Does vitamin D production in skin vary by season, latitude, skin exposure, sunscreen use, and age?

Vitamin D synthesis in skin is governed by both environmental UVB availability and skin-level biologic capacity. For a 71-year-old woman, these factors can make sunlight an unreliable sole source, particularly during winter or at higher latitudes.

Clinical and environmental evidence

• Season and latitude strongly determine vitamin-D-effective UVB. In direct skin testing, no previtamin D₃ production was detected in Boston from November–February; the nonproductive winter interval was longer in Edmonton. European data similarly show longer “vitamin D winters” with increasing latitude.
• More exposed skin generally increases potential production because a larger epidermal area contains the precursor for vitamin D₃ synthesis, although gains are nonlinear and depend on UVB dose and skin type.
• Properly applied sunscreen reduces UVB reaching the skin and can substantially suppress cutaneous production in controlled conditions. In usual practice, incomplete or thin application likely explains why observational studies often find little association between sunscreen use and circulating 25(OH)D.

Age and mechanism

• UVB converts epidermal 7-dehydrocholesterol to previtamin D₃, which becomes vitamin D₃ in skin. Aging is generally associated with lower cutaneous production, plausibly through reduced epidermal 7-dehydrocholesterol.
• Classic ex-vivo data found skin from adults aged 77–82 produced more than twofold less previtamin D₃ after UVB exposure than skin from those aged 8–18.
• This is not universal: a small 2024 controlled study of healthy ambulatory adults aged 65–89 found no significant difference from younger adults in skin 7-dehydrocholesterol or serum vitamin D₃ response after low-dose simulated sunlight.

Clinical implications

• Winter sun exposure at higher latitude should not be assumed to maintain vitamin D status, and sun protection should not be reduced to pursue vitamin D production.

Bottom line

• Season, latitude, exposed skin, sunscreen application, and age all materially alter cutaneous vitamin D production. Older adults on average have reduced capacity under comparable UVB exposure, but individual responses vary; dietary vitamin D or supplementation may be necessary when sunlight-derived synthesis is inadequate.