In a
meta-analysis including 11 studies with 3612 type 2 diabetes
cases and 55,713 controls, the RR for incident diabetes in the
fourth vs the first 25(OH)D quartile was 0.59 (95 % CI 0.52 to
0.67) [42•]. Numerous other cross-sectional and prospective
studies have largely, but not consistently, shown that low 25
(OH)D levels are associated with poor glycemic control and
insulin resistance [44–49]. When interpreting results of these
observational studies it should be underlined that vitamin D
deficiency is in part a consequence of obesity and low physical
activity: obesity leads to vitamin D deposition in the
adipose tissue, and low physical activity may be related to
less outdoor activities and thus less sun exposure leading to
low 25(OH)D levels [50, 51]. Despite careful adjustments for
possible confounders we cannot rule out residual confounding
related to such factors [50, 51]. Hypotheses that vitamin D
itself may reduce obesity have been generated but remain, at
present, speculative [