Magnesium deficit - overlooked cause of low vitamin D status?
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Conclusion
The present study must be praised for bringing potential interactions between individual nutrients into the focus of interest. Because the intake of Mg is often inadequate and several other factors are also known to impair Mg supply (for example, diuretics use, diabetes mellitus, chronic alcohol consumption, stress factors) [16, 17, 18], more attention should in future be paid to possible consequences of insufficient or deficient Mg supply in the general population. Further studies on the interactions between Mg supply and vitamin D status should include a more detailed assessment of individual Mg status (for example, by measuring biochemical parameters of Mg status or by adjusting statistical analysis for stress factors, diuretics use, alcohol consumption and concomitant diagnoses such as diabetes); a more detailed investigation of different components of the vitamin D-parathyroid hormone axis in general populations; clarification of the dose?response relationship; and the realization of randomized controlled trials to verify whether oral Mg is indeed able to improve vitamin D status and survival...."

http://bmcmedicine.biomedcentral.com/articles/10.1186/1741-7015-11-229