Vitamin D Levels Vary by Season

Americans' vitamin D levels vary throughout the year, peaking in August and bottoming out in February, a new study shows. The findings will contribute to understanding the vitamin's role in seasonal illnesses, according to the researchers from the University of California, Irvine, and the Mayo Clinic. Vitamin D, which is produced by the skin when exposed to sunlight, helps bones absorb calcium and can protect against osteoporosis. It's also believed to play a role in seasonal illnesses such as the flu. For this study, the researchers measured vitamin D levels in 3.4 million blood samples collected weekly in the United States between July 2006 and December 2011. The results appear in the journal PLoS One. "Even with food fortification, vitamin D levels in the population show a high level of seasonality due to the influence of sunlight," study first author Amy Kasahara, a UC Irvine graduate student in public health, said in a university news release. "In this study, we have shown that vitamin D levels lag the solar cycle, peaking in August and troughing in February," she said. http://www.nlm.nih.gov/medlineplus/news/fullstory_138061.html