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Armin Zittermann, Christian Trummer, Verena Theiler-Schwetz, Stefan Pilz

• \(\bf Purpose\) The upper tolerable intake level for vitamin D in the general population has been set at 4000 international units (IU) daily, but considerable uncertainty remains. We summarized reported harmful effects of a daily vitamin D supplement of 3200–4000 IU in trials lasting \(\geq\) 6 months. \(\bf Methods\) We performed a systematic review and meta-analysis of randomized controlled trials in several databases and identified 22 trials reporting safety data. Parameters of calcium metabolism, falls, hospitalization, and mortality were assessed. \(\bf Results\) The selected trials comprised a total number of 12,952 participants. All trials used supplemental vitamin \(D_3\). The relative risk (RR) of hypercalcemia in the vitamin D vs. control arm was 2.21 (95%CI: 1.26–3.87; 10 studies), with a vitamin D-induced frequency of hypercalcemia of 4 cases per 1000 individuals. Subgroup analysis in trials with > 100 and \(\leq\) 100 study participants revealed an RR of 2.63 (95%CI: 1.30–5.30; 7 studies) and 0.80 (95%CI: 0.24–2.62; 3 studies), respectively (\(P_{interaction}\) = 0.06). Risks of falls and hospitalization were also significantly increased in the vitamin D arm with an RR of 1.25 (95%CI: 1.01–1.55; 4 studies) and 1.16 (95%CI: 1.01–1.33; 7 studies), respectively. Risks of hypercalciuria, kidney stones, and mortality did not differ significantly between study arms. Quality assessment revealed high risk of incomplete reporting of safety-related outcome data. \(\bf Conclusion\) Supplemental vitamin D doses of 3200–4000 IU/d appear to increase the risk of hypercalcemia and some other adverse events in a small proportion of individuals, indicating that this dose is not completely safe. In future studies, rigorous reporting of safety-related outcomes is needed when using moderately high doses of vitamin D.