How much 25-hydroxyvitamin D can be replenished by sun exposure?
Dec 10, 2025
Source: Cainiu Health
Dr. Liu Wenmin
Introduction
Exposure to sunlight can provide 200 to 600 international units of 25-hydroxyvitamin D. With 15 to 20 minutes of sun exposure, adequate skin exposure, and moderate sunlight intensity, the amount synthesized approaches the daily requirement for adults. In summer, when sunlight is strong, synthesis efficiency is higher and the required amount can be achieved with short exposure. In winter, however, weaker and shorter sunlight leads to a significant reduction in vitamin D synthesis, which may be only one-third of that in summer.
Generally, sun exposure can provide 200 to 600 international units (IU) of 25-hydroxyvitamin D, with the exact amount depending on factors such as sunlight conditions. If in doubt, it is advisable to consult a healthcare professional beforehand. Detailed analysis is as follows:
When exposed to sunlight for 15 to 20 minutes with sufficient skin exposed and moderate sunlight intensity, vitamin D synthesis approaches the daily requirement for adults. In summer, when sunlight is strong, synthesis efficiency is higher, and even short exposures can meet daily needs. In winter, however, weaker and shorter sunlight results in significantly reduced synthesis—potentially only one-third of summer levels.
Skin condition and coverage also affect vitamin D production. Lighter skin absorbs ultraviolet (UV) rays more effectively and synthesizes vitamin D faster, while darker skin contains more melanin, which blocks UV radiation and requires longer sun exposure. Wearing excessive clothing or applying high-SPF sunscreen can prevent UV rays from reaching the skin, leading to inadequate synthesis.
To safely benefit from sun exposure, avoid peak sunlight hours, limit duration to prevent skin damage, and consider combining sun exposure with dietary sources of vitamin D. For personalized recommendations, consult a physician or registered dietitian.
