Vitamin D, Genetics and Blood Glucose Management
Did you read that a genetic study casts doubt that vitamin D prevents the development of type 2 diabetes? Well, don’t believe everything you read. The authors are quoted, “Our findings suggest that interventions to reduce the risk of type 2 diabetes by increasing concentrations of vitamin D are not currently justified.” That is one opinion. Here is a different perspective.
Ye and colleagues combined data from 22 observational studies with almost 100,000 participants to examine genetic markers, serum 25(OH)D3 concentrations and risk of developing type 2 diabetes. Several significant relationships between vitamin D-related single nucleotide polymorphisms (SNP) and vitamin D status were identified. The authors did not find an association with incident type 2 risk. Ergo, their conclusion that vitamin D status is not associated with risk of developing diabetes.
Ye and colleagues have overstated their conclusions about lack of causality. At best they were exploring associations among genetics, nutritional status and disease risk. Diabetes is a complicated disease and the cited studies required medical examination and clinical diagnosis. Were the same diagnostic criteria used to identify individuals with diabetes in all studies? What proportion of non-cases might be pre-diabetic?
A person’s genetics, measured as SNPs, does not change over time but nutritional status, especially vitamin D status, changes seasonally. According to Appendix Table 5, participants had mean serum 25(OH)D3 concentrations of 58, 61, 48, 43, 61, 57, 54, 46, 47 and 57 nmol/L. When was individual serum vitamin D status measured relative to time of diagnosis? Were serum 25(OH)D3 measurements adjusted for seasonal variation?
The study populations seemed to have middle-of-the-road vitamin D status. They were not vitamin D deficient (< 30 nmol/L) as are ~26 million Americans. Few study participants would have serum 25(OH)D3 concentrations > 80 nmol/L, associated with a 50% decreased risk in developing diabetes. The authors did not contrast high vs low serum 25(OH)Ds status. When Caucasians with high (≥ 81 nmol/L) were compared to those with low (<44 nmol/L) serum 25(OH)D3 concentrations, a 2-fold difference in diabetes risk was observed. Could vitamin D stratification resulted in a different conclusion? Who knows?
Sun exposure, dietary intake, and genetic variability in vitamin D-related genes are known to affect serum 25(OH)D status. Much remains to be learned about the role of vitamin D in maintaining pancreatic function, insulin sensitivity, and blood glucose management.
It is premature to conclude that vitamin D has no role in preventing diabetes.
Ye Z, Sharp SJ, Burgess S, Scott RA, Imamura F, Langenberg C, Wareham NJ, Forouhi NG. Association between circulating 25-hydroxyvitamin D and incident type 2 diabetes: a Mendelian randomization study. 2014 Lancet Diab Endocrinol doi: 10.1016/S2213-8587(14)70184-6
Brock KE, Huang W-Y, Fraser DR, Ke L, Tseng M, Mason RS, Stolzenberg-Solomon RZ, Freedman DM, Ahn J, Peters U, McCarty C, Hollis BW, Ziegler RG, Purdue MP, Graubard BI. Diabetes prevalence is associated with serum 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D in US middle-aged Caucasian men and women: a cross-sectional analysis within the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial. 2011 Br J Nutr doi: 10.1017/S0007114511001590
Scragg R, Sowers MF, Bell C. Serum 25-hydroxyvitamin D, diabetes and ethnicity in the Third National Health and Nutrition Examination Survey. 2004 Diab Care doi: 10.2337/diacare.27.2813