I got a blood test back that registered a 5 on my Vitamin D levels (30 being the desired floor, and 40 or 50 being closer to ideal). Everything else was perfect except for that VitD level. I was shocked, but after reading up on Vitamin D it made sense: I've had maybe two tans in 15 years, I don't get anywhere near enough sun exposure, and diet hardly makes a dent in the amount of VitD it takes to bring your blood levels up to a 40 or 50 range. I also felt fortunate after reading up on what very low levels of VitD can cause. I spent a few weeks reading just about everything I could get my hands on relating to VitD and what it does.

I don't have all of my sources handy now, but I can run through some of what I came to understand.

It has a role to play regarding depression, and or a general sense of malaise, having to do with serotonin levels. There are Vitamin D receptors in the areas of the brain linked to controlling depression. You'll find people often feel a significant pick-me-up when they correct low Vitamin D levels - often right from the first week of taking D3 supplements (whether that's a placebo effect or not, it seems to be very common).

VitD is now believed to be an important regulator when it comes to neurological diseases / conditions such as multiple sclerosis and alzheimers. They've found a direct link to low levels of VitD in children and M.S. - which you can see in the rate of M.S. in people in countries far north of the equator.

http://en.wikipedia.org/wiki/Vitamin_D_and_neurology#Vitamin...

There have been some links made to schizophrenia:

"A new study finds that vitamin D-deficient individuals are twice as likely to be diagnosed with schizophrenia as people who have sufficient levels of the vitamin."

http://psychcentral.com/news/2014/07/23/vitamin-d-deficiency...

People with cancer and heart disease frequently have particularly low levels of Vitamin D. This is a bit tricky though, because that can just as easily tie into lifestyle; but it's interesting non-the-less.

Common fatigue and bone pain often ties into very low levels of Vitamin D. As is well understood now, it has an important role to play in working with calcium and magnesium.

VitD is very important to the immune system:

"Deficiency in vitamin D is associated with increased autoimmunity as well as an increased susceptibility to infection."

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3166406/

VitD seems to play an important role in memory, and staving off age related memory loss (and perhaps keeping memory strong in general):

http://www.latimes.com/science/sciencenow/la-sci-sn-vitamin-...

There has been a variety of interesting discussion around whether Vitamin D has a role to play in type 2 diabetes. It's often found that type 2 patients suffer from especially low levels of VitD, but the counter arguments are that those low VitD levels have more to do with lack of physical activity:

http://www.nih.gov/news/health/oct2013/niddk-21.htm

http://care.diabetesjournals.org/content/34/6/1284.full

http://www.medicalnewstoday.com/articles/283275.php

It's claimed that Vitamin D can help reduce levels of LDL cholesterol:

http://well.blogs.nytimes.com/2014/03/13/vitamin-d-may-lower...