The D2d (Vitamin D and type 2 diabetes) trial previously suggested that vitamin D supplementation might reduce the rate of progression from prediabetes to type 2 diabetes; however, observed differences in the intention-to-treat analyses did not reach significance (Pittas et al, 2019). The present secondary analysis of D2d data sought to examine the effects of vitamin D supplementation on prediabetes progression and regression on a per-protocol basis; that is, in those participants who adhered to the study treatment protocol and who were not prescribed diabetes/weight loss medications, which might have obscured some of the effects of the intervention.
The 2423 participants were randomised to receive 4000 IU of vitamin D or placebo once daily. Over a median follow-up of 2.5 years, the hazard ratio for progression to type 2 diabetes was 0.84 (95% CI, 0.71–0.99) in the vitamin D group. The intervention group was also more likely to achieve normal glucose regulation (defined as two or three of the following measures falling below the ADA thresholds for diabetes: fasting plasma glucose ≥7.0 mmol/L, 2-hour glucose ≥11.1 mmol/L or HbA1c ≥48 mmol/mol [6.5%]) than the placebo group (12.4% vs 9.5% of participants; rate ratio 1.31).
The authors conclude that among participants who were adherent to the D2d study protocol, vitamin D supplementation lowered the risk of developing type 2 diabetes and increased the likelihood of regression to normal glucose regulation.