Sunlight Conversion Efficiency

DHCR7 codes for 7-dehydrocholesterol reductase, the enzyme that competes with sunlight for the precursor that becomes vitamin D in skin. Higher DHCR7 activity diverts more of the precursor to cholesterol synthesis; lower activity leaves more available for UVB conversion to vitamin D3. NADSYN1 sits alongside in the same pathway.

Carriers of common DHCR7 variants are associated with anywhere from typical sunlight-to-vitamin-D conversion to reduced efficiency, and the same sun exposure produces different vitamin D output across variant patterns. Skin pigmentation, age, and latitude all stack with the variant effect. By age 70, the same UV dose produces roughly a quarter of the vitamin D it did at age 20.

Latitude above ~37°N essentially eliminates UVB-driven vitamin D synthesis from October through March in most clients. Sunscreen blocks the conversion entirely (which is fine: UVA exposure carries the cancer risk, not the vitamin D production). Magnesium is required to activate the vitamin D made; deficiency stalls the pathway downstream of UVB.

The "I get plenty of sun" client and the "I work indoors" client are often closer in serum vitamin D than either expects. Which DHCR7 variant you carry, and where you live, decides whether sunlight is a meaningful vitamin D source or whether oral supplementation is the lever that actually moves levels.