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Vitamin D supplementation fails to lower HbA1c in type 2 diabetes
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Older adults with well-controlled type 2 diabetes assigned 4,000 IU daily vitamin D for 48 weeks saw no changes in insulin secretion rate or HbA1c vs. adults receiving placebo, study data show.
“Suboptimal vitamin D status has emerged as a potential contributor to the pathophysiology of type 2 diabetes, with several lines of evidence supporting a role for vitamin D in pancreatic beta-cell function and insulin sensitivity,” Edith Angellotti, PhD, a postdoctoral research fellow in the division of endocrinology, diabetes and metabolism at Tufts Medical Center in Boston, and colleagues wrote. Although several meta-analyses have suggested a beneficial effect of vitamin D supplementation on glycemia and insulin sensitivity in type 2 diabetes, trials were short and varied in quality, the researchers noted.
Angellotti and colleagues analyzed data from 127 older adults with well-controlled type 2 diabetes managed with lifestyle only or lifestyle plus metformin, participating in the Vitamin D for Established Type 2 Diabetes (DDM2) study, a double-blind, randomized, placebo-controlled trial conducted at Tufts Medical Center and the VA Medical Center in Cincinnati (mean age, 60 years; 30% women; 62% white; mean BMI, 30.9 kg/m²; mean HbA1c, 6.6%; 78% on metformin). Between March 2013 and July 2015, researchers examined the effect of 4,000 IU daily vitamin D3 supplementation (cholecalciferol; n = 66) or placebo (n = 61) for 48 weeks on pancreatic beta-cell function and HbA1c. Follow-up visits occurred at 16, 24, 36 and 48 weeks, with participants undergoing a 3-hour 75-g oral glucose tolerance test at baseline and week 24. Researchers assessed insulin secretion rate, estimated from peripheral plasma C-peptide levels after the OGTT and changes in HbA1c at weeks 16, 24, 36 and 48.
Within the cohort, baseline mean plasma 25-hydroxyvitamin D concentration was 26.6 ng/mL, and mean self-reported vitamin D intake was 399 IU daily. At week 24, plasma 25-(OH)D concentration was 47.1 ng/mL for participants in the vitamin D group, a mean increase of 20.5 ng/mL from baseline vs. a mean 25.5 ng/mL concentration for the placebo group, a mean decrease of 1.6 ng/mL. Mean vitamin D concentrations stayed consistent through week 48.
After the OGTT at week 24, researchers observed no between-group differences in total insulin secretion rate, Matsuda insulin sensitivity index or area under the curve for glucose or insulin.
During follow-up, HbA1c increased in both the vitamin D and placebo groups, with no between-group differences, according to researchers. Among the 28 participants assigned vitamin D and treated with lifestyle modification only, researchers observed a small improvement in HbA1c vs. those assigned placebo at week 24 (mean decrease, –0.1% vs. 0.3%; P = .034); however, there were no observed changes at any other time points.
“More patients treated with vitamin D also experienced improvement in the ‘glycemia’ composite outcome; however, the differences were not statistically significant,” the researchers wrote.
They noted that most of the cohort was prescribed metformin therapy, which may have masked a small effect of vitamin D supplementation on outcomes, and that vitamin D may have no detectable effect in people with well-controlled diabetes.
“These results indicate that any effect of vitamin D supplementation is expected to be relatively small and would be most noticeable among patients with early diabetes not requiring pharmacotherapy,” the researchers wrote. – by Regina Schaffer
Disclosures: The National Institute of Diabetes and Digestive and Kidney Diseases funded this study.
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