Vitamin D-binding protein, vitamin D status and serum bioavailable 25(OH)D of young Asian Indian males working in outdoor and indoor environments

Urban Asian Indians generally have low serum 25(OH) D. Information on serum bioavailable 25(OH) D and the effect of prolonged sun-exposure in them is not known. We assessed serum 25(OH) D and bioavailable 25(OH) D in males with varying durations of sun-exposure in Delhi during August-September. Serum 25(OH) D, vitamin D-binding protein (DBP), bioavailable 25(OH) D, free 25(OH) D index, iPTH, ionized calcium and sun-index were assessed in outdoor, mixed outdoor-indoor and indoor workers (n = 88, 32 and 74, respectively). The mean sun-index (12.0 +/- 6.25, 4.3 +/- 2.20 and 0.7 +/- 0.62, respectively; P < 0.001) was highest outdoors and lowest indoors. Serum 25(OH) D (29.0 +/- 8.61, 19.1 +/- 5.73 and 10.9 +/- 4.19 ng/ ml, respectively; P < 0.001), bioavailable 25(OH) D and free 25(OH) D index were maximum in outdoor workers followed by mixed-exposure and indoor workers. Their mean serum DBP levels (241.2 +/- 88.77, 239.3 +/- 83.40 and 216.6 +/- 63.93 mu g/ml, respectively; P = 0.12) were comparable. Mean serum iPTH was significantly lower in outdoor than indoor workers and showed inverse correlations with serum 25(OH) D, bioavailable 25(OH) D and free 25(OH) D index (r = -0.401, -0.269 and -0.236, respectively; P < 0.001 in all). Daily dietary-calorie intake was higher and calcium lower in outdoor than indoor workers. On regression analysis, sun-exposure was the only significant variable, increasing serum 25(OH) D by 2.03 ng/ml per hour of sun-exposure (95 % confidence interval 1.77-2.28; P < 0.001). Outdoor workers with prolonged sun-exposure were vitamin D-sufficient, with higher serum bioavailable 25(OH) D than the indoor workers during summer. Use of serum DBP levels did not affect the interpretation of their vitamin D status.