The study, which is the work of researchers at the University of California (UC), San Diego School of Medicine and Seoul National University in Korea, is not the first to link higher blood levels of vitamin D to a reduced risk of type 2 diabetes.
However, as they note in their journal paper, the authors explain that the evidence to date is "mixed" and omits blood levels of vitamin D that are "above the normal range."
Vitamin D is also important for cell growth, muscle function, fighting infection, and reducing inflammation.
How much vitamin D do we need?
The body obtains vitamin D from a few natural foods, some fortified foods, dietary supplements, and exposure to the sun. Once in the body, the vitamin undergoes some chemical changes to make it biologically useful.
The liver is the main producer of biologically active vitamin D; it converts the inert form into an active form called 25-hydroxyvitamin D (25[OH]D).
The level of 25(OH)D in the blood, which is known as "serum concentration," is considered "the best indicator of vitamin D status."
At present, there is much debate about what the ideal level of 25(OH)D should be to avoid disease and ensure optimum health.
The Institute of Medicine (IOM) suggest that 20 nanograms per milliliter is "adequate for bone and overall health in healthy individuals." Other groups have argued that the cut-off should be much higher, as much as 50 nanograms per milliliter.
The researchers behind the new study — including Cedric F. Garland, who is an adjunct professor in the Department of Family Medicine and Public Health at the UC, San Diego School of Medicine — defined the "normal range" to be 30 nanograms per milliliter — that is, 10 units above the level proposed by the IOM.
Low vitamin D, higher type 2 diabetes risk
Prof. Garland and colleagues examined data on 903 healthy adults — aged 74, on average — who had been enrolled in the Rancho Bernardo Study.
None of these participants had diabetes or prediabetes when they underwent exams and filled in questionnaires during study visits that took place in 1997–1999.
During these visits, they also gave blood samples from which the researchers could assess their vitamin 25(OH)D levels and other markers.
Over a follow-up period that finished in 2009, the group developed 47 cases of type 2 diabetes and 337 of prediabetes, which is a condition wherein blood sugar is higher than normal but not high enough to be classed as diabetic.
The results showed that the risk of developing type 2 diabetes in people with 25(OH)D blood levels lower than 30 nanograms per milliliter was five times that of those whose levels were higher than 50 nanograms per milliliter.
Prof. Garland says that to reach a 25(OH)D blood level of 30 nanograms per milliliter, people would have to take around 3,000 to 5,000 international units of vitamin D every day. Sun exposure amounting to some 10–15 minutes every day at around noon could lessen this amount.
He and his colleagues note that there are "still unresolved concerns" about whether 25 nanograms per milliliter should be the "desirable" blood level of vitamin D. However, they "tentatively suggest" that it should be no less than 40 nanograms per milliliter.
Vitamin D and diabetes mechanisms
Speculating on the mechanisms that might explain why higher levels of vitamin D might lower the risk of type 2 diabetes, the authors cite a number of studies.
One study, done in mice, revealed that lack of the vitamin D receptor was tied to higher rates of diabetes. A receptor is a cell protein that receives specific chemical signals in the cell's environment.
Other studies have found that insulin-producing cells in the pancreas have vitamin D receptors and that byproducts of vitamin D in the body "stimulate the pancreas to produce insulin."
They also cite other animal studies that have demonstrated that "active metabolites of vitamin D" can protect insulin-producing cells in the pancreas from receiving certain signals that trigger inflammation and cell death.
In their concluding remarks, the researchers point out that their study has a number of limitations.
An obvious one is that the group that they studied was not representative of the U.S. population: it consisted of "relatively healthy middle- to upper-middle-class Caucasians," all of whom had "good access to healthcare" and lived in places that had "sunny weather throughout the year."
Another limitation of the study is that some of the higher levels of vitamin D may have come about due to differences in laboratory techniques.
"Further research is needed on whether high 25-hydroxyvitamin D levels might prevent type 2 diabetes or the transition from prediabetes to diabetes."
Prof. Cedric F. Garland