Effects of magnesium citrate, magnesium oxide and magnesium sulfate on arterial stiffness 135 6 exhibited a greater increase in serum magnesium compared to organic formulations 18. This prompted us to compare the effects of organic with inorganic formulations, as bioavailability may differ between different formulations. The present study showed that the organic formulation magnesium citrate has a slightly higher bioavailability, assessed by 24-h urinarymagnesium excretion, compared with the inorganicmagnesium oxide and magnesium sulfate. However, the increase in plasma magnesium levels in the magnesium oxide and magnesium sulfate group was not significantly different from the increase in the magnesium citrate group during intervention. These findings are in line with the study of Coudray et al. showing that organic formulations were better absorbable than inorganic formulations, while no differences in terms of plasma magnesium were observed 17. Two studies reported higher 24-h urinary magnesium excretions as well as higher levels of serum and erythrocyte magnesium after magnesium citrate supplementation compared to magnesium oxide supplementation 16,25. The intestinal solubility likely plays an important role in bioavailability of magnesium. We previously observed that, during a 24-wk period, arterial stiffness was significantly improved following oral magnesium citrate supplementation, while the effect was not present at 3 months 10. In the current RCT, no effect of 24-wk oral magnesium supplementation on arterial stiffness was found. Similarly, Cuhna et al. recently reported no effect on arterial stiffness after 6-months of oral magnesium chelate supplementation administered at a dosage of 600 mg/d, which is 25% higher than the dose that was administered in our current study 26. Subgroup analyses of a recently published metaanalysis showed an effect of oral magnesium supplementation on endothelial function, which is another vascular function marker, was only present in RCTs with a study duration of at least 6-months 23. Thus, it is possible that a 24-wk treatment period is too short to find an effect of magnesium supplementation on vascular stiffness and that RCTs with longer treatment periods are required to provide evidence for an effect of oral magnesium supplementation on arterial stiffness. Although c-fPWV values were comparable with the previous observed values 10, in the present study we included more women and mean BMI and total cholesterol levels were lower at baseline, suggesting that the study population might have been slightly healthier than our previously selected study population.The meta-analysis of Marques et al. showed that magnesium was effective in RCTs including older and more overweight subjects 23. Indeed, in our subgroup analyses,the effect of oral magnesiumsupplementation on arterial stiffness was modified by age and BMI, in such a way that the effect was more pronounced in older and more overweight participants. However, the effects in these subgroups were
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