Seasonality of Vitamin D
June 23, 2013

Vitamin D Levels Highest In Late Summer, Lowest In Late Winter

redOrbit Staff & Wire Reports - Your Universe Online

Vitamin D levels among Americans are at their highest during the month of August and at their lowest in February, according to new research appearing in the journal PLOS One.

The nutrient, which helps bones absorb calcium and can help prevent osteoporosis, is produced in the skin following exposure to the sun’s ultraviolet B (UVB) rays, researchers from the University of California, Irvine and the Mayo Clinic said. In addition, low levels of vitamin D are also believed to weaken the body’s “innate immunity” or first line of defense against seasonal illnesses such as the flu.

While vitamin D can be obtained through nutritional supplements or certain types of food, including egg yolks and oil-rich fish such as salmon and mackerel, the study authors state that solar exposure remains the most important way people acquire the nutrient. In order to further study the link between the vitamin and seasonal illnesses, they set out to come up with good estimates of the substance’s cyclicality.

“Even with food fortification, vitamin D levels in the population show a high level of seasonality due to the influence of sunlight,” explained first author Amy Kasahara, a graduate student at UC Irvine. “The exact biochemical pathways from UVB rays to vitamin D were discovered in the 1970s. In this study, we have shown that vitamin D levels lag the solar cycle, peaking in August and troughing in February.”

According to Andrew Noymer, senior author of the study and an associate professor of public health at the university, the correlation between the seasons and vitamin D has been known for quite a while.

However, he claimed his team’s study has been able to create more precise estimates of the nutrient’s seasonality. “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,” he explained.

Noymer, Kasahara, and Ravinder J. Singh of the Mayo Clinic measured the 25-hydroxyvitamin D levels of 3.4 million blood samples collected from US residents each week from July 2006 through December 2011. They developed population averages based on that information, meaning it is not necessarily reflective of any one individual’s vitamin D levels at any given point in time.