Seasonal changes in plasma 25-hydroxyvitamin D concentrations of young American black and white women.

Seasonal changes in 25-hydroxyvitamin D concentrations were studied in 51 black and 39 white women aged 20-40 y from Boston. Individual measurements were made in February or March (February-March), June or July (June-July), October or November (October-November), and the following February or March (February-March). Samples from the four visits were analyzed in batches at the end of the study. Plasma 25-hydroxyvitamin D was substantially lower in black than in white women at all the time points, including February-March when values were lowest (30.2 +/- 19.7 nmol/L in black and 60.0 +/- 21.4 nmol/L in white women) and June-July when they were highest (41.0 +/- 16.4 nmol/L in black and 85.4 +/- 33.0 nmol/L in white women). Although both groups showed seasonal variation in 25-hydroxyvitamin D concentrations, the mean increase between February-March and June-July was smaller in black women (10.8 +/- 14.0 nmol/L compared with 25.4 +/- 29.8 nmol/L in white women, P = 0.006) and their overall amplitude of seasonal change was lower (P = 0.001). Concentrations of serum parathyroid hormone in February-March were significantly higher (P < 0.005) in black women (5.29 +/- 2.32 pmol/L) than in white women (4.08 +/- 1.41 pmol/L) and were significantly inversely correlated with 25-hydroxyvitamin D in blacks (r = -0.42, P = 0.002) but not in whites (r = -0.19, P = 0.246). Although it is well established that blacks have denser bones and lower fracture rates than whites, elevated parathyroid hormone concentrations resulting from low 25-hydroxyvitamin D concentrations may have negative skeletal consequences within black populations.