We read with interest the article by Krul-Poel et al. (1) that reported the results of a randomized controlled trial on the effect of vitamin D supplementation on glycemic control in patients with type 2 diabetes. The authors did not find any significant improvement of HbA1c in the group receiving vitamin D supplementation (129 patients) as compared with the group receiving placebo (132 patients) and concluded that intermittent high-dose vitamin D supplementation did not improve glycemic control in type 2 diabetes. The significant reduction of HbA1c obtained in the small subgroup of patients with diabetes with severe vitamin D deficiency (<30 nmol/L) is interesting but not concordant with previous findings, as acknowledged by the authors themselves, which leaves the matter still open.
The major predictor of glycemic improvement after any pharmacological therapy in type 2 diabetes is the starting HbA1c level (2,3); it would be interesting to assess whether this relation (the higher the baseline HbA1c level, the higher the HbA1c response to vitamin D supplementation) is also present in patients treated with vitamin D. This finds some support from the results by Soric et al. (4) that found a higher decrease of HbA1c (−1.4%) during vitamin D treatment in patients with baseline HbA1c >9% than in patients with baseline HbA1c between 7.0 and 8.9% (0.3%).
Unfortunately, Krul-Poel et al. (1) did not measure the circulating testosterone in the male patients, who made up the majority of their sample. Testosterone levels are often reduced in male patients with type 2 diabetes as a consequence of hypogonadotropic hypogonadism. A high prevalence of vitamin D deficiency has been described in patients with type 2 diabetes with hypogonadism (5); the possibility remains that some men with diabetes and vitamin D deficiency (38% of those evaluated by Krul-Poel et al.) may have associated hypogonadism.
Article Information
Duality of Interest. No potential conflicts of interest relevant to this article were reported.