Vitamin D in the U.S. Population At Its Peak in August and Lowest in February

Researchers from UC Irvine and Mayo Clinic have discovered that the levels of Vitamin D in the U.S. population are at its peak in August and lowest during the month of February.

Exposure to ultraviolet B rays from the sun produces vitamin D in the skin.  This vitamin is necessary for good health and sturdy bones as it absorbs calcium and protects against osteoporosis. In seasonal illnesses such as flu, Vitamin D is known to play a major role.

When the level of vitamin D drops it impairs innate immunity.  One of the best ways to acquire vitamin D is by exposing oneself to sunlight. This vitamin can also be obtained through a diet of salmon, egg yolks, sardines, milk and cereals.

"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 graduate student in public health and first author on the paper. "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."

For this study, the researchers analyzed the level of 25-hydroxyvitamin D in 3.4 million blood samples that were collected regularly between July 2006- December 2011.

Andrew Noymer, associate professor of public health and senior author of the article, said that the association between seasons and vitamin D has been known for some time, but in the new study a lot of precision has been added to the estimates of vitamin D seasonality.

The study appears in the journal PLOS ONE.