Vitamin D supplementation reduces insulin resistance in South Asian women living in New Zealand who are insulin resistant and vitamin D deficient – a randomised, placebo-controlled trial

Abstract

Low serum 25-hydroxyvitamin D (25(OH)D) has been shown to correlate with increased risk of type 2 diabetes. Small, observational studies suggest an action for vitamin D in improving insulin sensitivity and/or insulin secretion. The objective of the present study was to investigate the effect of improved vitamin D status on insulin resistance (IR), utilising randomised, controlled, double-blind intervention administering 100 mg (4000 IU) vitamin D 3 (n 42) or placebo (n 39) daily for 6 months to South Asian women, aged 23-68 years, living in Auckland, New Zealand. Subjects were insulin resistant -homeostasis model assessment 1 (HOMA1) . 1·93 and had serum 25(OH)D concentration , 50 nmol/l. Exclusion criteria included diabetes medication and vitamin D supplementation . 25 mg (1000 IU)/d. The HOMA2 computer model was used to calculate outcomes. Median (25th, 75th percentiles) serum 25(OH)D 3 increased significantly from 21 (11, 40) to 75 (55, 84) nmol/l with supplementation. Significant improvements were seen in insulin sensitivity and IR (P¼0·003 and 0·02, respectively), and fasting insulin decreased (P¼ 0·02) with supplementation compared with placebo. There was no change in C-peptide with supplementation. IR was most improved when endpoint serum 25(OH)D reached $80 nmol/l. Secondary outcome variables (lipid profile and high sensitivity C-reactive protein) were not affected by supplementation. In conclusion, improving vitamin D status in insulin resistant women resulted in improved IR and sensitivity, but no change in insulin secretion. Optimal vitamin D concentrations for reducing IR were shown to be 80 -119 nmol/l, providing further evidence for an increase in the recommended adequate levels. Registered Trial No. ACTRN12607000642482. Vitamin D: Type 2 diabetes: Insulin resistanceThere is mounting interest in the role of vitamin D in the aetiology of type 2 diabetes, and the most commonly preceding conditions, reduced insulin sensitivity and compromised b-cell function.Low serum 25-hydroxyvitamin D (25(OH)D) concentrations have been shown to correlate with impaired glucose tolerance and an increased risk of type 2 diabetes (1 -5) , while a correlation between hypovitaminosis D and insulin resistance (IR) has been identified in pregnant women and obese adolescents (6,7) . A 10-year prospective study identified an inverse relationship between baseline serum 25(OH)D concentrations and later risk of IR (8)

The effect of different doses of vitamin D3 on markers of vascular health in patients with type 2 diabetes: a randomised controlled trial

“…This finding also argues against the possibility that reduced renin levels drove the fall in blood pressure observed in this study. Several studies have demonstrated that vitamin D can improve insulin resistance in patients with prediabetic states including insulin resistance and impaired glucose tolerance [10,11,25,26]. However, studies examining the effect of vitamin D on insulin resistance and glycaemic control in patients with established type 2 diabetes have been negative [9,10], and the large Women's Health Initiative study [27] found no reduction in new diagnoses of diabetes in participants receiving calcium plus a small (400 IU per day) dose of vitamin D 3 .…”

“…A number of small studies failed to show any improvement in glycaemic control or indices of insulin sensitivity during supplementation [2,9,10], although supplementation in patients with impaired glucose tolerance but without frank diabetes did successfully improve insulin resistance in two trials [10,11].…”

Vitamin D and risk of CVD: a review of the evidence

“…Currently, the best available inflammatory biomarker is CRP, an acute phase reactant that is a reliable, independent predictor of the risk of myocardial infarction, stroke and cardiovascular death (111) . Clinical trials of vitamin D supplementation show inconsistent results regarding an effect on CRP concentrations: most report no effect (77,80,97,(112)(113)(114)(115)(116) , whereas some show reductions in CRP concentrations (74,117,118) . Some trials have measured highsensitivity CRP (hsCRP), which can detect low-grade inflammation.…”

“…Several large epidemiological studies have found 25(OH) D concentrations to be associated with improved insulin resistance and sensitivity (120)(121)(122) , but RCT have found mixed results; some have found a significant beneficial effect of supplementation (78,116,123) , while others have shown no effect (76,124) . An RCT in 100 apparently healthy, but centrally obese men observed an improvement in postprandial insulin sensitivity measured by the oral glucose insulin sensitivity index after supplementation with 3000 μg D 3 once a fortnight for 6 weeks: mean difference in change in oral glucose insulin sensitivity between placebo and the D 3 group was 41·1 (SE 15·5), P = 0·01 (78) .…”

Nonfat milk attenuates acute hyperglycemia in individuals with android obesity: A randomized control trial

“…Oral magnesium supplementation, for example, improves insulin sensitivity, glucose homeostasis, and HbA1c levels in diabetic patients (Bo & Pisu, 2008). Several intervention studies have shown improved glycemic responses with vitamin D treatment, but these effects may be specific to individuals who were deficient at baseline or those who had preexisting metabolic disorders (von Hurst, Stonehouse, & Coad, 2010; Rice et al., 2011). The micronutrient profile of dairy could contribute to attenuated hyperglycemia as the cellular influx of calcium plays a pivotal role in nutrient intake and in endothelial‐dependent vasodilation, and magnesium is essential for optimal coupling and signaling through the insulin receptor (Rice et al., 2011; Zemel, 2001).…”