Vitamin D, also known as the "sunshine vitamin," is a steroid precursor. It was originally thought to play a role only in the mineralization of bones and teeth by maintaining the correct phosphorous/calcium ratio. But over time research has linked low vitamin D levels with obesity, diabetes, cardiovascular disease, autoimmune disease, osteoporosis, and cancer.

According to the CDC, in 2006 a whopping one-fourth of the population was deficient in vitamin D. Eight percent were "at risk" for vitamin D deficiency illnesses and one percent had levels that were considered imminently harmful. According to Natural News, vitamin D is "perhaps the single most underrated nutrient in the world of ."

What’s causing these low vitamin D levels? One theory is that we are not outside as much as prior generations, and when we are, we slather on the sunscreen that prohibits UVB (the rays responsible for suntans) from penetrating the skin. These same UVB rays naturally produce vitamin D.

The time of day, the season, the altitude, the latitude, and other factors come into play to determine how much UVB reaches the skin. Vitamin D levels can become depleted without enough sunshine, and this is especially true during the winter months when we stay inside more and the sun is not as intense.

As we move into fall, it's a good time to have your vitamin D level checked by a simple blood test. A normal 25-hydroxyvitamin D blood test will register above 30 nanograms per milliliter (ng/mL). Previously it was thought that levels 21 and below needed treatment, though more recently anything below 35 is addressed by many forward-thinking practitioners. Using these levels, it is estimated that one billion people are deficient in vitamin D.

What does this have to do with ? Glad you asked. Recent studies by Springer, and research results reported in the New England Journal of Medicine and by the Vitamin D Council, are indicating a link to . Of note: Canadian researchers reviewed 14 studies involving 31,424 participants and found a strong correlation between depression and a lack of Vitamin D. The lower the Vitamin D level, the greater the chance of depression. But the big question is still causality. Does one get depressed because of a deficiency of Vitamin D, or does depression lower the vitamin level?

Seasonal Affective Disorder (SAD) research is interesting as well. The National Institute of Health cites several studies where sunlight markedly improves mood. Of course, even though we know sunlight increases vitamin D levels, this doesn’t prove that the increase in vitamin D is what is responsible. It also doesn’t prove that sunlight will work in those who are depressed and do not have SAD.

Here are a few important things to note:

(1) The healing properties of natural sunlight cannot penetrate glass. That's right. You cannot sit inside your home or car and reap the benefits of sitting in a sunny spot. You must go outside.

(2) If you have dark skin, you'll need about 25 times more exposure time as a light-skinned individual to produce the same amount of vitamin D.

(3) Your body cannot absorb calcium without enough vitamin D. You can take all the calcium you want, but will receive no benefit unless vitamin D is present.

(4) A vitamin D deficiency is not reversed immediately. You're looking at months of sunlight and/or supplements before levels return to normal.

(5) Your kidneys and liver activate vitamin D. Having kidney disease or a damaged liver will hinder the ability to activate vitamin D when needed.

(6) Now for the bad news. There has to be some bad news, right? Well, here it is. Sunscreens drastically reduce the amount of vitamin D the body makes. In light of this, there is a theory that more individuals are depressed these days because everyone uses sunscreen and they're not taking vitamin D supplements.

A personal note here is that a local internist in my community started ordering vitamin D blood tests during routine physical exams. I know three of his patients who had a history of mild to moderate depression, though they were not taking meds. All three had low vitamin D levels, were started on replacement and within three months reported a marked improvement in energy and mood and virtually no depression. Not a rigorous scientific study, but definitely more food for thought.

Vitamin D supplements can be found over the counter in just about any corner store. However, the high dose form, which contains 50,000 units of the vitamin and is taken once or twice a week, can only be obtained by prescription.

A prescription for 50,000 units of vitamin D could be a simple remedy if your level of the vitamin is low. Of course, this should be discussed with your doctor. Many new studies are in the works looking at this intriguing connection. But in the meantime, the potential benefits of vitamin D for depression should not be ignored.