Vitamin D’s immune benefits: Cold, flu and beyond

As part of our special edition on immune health, we look into the wider immune health benefits of vitamin D.

Results of the VIDARIS Randomized Controlled Trial indicated that a monthly dose of 100,000 IUs of vitamin D3 for 18 months was associated with the same number of URTIs per participant, the duration of symptoms, and the number of days of missed work as the placebo group (JAMA​, Vol. 308, pp. 1333-1339).

In an accompanying editorial, Jeffrey Linder, MD, from Brigham and Women’s Hospital and Harvard Medical School, Boston, said the results of the VIDARISstudy “suggest that vitamin D should join the therapies listed in the Cochrane reviews as being ineffective for preventing or treating upper respiratory tract infections in healthy adults.”​

Such statements could be interpreted as dismissing the overall immune health benefits of vitamin D. The Office of Dietary Supplements (ODS) confirms that vitamin D a role in immune function.

The vitamin​

Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive.

Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.

A recent review by scientists from the University of Catania, Italy examined the role of 1,25(OH)2D as an immuno-modulator (Immunology​, Oct 2011, Vol. 134, pp. 123-139).

“Immune cells are not only targets for active 1,25(OH)2D3, but are able to activate this hormone in a local fashion, arguing for its […] role within the immune system,” ​wrote the Catania-based reviewers.

First reports​

The importance of vitamin D for activating the immune system was reported for the first time in 2010 by scientists from the University of Copenhagen. Writing in Nature Immunology​ (doi: 10.1038/ni.1851), the scientists reported vitamin D is necessary to trigger T cells – the immune system’s killer cells – into action, and insufficient levels of the vitamin mean the cells remain dormant and inactive.

And despite the recent JAMA​ results, the the first population-based study to evaluate and demonstrate an association between serum 25(OH)D level and upper respiratory tract infections was published in 2009, and it found that increased levels of vitamin D may protect against common respiratory infections such as cold and flu.

Writing in the Archives of Internal Medicinem researchers from the University of Colorado, Denver reported that people with the lowest average levels of vitamin D were about 40% more likely to have a recent respiratory infection, compared to those with higher vitamin D levels.

“The association seems to be robust, with a clinically and statistically significant association present in all seasons and when controlling for potential confounders,” ​they wrote.

Inadequate science?​

The science is developing but it seems to have a ways to go before it is adequate to force a change in policy.

According to the ODS, recent discussion over changes to DRIs for vitamin D concluded that the health relationships examined, including immune response, were “either not supported by adequate evidence to establish cause and effect, or the conflicting nature of the available evidence could not be used to link health benefits to particular levels of intake of vitamin D or serum measures of 25(OH)D with any level of confidence”​.