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SA JOURNAL OF DIABETES & VASCULAR DISEASE
like obesity. Also, in case-control and cross-sectional studies, the
measurement of vitamin D is done after the diagnosis of diabetes
and not before, making impossible the proving of causality.
A meta-analysis looking at 11 prospective studies concluded
that there was a combined relative risk for diabetes comparing the
highest with the lowest quartiles of 25-OH vitamin D of 0.59 (0.52-
0.67).
19
Also, an analysis of 25-OH vitamin D levels done during the
DPP, found a higher concentration was associated with lower risk
of incident diabetes, after adjusting for lifestyle interventions (HR
0.72, 95% CI: 0.56-0.90).
20
Interventional studies are mostly lacking and there is variability
in reporting. No clear benefit of supplementation of vitamin D
with the aim to prevent diabetes has been demonstrated yet.
21
It
was shown that there may be a small benefit on insulin resistance
markers in patients who already have impaired fasting glucose.
22
A
randomized controlled trial evaluating the secretion and sensitivity
of insulin with vitamin D supplementation showed a potential
benefit,
23
but another trial in obese African American patients
demonstrated no improvement in glycaemia after 3 months.
24
Diabetes complications
An increase in both the all-cause mortality as well as cardiovascular mor-
tality in T2DM is associated with lower 25-OH vitamin D levels.
25
How-
ever, in this study severe vitamin D deficiency at baseline did not predict
progression to micro- or macroalbuminuria. Also, a systematic review
of 15 trials concluded that there was insufficient evidence to draw con-
clusions regarding micro- and macrovascular events in T2DM.
22
Similarly, though an increased mortality has been demonstrated
in T1DM with a lower vitamin D level, severe vitamin D deficiency
at baseline did not predict the development of microvascular
complications such as nephropathy and retinopathy.
26
Cardiovascular outcomes
Low 25-OH vitamin D levels have been associated with stroke,
peripheral vascular disease, chronic heart failure and myocardial
infarction in cross-sectional and longitudinal data.
27
The question
remains whether this is indeed cause or effect. To date, interventional
studies have been inconclusive and results of larger trials such as
the VITAL and VIDAL studies are awaited.
Recommendations and conclusion
Guidelines as to which patients to test for vitamin D deficiency,
as well as recommendations for vitamin D intake in various
subgroups, are available.
28
The most effective source of vitamin
D remains sunlight exposure, but recent concerns regarding the
disadvantages of UV exposure has made this undesirable. However,
it must be said that the amount of sunlight needed to produce a
significant amount of vitamin D is very little.
28
In conclusion, vitamin D has proven effects on insulin secretion
and sensitivity. Observational data supports an association between
vitamin D deficiency and diabetes, but this does not prove causality.
Interventional trials to resolve the question of benefit, either on
development or outcome of diabetes, are still largely lacking. There-
fore, testing and subsequent vitamin D supplementation for diabetic-
related outcomes is at presently not recommended in patients.
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