamin D levels. Additionally, the amount of vitamin D given to patients was arguably inadequate — half the daily amount recommended by The Endocrine Society.5,6

Other clinical trials have shown a positive results with vitamin D supplementation. A trial with Mongolian schoolchildren found vitamin D supplementation halved the risk of acute respiratory infections.7 A trial on patients with increased susceptibility to respiratory tract infections but no diagnosis found that vitamin D status significantly reduced respiratory infections.8 A trial supplementing schoolchildren in Japan found that the supplemented group experienced incidence of influenza A at 58% the rate of the placebo group.9

More research is needed. None of these studies are conclusive, but they strongly suggest that there is an immune system benefit to making sure you have sufficient vitamin D levels.

And, after all, who has time for the flu?

1: Vitamin D in the prevention of acute respiratory infection: Systematic review of clinical studies

2: Vitamin D and Immune Function

3: Murdoch, D. R., Slow, S., Chambers, S. T., Jennings, L. C., Stewart, A. W., Priest, P. C., … & Scragg, R. (2012). Effect of vitamin D3 supplementation on upper respiratory tract infections in healthy adults: the VIDARIS randomized controlled trial. Jama, 308(13), 1333-1339.

4: “Vitamin D status: United States, 2001-2006.” (2011): 1-7.

5: Vitamin D3 Supplements Do Not Lessen Cold, Influenza Risk

6: Holick, M. F., Binkley, N. C., Bischoff-Ferrari, H. A., Gordon, C. M., Hanley, D. A., Heaney, R. P., … & Weaver, C. M. (2011). Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. The Journal of Clinical Endocrinology & Metabolism, 96(7), 1911-1930.

7: Camargo, C. A., Ganmaa, D., Frazier, A. L., Kirchberg, F. F., Stuart, J. J., Kleinman, K., … & Rich-Edwards, J. W. (2012). Randomized trial of vitamin D supplementation and risk of acute respiratory infection in Mongolia. Pediatrics, 130(3), e561-e567./a>

8: Vitamin D3 supplementation in patients with frequent respiratory tract infections: a randomized and double-blind intervention study

9: RUrashima, M., Segawa, T., Okazaki, M., Kurihara, M., Wada, Y., & Ida, H. (2010). Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. The American journal of clinical nutrition, 91(5), 1255-1260./a>