Study
A recent study, published in the journal PLOS One (Public Library of Science), showed an increased chance of Type II Diabetes with a long-term low blood Vitamin D Levels. The study followed the vitamin D levels of 903 participants over a period of 12 years.
At the onset of the study (1997), none of the participants were diabetic or pre-diabetic. At the end of the study (2009), 47 of all participants had Type II diabetes, and 337 of the patients were pre-diabetic.
Findings
Patients with a plasma vitamin D level, actual test was 25(OH)D, greater than 30 ng/ml had an approximately 70% lower incidence of diabetes compared with those with a level less than 30 ng/ml. Those with an even higher plasma Vitamin D level of more than 50 ng/ml, had an 81% lower incidence of diabetes.
While those with Vitamin D levels of more than 50 ng/ml were significantly less likely to have pre-diabetes, there was only a weak association with those with normal to middle levels (30 ng/ml - 50 ng/ml) compared to those that were low (less than 30 ng/ml).
Explanations
The authors discussed a couple of reasons for the association.
First, they referenced research that shows that vitamin D will stimulate the pancreas to produce insulin. Second, they refer to a study that shows that Vitamin D is a precursor to metabolites that will protect pancreatic cells from inflammation and cell death.
Caution
The authors caution that the subjects of their study do not represent the broader U.S. population. The study took place in an affluent and mostly white town in southern California, where the participants had good access to healthcare.
Other Thoughts
As a doctor, this is just another reason to remind patients to stay on vitamin D during the three colder seasons. I find that most (though not all) patients that are active outside in the summer don't need to supplement during those three months. Other research shows that vitamin D can help with infections (cold/flu) as well as autoimmune disorders.
How Much Vitamin D to Take? 
The Vitamin D Council recommends 5,000 IU a day for adults, which is what I recommend for my patients. If their blood tests are low (less than 30 ng/ml) or are fighting an illness, I will recommend a higher dose (along with magnesium and vitamin K.)
While the Institute of Medicine (IOM) recommends 4,000 IU daily as the most one should take, the Endocrine Society Practice Guidelines state that up to 10,000 IU of vitamin D3 daily is safe for most adults.
Testing
Make sure you get the Vitamin D, 25 Hydroxy test. Try to get your MD or DO to order it, and it should get covered by insurance. If your insurance won't cover it, I use Direct Labs (scroll down to the bottom). For about the same price, the Vitamin D Council has a test kit.
Interestingly, this study shows that it is really rare to have vitamin D levels that are problematic.
Yours in health,
Dr. Michael