Does Vitamin D Supplementation Improve Glycemic Control in Type 2 Diabetes?

Diabetes is caused by the body's inability to properly produce and handle insulin. Type 2 diabetes is the most common form of the disease, with over one million new cases diagnosed in the United States every year (Pittas, Lau, Hu, & Dawson-Hughes, 2007, 2017). The effects of type 2 diabetes can be devastating, for the patient as well as society as a whole.

Diabetes is a leading cause of kidney failure, nontraumatic lower-limb amputations, and new cases of adult blindness, as well as a major cause of heart disease and stroke and the seventh leading cause of death in the United States (National Diabetes Information Clearinghouse, 2011). Besides the personal toll diabetes can exert on those afflicted, it is estimated that the disease costs the American healthcare system $174 billion a year, with only an estimated 25% of patients receiving the care necessary to treat and control the disease (Konrad, 2010).

This staggering cost is the result of expensive treatments as well as the size of the patient population; it is estimated that 8.3% of the population, or 25.8 million people, are affected by diabetes (National Diabetes Information Clearinghouse, 2011). Type 2 diabetes is more common in some populations than in others; these population groups include African-Americans, Latinos, Native Americans, and Asian-Americans, Native Hawaiians and other Pacific Islanders, as well as the aged population (American Diabetes Association, 2012).

Despite the elevated risk of developing the disease found in some populations, epidemiological data suggests that nine of ten cases of type 2 diabetes can be attributed to habits and other forms of modifiable behavior, including obesity (Pittas, Lau, Hu, & Dawson-Hughes, 2007, 2017). In this sense, type 2 diabetes is primarily a lifestyle disease, as it typically results from the health and nutrition choices of the patient, although family history and genetics increase one's vulnerability to development of the disease.

In fact, up to 80% of type 2 diabetes cases in the United States can be prevented by three steps: smoking cessation, healthy diet, and regular exercise (Konrad, 2010). Over the last decade or so, evidence has emerged identifying and acknowledging the role of vitamin D in metabolic function extending beyond calcium regulation, bone mineralization, and cancer prevention to include glucose metabolism and a relationship with obesity (Davis, 2011, 148). Low levels of vitamin D have been associated with insulin resistance and obesity (Schwalfenberg, 2008, 864).

This discovery has ramifications for diabetes care, as metabolism and obesity are both related to the development of type 2 diabetes. There are many risk factors for vitamin D deficiency, including a lack of exposure to the sun, inadequate dietary intake, darker skin color, age, obesity, and the use of various medications (Schwalfenberg, 2008, 865). Additionally, poverty, urban living conditions and lactose intolerance "promote opportunities for vitamin D deficiency to manifest and attenuate opportunities for participation in health-promoting behaviors by those affected (Davis, 2011, 148).

Vitamin D may have a beneficial effect on insulin action directly, through stimulating the expression of insulin receptor, enhancing insulin responsiveness for glucose transport, or indirectly through its role in regulating extracellular calcium and ensuring normal calcium influx through cell membranes and adequate intracellular cytosolic calcium (Pittas et al., 2007, 2018).

While the evidence, on the whole, suggests that Vitamin D supplementation offers the possibility of improving glycemic control in patients with type 2 diabetes, it is also clear that more research needs to be done on the subject, as not all patients or patient groups will experience the same improvement in glycemic control with vitamin D supplementation, particularly since vitamin D receptor genotypes are determinants of insulin secretory capacity in different ethnic groups; for example, one study reported that glycemic control worsened in three Asian patients following such supplementation (Schwalfenberg, 2008, 865).

Clearly, a three patient sample is insufficient from which to draw substantive conclusions, but such results do point to a need for more research on the topic. Despite the need for more conclusive research, one could tentatively conclude that Vitamin D and calcium deficiency may negatively influence glycemia, while a combined supplementation program of both nutrients may prove beneficial for optimizing glucose metabolism (Pittas et al., 2007, 2017). Yet more research is clearly needed in order to establish the efficacy and applicability of vitamin D supplementation as a treatment for type 2 diabetes.

While there are currently more than 1,000 ongoing studies examining the benefits of vitamin D, only one interventional study will be large enough to assess the balance of benefits and risks of vitamin D for multiple chronic disease outcomes, and those results will not be available until 2015 (Endocrine Today, 2010). There are clear gaps in the relevant literature; for instance, while various studies have shown the benefits of vitamin D on a reduced risk for.