Response to Pittas et al.
In their recent article, Pittas et al. (1) reported on the effect of calcium and vitamin D supplementation on glycemic status in nondiabetic elderly subjects. Because vitamin D supplementation, though mainly in subjects with relatively low vitamin D levels, improves muscle function (2), it is possible that some of the favorable effects on fasting glucose levels in the impaired fasting glucose group might be related to changes in physical activity. Furthermore, the authors state that while they adjusted the end points (e.g., percentage change in fasting plasma glucose [FPG]) for baseline values, there was still interaction between the baseline FPG and treatment groups. Thus, they examined the treatment effect according to established criteria of glucose tolerance and not according to median FPG levels or tertiles of baseline FPG, an approach that would be more physiologically relevant because the risk of future diabetes increases at lower FPG levels (3). Finally, because vitamin D appears to exert a threshold effect, it would be interesting to examine whether there was an association of attained vitamin D levels and glycemia or insulin sensitivity. We believe that these are critical issues for future trials that examine the effect of calcium and vitamin D supplementation on the progression or prevention of diabetes.
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