Human Study Suggests: Preload + 2,000IU/Day Maintenance Dose of Vitamin D May Prevent Progression of Prediabetes

Without sun exposure even a D-rich diet won't contain enough vitamin D (photo from WebMD Slideshow)

Hitherto, studies on the effect of vitamin D supplementation on type II diabetes yielded mixed results with the majority of them providing insufficient evidence to support any of the miraculous effects you will read about on the Internet.

Next to the often-heard claim that the corresponding studies simply weren't using enough vitamin D3, another, actually more reasonable hypothesis why the trials fail is that the provision of a substance that may protect you from developing type II diabetes when you've already messed yourself up is nonsensical.
Against that background studies like the one researchers from the Sher-I-Kashmir Institute of Medical Sciences in Kashmir conducted recently are particularly interesting, because in contrast to the subects in the average US study, Kushay's et al.'s subjects were 137 men and women that were diagnosed as with "prediabetes". All of them had to make standard lifestyle changes, but only 68 received 60,000 IU weekly for 4 weeks and then 60,000 IU monthly over the 12 months study.

Fasting plasma glucose (FPG), 2‑h plasma glucose and A1C levels were estimated at 0, 6 and 12 months. Changes in FPG, 2‑h plasma glucose, A1C level and the proportion of subjects developing diabetes were assessed among 129 subjects.

Figure 1: Rel. changes in BMI, fasting blood glucose (FPG), 2h post-prandial glucose and A1C (left) as well as relative incidence of type II diabetes development in subjects over the 12-months study period (Kuchay. 2015).

As you can see in Figure 1 there were significant differences in terms of the blood values that are indicative of developing type II diabetes; and still, the supplement was not able to save significantly more subjects from developing type II diabetes (p = 0.57 for the difference between vitamin D and no vitamin D and the development of type II diabetes).

Figure 2: Subgroup analysis of the effects of D-supplementation on subjects with baseline low vs. high 25OHD (Kuchay. 2015)

As in previous studies, the benefits were significantly more pronounced in the subjects in the "deficiency range" for 25OHD (<30ng/ml). Or, as the authors say it: "Those subjects who had baseline serum 25‑OHD levels above 30 ng/mL also showed statistically significant change in
FPG, 2‑h plasma glucose and A1C levels at 12 months.

This change was, however, less pronounced in those subjects who had baseline Vitamin D levels above 30 ng/mL" (Kuchay. 2015).
References:

• Kuchay, M. S., et al. "Effect of Vitamin D supplementation on glycemic parameters and progression of prediabetes to diabetes: A 1-year, open-label randomized study." Indian Journal of Endocrinology and Metabolism 19.3 (2015): 387.