Researchers from UC Irvine and Mayo Clinic have found fluctuating levels of vitamin D in American citizens; the highest being in August and the lowest in February.

Vitamin D, one of the most essential vitamins for bone and musculoskeletal growth, is produced naturally in the human body after exposure to ultraviolet B rats of the sun. It role in preventing osteoporosis and strengthening the innate immunity (the body's first line of defense against pathogens) is well known.

However, the results obtained from this study, which reveal fluctuating levels of vitamin D, also shed light about how it may play an important role in seasonal illnesses, such as flu.

"Even with food fortification, vitamin D levels in the population show a high level of seasonality due to the influence of sunlight," graduate public health and an author of the paper, Amy Kasahara, explained.

"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."

This study has helped confirm the co-relation between seasons and vitamin D levels in the population, and may help make precise decisions about the estimate of vitamin D 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," senior author of the study and an associate professor of public health Andrew Noymer, added.

To calculate vitamin D levels, the researchers collected around 3.4 million blood samples and measured the 25-hydroxyvitamin D level in each of the samples.

Since this study was based completely on population averages, individual study would prove to be irrelevant.

Blood tests can help measure vitamin D levels directly, and using supplements may help individuals combat and prevent associated deficiency conditions. 

The study has been published in the journal PLOS ONE.