wo ist vitamin d drin — which blood markers matter?

Vitamin D is found in fatty fish (salmon, mackerel, herring), liver, egg yolks, and fortified foods (margarine, milk). However, the primary source is endogenous synthesis via sunlight (UVB). The key blood marker is 25-hydroxyvitamin D (25(OH)D), which reflects total vitamin D status. A level of 20 ng/mL is considered borderline insufficient per IOM/DGE guidelines. Parathyroid hormone (PTH) and calcium can be measured additionally to detect regulatory disturbances. Genetic variants like VDR rs2228570 (from your MyBody-X test) affect vitamin D metabolism, but evidence for direct impact on skin aging is mechanistic and not supported by human studies. Blood markers are more reliable than genetic predictors. Supplementation with 800–2000 IU daily is common for insufficiency, but medical consultation is advised. Caveat: The cited sources (PMC13050347, PMC12862355) relate to VDR and skin aging, not directly to dietary sources.