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Americans' vitamin D levels are highest in August and lowest in February, a new study revealed.

The "sunshine vitamin" which is vital for healthy bones is produced in the skin upon exposure to ultraviolet B rays from the sun. People can also get vitamin D from foods like egg yolks, oily fish like mackerel, salmon, sardines and herring and vitamin D-enriched milk and cereal.

Not only does vitamin D help bones absorb calcium, it may also play an essential role in bolstering immunity. Previous studies found that low levels of vitamin D may impair "innate immunity" or the body's first line of defense against pathogens.

Researchers said to further understand the link between vitamin D and immunity, good estimates of the cyclicality of the vitamin are needed.

Researcher Amy Kasahara, a UC Irvine graduate student in public health, explained that solar exposure is the most important way people acquire vitamin D.

"Even with food fortification, vitamin D levels in the population show a high level of seasonality due to the influence of sunlight," Kasahara said in a news release.

"The exact biochemical pathways from UVB rays to vitamin D were discovered in the 1970s," she said. "In this study, we have shown that vitamin D levels lag the solar cycle, peaking in August and troughing in February."

While scientists have long known the correlation between seasons and vitamin D intake, the latest findings "puts a lot more precision on the estimates of vitamin D seasonality," Andrew Noymer, associate professor of public health and senior study author said in a statement.

"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," Noymer added.

For the study, researcher measured the level of 25-hydroxyvitamin D in 3.4 million blood samples collected weekly in the U.S. between July 2006 and December 2011.

Researchers noted that the study looked at population averages, so people shouldn't make assumptions about their own levels of vitamin D based on the calendar.