Overview
- Authorities highlight that in Germany the body can synthesize Vitamin D from sunlight mainly March to October, so winter checks of 25(OH)D are advised before supplementing and labs use differing cutoffs and units.
- For adults, DGE recommends about 800 IU daily when needed and EFSA sets a tolerable upper limit of 4,000 IU, with experts warning that unsupervised high doses can cause kidney stones, calcifications and heart rhythm problems.
- Clinicians advise verifying levels especially above roughly 2,000 IU per day and adjusting for individual factors, noting Vitamin D is fat‑soluble and best taken with fat when in tablet form.
- Infants are to receive 400–500 IU daily from the second week for at least a year, extending to 18 months for babies born in autumn or winter, and older children should supplement only on medical advice after testing.
- Nutrient interactions are nuanced: D3 and K2 or magnesium can be useful but are not routinely required, K2 needs caution with certain anticoagulants, and B12 remains essential with low toxicity yet warrants testing in at‑risk groups such as vegans or those with absorption disorders.