EFFECTS OF SKIN THICKNESS, AGE, BODY-FAT, AND SUNLIGHT ON SERUM 25-HYDROXYVITAMIN-D

We tested the hypothesis that the age-related decline in skin thicknes s may contribute to the age-related decline in serum 25-hydroxyvitamin D [25(OH)D]. We measured skinfold thickness on the back of the hand, serum 25(OH)D, height, and weight in 433 normal postmenopausal women. We also noted the average daily hours of sunlight in the month in whic h the observations were made and in the preceding 2 mo. Serum 25(OH)D was positively related to hours of sunlight (with a time lag of 2 mo) and to skin thickness, and negatively to body mass index (wt/ht2). Ser um 25(OH)D fell significantly after age 69 y. Seasonal variation of se rum 25(OH)D was greater in lean than in fat subjects, which we attribu ted to the larger fat mass and consequent larger pool size in the latt er group. The results suggest that the tendency for serum 25(OH)D to f all with age is due in part to the age-related decline in skin thickne ss.