Diabetes
There is a growing interest in the role of vitamin D in the
aetiology of type 2 diabetes based on several reports
finding vitamin D deficiency in impaired glucose tolerance
and type 2 diabetes [9]. The association of vitamin D deficiency
and type 2 diabetes could be easily predicted by
their common link to obesity; however, several in vitro and
epidemiological studies suggest a direct relationship
between these two apparently disparate pathways. For
example, a potentially direct beneficial effect of vitamin D
on insulin action has been suggested by an increase in
insulin receptors following 24 h of treatment with
1,25(OH)2D3 in U-937 human promonocytic cells [10].
Furthermore, the role of vitamin D in ensuring adequate
calcium influx through cell membranes and intracellular
cytosolic calcium is of paramount importance to preserve
the insulin-mediated insulin process in insulin-responsive
tissues, such as skeletal muscle and adipose tissue [11].
Other researchers, however, have reported that vitamin D
deficiency may blunt insulin action through elevated parathyroid
hormone (PTH) levels [12]. In addition, it has been
reported that an intriguing correlation between vitamin D
deficiency and type 1 diabetes exists [13], may be due to
the ability of vitamin D in preserving insulin release
modulating the extracellular and intracellular b calcium
pools [14]. A recent small, but well-conducted, study [15]
suggested that cholecalciferol supplementation might
increase the disposition index (the product of insulin
secretion and action) in subjects at high risk of developing
diabetes, warranting the need for larger studies. ‘Recently,
an interesting association has also been reported between
hypovitaminosis D and the severity of non-alcoholic fatty
liver disease (NAFLD) in terms of steatosis, necroinflammation
and fibrosis. In addition, hypovitaminosis D is
also thought to be a risk factor for NAFLD independently of
the other components of the metabolic syndrome, although
the molecular mechanism by which it would act is still
unknown