A balanced diet includes all of the essential vitamins and nutrients our bodies need to function normally. But what happens when one of those vitamins comes from the sun?

It has long been established that vitamin D helps the body absorb and use calcium. While this vitamin can be taken as a supplement to one's diet, or can come from a diet in the form of eggs and fish, the body can make the vitamin itself when exposed for a minimum of ten to fifteen minutes to the sun.

However, if a vital nutrient relies on the sun to be made, what happens in colder months?

The correlation between the seasons and vitamin D has been known for some time, but not proven. In a recent study, vitamin D levels were measured in 3.4 million blood samples collected weekly in the United States between 2006 and 2011. This has been the largest and most conclusive study of the effect of season changes on vitamin D in the body.

The researchers found that vitamin D levels are not completely in synch with the season — they peak at the end of summer in August and drop to their lowest levels at the end of winter, in February. Originally, it was expected that vitamin D would peak when the sun was at its highest UV index and most direct angle to the Earth, which is usually in June and July. However, vitamin D levels build up when more sunlight is available. Therefore, even though it is summer and the sun is shining, vitamin D levels will not build up to a high level until the end of summer.

Vitamin D levels in the blood have been linked to better defense against seasonal ailments like the flu. This could potentially explain the phenomena seen almost every winter, when flu infections seem to explode. Sun exposure, even for a short time, is the best way to ensure one's body has enough vitamin D. Without it, one is susceptible to illness, osteoporosis, and more frequent aches and pains.

Amy Kasahara, Ph.D., a graduate student who aided this research, warned, "Even with food fortification, vitamin D levels in the population show a high level of seasonality due to the influence of sunlight." Her colleague Andrew Noymer, Ph.D., lead author of their publication, said, "What we have been able to do is put a lot more precision on the estimates of vitamin D seasonality." This is of importance because it reveals when people should supplement their diets with vitamin D.

The researchers admit that their conclusion is based on average vitamin D levels in the population, and individuals should not try to medicate themselves based solely on the season instead of on their own vitamin D levels. The Vitamin D Council says that some groups of people are likely to have vitamin D deficiencies — these include people spending a lot of their daytime hours indoors, people that live in northern parts of the United States or Canada, and people who protect their skin form the sun with sunscreens or clothing.

However, the Vitamin D Council also warns that people should not self-medicate as a result of this information; the only way to decipher your vitamin D levels is to speak to a medical professional and have a simple blood test performed.

Source: Kasahara AK, Singh RJ, Noymer A. Vitamin D (25OHD) Serum Seasonality in the United States. PLOS ONE. 2013.