A bright spot for our health this summer is that we can get a dose of the sunshine vitamin.
Even though our bodies can produce the most vitamin D in June, when the sun is highest in the Northern Hemisphere, vitamin D levels in the USA peak in August, according to a new study. The sun is lowest in December, but Americans' vitamin D levels bottom out in February.
Researchers from theUniversity of California-Irvine and the Mayo Clinic in Rochester, Minn., measured the level of 25-hydroxyvitamin D in 3.4 million blood samples collected weekly from July 2006 to December 2011 in the USA. The study was published in the journal PLOS ONE.
Vitamin D helps bones absorb calcium, and some research suggests it boosts immunity against seasonal illnesses, such as the flu. Vitamin D is produced by the body when the skin is exposed to the sun's ultraviolet B (UVB) rays. People also can get the nutrient from foods that naturally contain Vitamin D (egg yolks and oily fish), fortified foods (cereals and milk) and supplements.
"Even with food fortification, vitamin D levels in the population show a high level of seasonality due to the influence of sunlight," Amy Kasahara, lead author of the paper, says in a statement.
"The exact biochemical pathways from UVB rays to vitamin D were discovered in the 1970s," says Kasahara, a U.C.-Irvine graduate student in public health. "In this study, we have shown that vitamin D levels lag the solar cycle."
Andrew Noymer, a senior author of the article, says, "What we have been able to do is put a lot more precision on the estimates of vitamin D seasonality.
"Vitamin D is fat-soluble so it can be stored in the body," says Noymer, an associate professor of public health at U.C.-Irvine. "Over the summer, we accumulate vitamin D."
John Cannell, executive director of the not-for-profit Vitamin D Council, based in San Luis Obispo, Calif., says the research shows that some Americans' vitamin D levels are not adequate in the winter. "If they don't supplement, they are going to be vitamin D-deficient," says Cannell, who was not involved in the study.
There has been heated debate about how much vitamin D a person needs. Theindependent Institute of Medicine recommends that individuals ages 1-70 get 600 IUs (international units) of vitamin D daily.
In children, vitamin D deficiency can cause rickets, a softening of the bones, according to theOffice of Dietary Supplements of the National Institutes of Health. In adults, the deficiency can lead to osteomalacia, which causes bone pain and muscle weakness.
Noymer says the study looked at population averages, so people should not make assumptions about their vitamin D levels based on the calendar. People can get their blood tested to measure their vitamin D levels.
"If one is concerned about having a low level of vitamin D, then February would probably be the best time to have the blood tested," Noymer says. "If you are not sufficient then, chances are you are never sufficient throughout the year."