Type 2 diabetes (T2D) is a manifestation of metabolic syndrome and has become a prominent public health problem, worldwide.1 Within the last decade, research has suggested vitamin D’s (VDs) effects upon physiological processes extend beyond its most notable roles (i.e., calcification of bone/bone density) to include adiposity, lipid metabolism, blood pressure, glucose intolerance, and insulin resistance (IR).1(1) Furthermore, research linked low levels of VD/diabetes and the beneficial effects of VD supplementation to improve T2D symptoms. However, little research has explored VD supplementation among individuals with pre-diabetes.
Studies have elucidated associations between VD and pancreatic beta cell (produces insulin) function and IR via monitoring fasting insulin, and hemoglobin A1C (HBA1C).1(1-2) However, as mentioned in the introduction, limited research has considered VD and its relationship to IR, insulin secretion, glucose homeostasis, and ultimately T2D prevention. Thus, Tang et al1(2) conducted a meta-analysis to explore and quantify direct effects of VD upon glucose/insulin homeostasis and incidence of T2D among non-diabetic adults.
Tang et al1(2) selected studies that met the following criteria:
- Randomized control trials
- Adults >18 years of age without diabetes
- Oral VD (vitamin D2 or vitamin D3) supplementation with/without calcium (Ca)
- Placebo or no treatment, with/ without Ca as a comparator
- Reporting at least one of the researchers’ outcomes of interest
Ultimately, the researchers were interested in the incidence of diabetes and changes in 25-hydroxyvitamin D, or 25OHD (a measure of active VD in the blood), between baseline and the end of the trial. Secondly, the researchers also wanted to extract additional outcomes such as insulin resistance, glucose tolerance, fasting insulin, insulin secretion, insulin sensitivity, and beta cell function (releases insulin)1(2) After inclusion criteria and exclusion criteria were addressed, 47 randomized controlled trials covering the above matters of interest remained.1(2)
The meta-analysis (from 40 trials) indicated that VD supplementation resulted in an average increase in 25OHD levels of 16 ng/ml compared to placebo groups, with a linear trend based on dose-response.1(3) Glucose metabolism was also improved in the presence of increased VD levels with a significantly decreased fasting insulin of 1.47 mIU/L and a fasting glucose decrease of 0.11 mmol/L when comparing to the placebo group.1(3) Furthermore, VD supplementation was significantly associated with reductions in insulin resistance as measured by homeostatic model assessment-insulin resistance (HOMA-IR).1(3) Specifically, said improvements remained significant when VD supplementation was over 4000 IU/day; a level considered to be in excess of the official Tolerable Upper Intake Level of 4000 IU/day.1(3),2
Changes in insulin secretion and beta cell function were also noted by Tang et al1(5); evidence from homeostatic model assessment for beta cell function (HOMA-B) indicated significant decreases in HOMA-B in the presence of both lower doses (1000-4000 IU/day) and higher doses (>4000 IU/day)1(5) Furthermore, 6 trials which encompassed over 39,000 participants were reviewed who had diabetic and pre-diabetic individuals and the relationship to VD supplementation. Interestingly, within said population, it was found that VD doses of >4000 IU/day did not decrease risk of developing diabetes from pre-diabetes.
In conclusion, T2D is a manifestation of metabolic syndrome and has become a prominent public health problem, worldwide. Within the last decade, research has suggested vitamin D’s (VD) effects upon physiological processes extend beyond its most notable roles of improving bone density. Research has suggested that VD supplementation can exert the bulk of its influence upon fasting levels of glucose, insulin, and HOMA-IR.1(6)Overall, the meta-analysis (among non-diabetic participants) by Tang et al1(6)has suggested that vitamin D supplementation with doses > 4000 IU/day elicited said response, and thus, might be used as a preventative intervention. However, the influence of VD on the incidence of diabetes appears insignificant.
References
1. Tang H, Li D, Zhang X, et al. Effects of vitamin D supplementation on glucose and insulin homeostasis and incident diabetes among nondiabetic adults: A meta-analysis of randomized controlled trials. Int J Endocrinol. 2018:1-9. doi:https://doi.org/10.1155/2018/7908764.
2. Gropper SS, Smith JL, Carr TP. Advanced Nutrition and Human Metabolism. 7th ed. Boston, MA: Cengage Learning; 2018.
-Michael McIsaac