Dear Doctor: I'm hearing that vitamin D can reduce the risk of colds and flu -- and that more foods should be fortified with it. What's the evidence for that?
Dear Reader: As a doctor, I'll confess I'm skeptical about such a broad claim. As I wrote in a prior column, vitamin D is not so much a vitamin as a hormone within the body. It's produced by the body and, further, many foods are rich in the substance. If your vitamin D levels are normal, ingesting additional vitamin D wouldn't seem to help prevent the common cold.
That's not to say such thinking is new. The famed English general practitioner Robert Edgar Hope-Simpson, noting that the influenza virus occurred predominately in the winter, surmised that lack of exposure to the sun may make people more susceptible to influenza. He also noted that in tropical climates, influenza occurs more often during the rainy season when there is a decrease in exposure to the sun. To produce vitamin D, the human body needs sun exposure, so the purported connection between vitamin D and colds seems to have started with that correlation.
Further, a deficiency of vitamin D can lead to decreased white blood cell function, impeding the body's ability to fight off infection. Additional vitamin D stimulates the production of antimicrobial peptides.
As for scientific research on the topic, a 2012 study of 30 patients with chronic lung disease and very low levels of vitamin D (less than 10 ng/ml) showed a 43 percent reduction in exacerbations of the disease when they were given 100,000 units of vitamin D monthly (average 3,200 units per day) for one year. Commonly, these exacerbations are related to infection. A 2015 study showed similar findings of a 43 percent reduction in moderate to severe exacerbations among lung disease patients with vitamin D levels below 20 ng/ml.
However, vitamin D supplementation did not change the upper respiratory infection rate. Also, in both studies, vitamin D supplementation had no effect on people with normal vitamin D levels, i.e., greater than 20 ng/ml.
In another 2012 study, patients with normal vitamin D levels were randomized to receive 100,000 units of vitamin D or a placebo monthly for 18 months. The authors concluded that there was no difference in the rates, or the severity, of upper respiratory infections between those who took vitamin D and those who took a placebo.
The latest study, to which I believe you are referring, was a combined analysis of 25 studies from around the world, including Afghanistan and Mongolia. This analysis linked vitamin D supplementation to a 12 percent reduction in the number of people who developed at least one respiratory infection.
However, in further analyzing the data, the authors found that most of the benefit for vitamin D supplementation was seen in those with levels less than 10 ng/dl, which is very low. The authors did not find any statistical benefit in vitamin D supplementation among people with levels greater than 10 ng/ml. The authors also found that daily or weekly intake of vitamin D had a greater benefit than larger doses given monthly.
In summary, vitamin D does seem to have benefits in decreasing colds and flu in those who have exceedingly low levels of vitamin D -- numbers not seen in the majority of the population. Further, patients with chronic illnesses, elderly people and residents with long, cloudy winters may find that vitamin D supplementation can decrease the frequency of upper respiratory infections.
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