Evidence check: Vitamin D 25-OH in the context of Fruchtbarkeit

The role of vitamin D (25-OH) in fertility is increasingly discussed, but the evidence is mostly moderate. Observational studies link low vitamin D levels to reduced fertility in women (e.g., longer cycle length, lower oocyte quality) and men (lower sperm quality). Randomized controlled trials (RCTs) are fewer and show mixed results: some report improved live birth rates after supplementation, others no significant effect. The German Nutrition Society (DGE) recommends a 25-OH-D level of at least 50 nmol/l, but no specific fertility guidelines exist. Important: deficiency should be confirmed by blood test before supplementing, as long-term high doses (e.g., >100 µg/day) can be toxic. Individual genetics (e.g., VDR variants) may influence vitamin D metabolism, but the effect is small. Overall, maintaining adequate vitamin D is sensible but not a magic bullet. Evidence is based on human studies of moderate quality, not strong RCTs.