Vitamin D intake needed to maintain target serum 25-hydroxyvitamin D concentrations in participants with low sun exposure and dark skin pigmentation is substantially higher than current recommendations

Abstract

Cutaneous cholecalciferol synthesis has not been considered in making recommendations for vitamin D intake. Our objective was to model the effects of sun exposure, vitamin D intake, and skin reflectance (pigmentation) on serum 25-hydroxyvitamin D (25[OH]D) in young adults with a wide range of skin reflectance and sun exposure. Four cohorts of participants (n = 72 total) were studied for 7-8 wk in the fall, winter, spring, and summer in Davis, CA [38.5 degrees N, 121.7 degrees W, Elev. 49 ft (15 m)]. Skin reflectance was measured using a spectrophotometer, vitamin D intake using food records, and sun exposure using polysulfone dosimeter badges. A multiple regression model (R(2) = 0.55; P < 0.0001) was developed and used to predict the serum 25(OH)D concentration for participants with low [median for African ancestry (AA)] and high [median for European ancestry (EA)] skin reflectance and with low [20th percentile, approximately 20 min/d, approximately 18% body surface area (BSA) exposed] and high (80th percentile, approximately 90 min/d, approximately 35% BSA exposed) sun exposure, assuming an intake of 200 iu/d (5 ug/d). Predicted serum 25(OH)D concentrations for AA individuals with low and high sun exposure in the winter were 24 and 42 nmol/L and in the summer were 40 and 60 nmol/L. Corresponding values for EA individuals were 35 and 60 nmol/L in the winter and in the summer were 58 and 85 nmol/L. To achieve 25(OH)D > or =75 nmol/L, we estimate that EA individuals with high sun exposure need 1300 iu/d vitamin D intake in the winter and AA individuals with low sun exposure need 2100-3100 iu/d year-round.

FIGURE 1

Maximum potential UV-B exposure measured by PS dosimeter badges exposed in the direct sun on a horizontal surface on the same day each week that participants wore personal dosimeter badges. One badge was exposed each day. Each point represents a single badge (n = 31). Study cohort dates: fall, 10/29–12/15; winter, 1/17–3/16; spring, 4/11–6/8; and summer, 7/18–9/14. Study cohorts without a common letter differ, P < 0.05.

FIGURE 2

Individual mean UV-B exposure measured by PS dosimeter badges worn by participants once a week for each cohort. Each point represents the mean dose for each participant over 7–8 wk, adjusted for BSA exposed to the sun (n = 72). Study cohort dates: fall, 10/29–12/15; winter, 1/17–3/16; spring, 4/11–6/8; and summer, 7/18–9/14. Study cohorts without a common letter differ, P < 0.05.

FIGURE 3

Forehead skin reflectance varies by ancestry. The L* scale for skin reflectance extends from 0 (black) to 100 (white). Each point represents the mean forehead skin reflectance for each participant, n = 72. Groups without a common letter differ, P < 0.05.

FIGURE 4

Association between the serum 25(OH)D concentrations measured by LC-MS and RIA for all samples from 72 participants, n = 216. Regression equation: LC-MS 25(OH)D^0.1 = 1.02873 + (0.13195)(ln RIA 25(OH)D); R² = 0.8351; P < 0.0001. Dashed lines = 95% CI.

FIGURE 5

Distribution of mean (baseline, 4 and 7 or 8 wk) serum 25(OH)D2 and 25(OH)D3 concentrations in the 72 study participants measured by LC-MS. The X-axis indicates 20 nmol/L increments from 0 to 220 nmol/L.

FIGURE 6

Multiple, linear regression model prediction of serum 25(OH)D in 4 groups based on sun exposure [20th (low) or 80th (high) percentile for each season in Joules] and skin reflectance [median forehead reflectance for participants of African (dark skin) and European (light skin) ancestry]. Study cohort dates: fall, 10/29–12/15; winter, 1/17–3/16; spring, 4/11–6/8; and summer, 7/18–9/14. Vitamin D intake was set at 200 iu(5 μg)/d. The equation is shown in Table 4, footnote 1.