THURSDAY, June 20 (HealthDay News) -- 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
"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
The connection between seasons and vitamin D levels has been
known for some time, but this study provides more precise data
about vitamin D levels at different times of the year.
"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," study senior author Andrew Noymer, an associate
professor of public health at UC Irvine, said in the news
Vitamin D is found in foods such as egg yolks and oil-rich fish
such as mackerel, salmon, sardines and herring. Milk and cereal
often are fortified with vitamin D. Another option to boost your
vitamin D intake is to take supplements.
The Harvard School of Public Health has more about
vitamin D and health.