By Banaz Al-khalidi
November is National Diabetes Month and World Diabetes Day takes place yearly on November 14 to engage millions of people worldwide in diabetes advocacy and awareness. The International Diabetes Federation estimates that 382 million adults (20-79 years old) suffered from diabetes in 2013, which equates to a prevalence of 8.3%. To provide a better perspective by nation, the 10 countries with the highest prevalence of diabetes in 2013 were as follows: Tokelau (37.5%), Federated States of Micronesia (35%), Marshall Islands (34.9%), Kiribati (28.8%), Cook Islands (25.7%), Vanuatu (24%), Saudi Arabia (24%), Nauru (23.3%), Kuwait (23.1%), and Qatar (22.9%). However, if we were to look at the 3 countries with the greatest number of people with diabetes, China ranks the highest (98.4 million), followed by India (65.1 million) and USA (24.4 million). These figures are quite alarming.
Of those suffering from diabetes, type 2 diabetes comprises almost 90% of people with diabetes around the world. As such, type 2 diabetes is one of the fastest growing health problems in the world. So what could be driving this epidemic?
Evidence from observational studies have consistently shown us that low blood levels of vitamin D are associated with an increased risk of type 2 diabetes. The results of numerous observational studies led to speculation that the development of type 2 diabetes is associated with vitamin D insufficiency. Going back to the figures presented earlier, if vitamin D insufficiency is a risk factor for type 2 diabetes, one might also speculate that countries with higher prevalence of diabetes are facing a coexisting problem of type 2 diabetes and vitamin D insufficiency. For example, Tokelauans (the nationals of Tokelau) who have the highest prevalence of type 2 diabetes, may also be at risk for vitamin D insufficiency despite having a tropical and marine climate. The question then becomes, could vitamin D be a causal factor in the development of type 2 diabetes? While this might sound too simplistic, I assure you it’s not.
Interpretation of evidence on vitamin D and type 2 diabetes is complicated for a number of reasons. First and foremost, observational studies do not tell us anything about the cause-effect relationship between vitamin D and type 2 diabetes because of possible uncontrolled confounding factors, such as physical activity, that may affect both vitamin D levels and the risk of type 2 diabetes. Second, observational studies cannot inform us about reverse causation. In other words, which comes first, the chicken or the egg? Third, there are a myriad of factors that affect vitamin D levels, including environmental, cultural, genetic and physiological factors. It remains unclear then whether there is a causal link between vitamin D and type 2 diabetes.
To answer this question, a large genetic study published in The Lancet Diabetes and Endocrinology journal looked at the causal association between low blood levels of vitamin D and risk of type 2 diabetes. The study concluded that the association between vitamin D and type 2 diabetes is unlikely to be causal. The research, which was a Mendelian randomization study, examined the link between type 2 diabetes risk and vitamin D, by assessing the genes that control blood levels of vitamin D. Most importantly, the design of this study has a powerful control for confounding factors and reverse causation which are issues of concern in observational studies. This may partly explain the discrepancy between results from earlier observational studies and this study in question. However, we still need to be cautious about interpreting the results from mendelian randomization studies as some of the underlying assumptions in the study might remain untested.
The take home message is that no special recommendations could be made about vitamin D levels or supplementation for people with type 2 diabetes. However, long-term randomized trials of vitamin D supplementation remain important to elucidate vitamin D’s role in type 2 diabetes.
As we recognize National Diabetes Awareness this month, it is important to remind patients that diabetes is a progressive chronic lifestyle disease that can be controlled by making healthy lifestyle changes- such as partaking in regular physical activity, eating a balanced diet, maintaining a healthy body weight, taking prescribed medications, joining a smoking cessation program, and improving sleeping patterns.
Zheng Ye, Stephen J Sharp, Stephen Burgess, Robert A Scott, Fumiaki Imamura, Claudia Langenberg, Nicholas J Wareham, Nita G Forouhi. Association between circulating 25-hydroxyvitamin D and incident type 2 diabetes: a mendelian randomisation study. The Lancet Diabetes & Endocrinology, 2014; DOI: 10.1016/S2213-8587(14)70184-6.