fortified vs the plain yoghurt drink group. In another RCT
among 70 patients with type 2 diabetes, calcitriol treatment
(0.5 μg daily) for 3 months was associated with a significant
attenuation in glucose increase compared with the placebo
group [89]. In a randomized but not placebo controlled trial
in women with gestational diabetes, study participants were
allocated to receive either a single postpartum intramuscular
injection of 300,000 IU vitamin D or no injection [88]. After
3 months of follow-up, insulin resistance was significantly
improved in the vitamin D vs the control group. In summary,
there were some moderate vitamin D effects on glucose
metabolism in recent RCTs among diabetic patients that
warrant further investigations. The fact that these studies
were exclusively performed in Iran deserves further investigations
with reference to potential effect modification related
to ethnic and socio-cultural differences. This might be
of particular interest when considering that previous RCTs
in Caucasian diabetic patients failed to show vitamin D
effects on glycemic control [54, 62, 69]. It should also be
noted that 2 of the recent RCTs showing improved glucose
metabolism following vitamin D treatment included additional
calcium supplementation [85–88]. This may suggest
an interaction of vitamin D and calcium, hypothetically
related to the fact that calcium intake is known to increase
the half-life of 25(OH)D and that both calcium and vitamin
D metabolites decrease PTH secretion [91]. Additional attention
should also be paid to observations that calcium
intake itself may modulate energy metabolism and increase
fecal fat excretion [92].