Determinants of vitamin D levels from sun exposure

Vitamin D deficiency, generally defined as 25-hydroxyvitamin D (25(OH)D) concentration <50 nmol/L, affects nearly half the world's population. However, optimal 25(OH)D concentration is much higher, depending on the particular outcome. Solar ultraviolet B (UVB) exposure is the primary source of vitamin D for most people. Wintertime 25(OH)D concentrations are maintained at 60%–80% of summertime concentrations through a combination of recirculating 25(OH)D stored in muscle cells, oral intake, and at lower latitudes, some UVB exposure. Many factors affect 25(OH)D concentrations related to solar UVB exposure, including skin pigmentation, solar zenith angle, atmospheric aerosols and clouds, time spent in the sun, amount of skin surface area exposed, use of sunscreen, age, and body mass index. Cultural and lifestyle differences—such as beauty standards, including high regard for fair skin in darker-skinned populations and avoidance of wrinkling, occupation, religion-related clothing, urban/rural residence, and fear of developing skin cancer or melanoma—also affect some of those factors. Thus, fortification of food with vitamin D and vitamin D supplementation would have to be employed to compensate for unavoidable or inconvenient lack of solar UVB exposure.