The summer solstice was Friday, June 21, marking the longest day of the year and the first official day of summer. As the northern hemisphere tilted closest toward the sun, a new UC Irvine/Mayo Clinic Study for the first time in data from the United States finds that vitamin D levels in the population peak in August and are at their lowest in February. This essential vitamin—necessary for healthy bones—is made in the skin upon exposure to ultraviolet B (UVB) rays from the sun.
Vitamin D absorption is thought to play a role in the seasonality of a number of diseases and adverse health conditions. To further study the links between Vitamin D and seasonal diseases such as the flu, good estimates of the cyclicality of the vitamin are necessary.
Solar exposure is the most important way people acquire the nutrient. Foods that contain vitamin D include egg yolks and oily fish such as mackerel, salmon, sardines and herring. Some foods such as milk and cereal are fortified with vitamin D. Vitamin D also helps bones absorb calcium and can protect against conditions like osteoporosis.
“Even with food fortification, vitamin D levels in the population show a high level of seasonality due to the influence of sunlight,” said Amy Kasahara, a UC Irvine masters of public health student and the study’s first author.
“The exact biochemical pathways from UVB rays to vitamin D were discovered in the 1970s,” she said. “In this study we have shown that vitamin D levels lag the solar cycle, peaking in August and troughing in February.” The findings appear in the journal PLOS ONE http://dx.plos.org/10.1371/journal.pone.0065785
The correlation between the seasons and vitamin D has been known for a long time. “What we have been able to do is put a lot more precision on the estimates of vitamin D seasonality,” said Andrew Noymer, UC Irvine associate professor of public health and senior author of the study.
Although the positive effect of vitamin D is well understood in terms of skeletal health, there are a number of unproven hypotheses about its role in other aspects of health, such as the seasonality of influenza infection.
“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,” noted Noymer.
Vitamin D in the study was measured as blood levels of 25-hydroxyvitamin D, also called 25OHD. The study analyzed 3.4 million blood samples collected weekly in the United States from July 2006 to December 2011.
The research looked at population averages, so individuals should not make direct relations between calendar date and their own levels of vitamin D. Healthcare providers can perform individual blood tests to measure vitamin D directly, and supplements are available for people who cannot or do not receive exposure to sunlight.
Ravinder Singh of the Mayo Clinic co-authored the study.
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Press release from University of California Irvine.