Effects of magnesium citrate, magnesium oxide and magnesium sulfate on arterial stiffness 117 6 Somatic and GI symptom severity Participants completed a Patient Health Questionnaire (PHQ-15) prior to the baseline, 12-wk and 24-wk visits. Severity was rated for the last four weeks (baseline), or since the last visit (follow-up) as 1 (“not bothered at all”), 2 (“slightly bothered”), or 3 (“remarkably bothered”). The item “menstrual cramps” was removed, as only post-menopausal women participated in the current study. The somatic symptom severity score was calculated by taking the mean score of the 14 somatic symptoms of the PHQ-15. The PHQ-15 includes three gastro-intestinal (GI) items: 1) stomach pain, 2) constipation, loose bowels, or diarrhea, and 3) nausea, gas or indigestion. Scores for the three items were added up in a composite GI score, with scores ranging from 1-6. Statistical analyses Given our two main research questions, we evaluated whether magnesium citrate is superior to placebo to improve c-fPWV (primary confirmatory objective), and explored whether magnesium oxide and magnesium sulfate are non-inferior to magnesium citrate to improve c-fPWV (secondary objectives). Because the primary objective was confirmation of a previous finding, sample size calculation was based on a two-sided type-I error of 0.05 and a power of at least 95%. Using the standard deviation of change in c-fPWV over time of the primary study of 0.9 m/s, it was calculated that 24 participants were required for both the magnesium citrate and the placebo group to allow for detection of a treatment effect of at least 1.0 m/s in c-fPWV. To account for potential drop out by 10%, group sizes were increased to 26 participants. For the exploratory secondary objective sample size calculation was based on a one-sided type-I error of 0.05 and a power of 80% and a non-inferiority design. Using the standard deviation of change in c-fPWV over time of the primary study of 0.9 m/s, it was calculated that 41 participants were required for the comparison of magnesium oxide with magnesium citrate and the comparison of magnesium sulfate with magnesium citrate with a non-inferiority margin of 0.5 m/s in c-fPWV. To account for potential drop out by 10%, group sizes were increased to 46 participants. Because we included 46 participants in the magnesium citrate group to provide power for the secondary analyses, the power for the primary analysis consisting of comparison of magnesium citrate with placebo treatment became 99%.All main analyses were performed according to the intention-to-treat principle. Additional per-protocol analyses were performed to examine whether exclusion of noncompliant subjects influenced the results. Normally distributed variables are presented as mean±standard deviation (SD) and nonnormally distributed variables as median (interquartile range). Data on somatic and GI symptom severity (PHQ-15) are presented as mean ± standard error (SEM). Longitudinal
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