Can Sunshine Stave Off the Flu?

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Here’s another reason to soak up the sun this summer: new research shows that Americans’ vitamin D levels fluctuate with the seasons, putting you at a higher risk of a deficiency during the winter months.

That may explain the flux in flu symptoms during the winter in the US, since low levels of vitamin D may impair the body’s “innate immunity”, or the first line of defense against pathogens. But to understand how vitamin D affects seasonal illness, researchers first needed to uncover how levels of the vitamin change over the course of the year.

While vitamin D occurs naturally in foods like egg yolks and oily fish, the bulk of our vitamin D comes from the sun. Researchers from the Mayo Clinic and the University of California at Irvine have found that Americans vitamin D levels peak in August and dip drastically by February.  They discovered this by analyzing vitamin D levels in 3.4 million blood samples taken weekly in the US between July 2006 and December 2011.

Scientists have known for some time that vitamin D levels change with the seasons, but researchers believe this will allow doctors to predict vitamin D levels with more precision.

“Our analysis, combined with other data, will help contribute to understanding the role of vitamin D in all seasonal diseases, where the simple winter/spring/summer/fall categories are not sufficient,” said Dr. Andrew Noymer, associate professor of public health and lead author of the study, in a press release.

The researchers said their study looked at population averages, so people won’t be able to judge their vitamin D levels based on the calendar. Health-care providers can perform blood tests to determine vitamin D deficiencies, and prescribe supplementation in individuals who don’t get enough sun exposure.

Patients with back pain and other musculoskeletal conditions may also benefit from vitamin D supplementation, since the vitamin can improve the bone’s ability to absorb calcium.


Kasahara AK, et al. Vitamin D (25OHD) serum seasonality in the United States. PLoS One 2013; 8(6): e65785. doi:10.1371/journal.pone.0065785.

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