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

Abstract

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 µg/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.

Absence of vitamin D deficiency among common outdoor workers in Delhi

“…The examples include farmers of Nebraska, coastal guards, field workers in Florida, and countries with plenty sunshine as Hawaii, Israel and Australia, where serum 25(OH)D ranged from 75 to 160 nmol/L . The vitamin D status of traditional hunters in Tanzanian, Caribbeans and Indian construction workers too ranged from 75 to 115 nmol/L …”

“…Interestingly, our recent study showed normal vitamin D status in outdoor construction workers of Delhi . Absence of VDD in the construction workers was explained by compulsory long hours of sun exposure.…”

Vitamin D sufficiency through sunshine in infants?

“…A recent observational study on adult Indians has documented that sunshine exposure is associated with higher serum 25(OH)D levels [9]. Outdoor manual workers in Delhi had normal vitamin D status with mean serum 25(OH)D values between 20 and 30 ng/mL [9].…”

“…Outdoor manual workers in Delhi had normal vitamin D status with mean serum 25(OH)D values between 20 and 30 ng/mL [9]. It is possible that the lack of adequate sunshine exposure among mothers and their offspring is resulting in a high prevalence of biochemical vitamin D deficiency.…”

Exposure to sunlight and Vitamin D synthesis

“…In another study from north India,[4] authors compared serum 25(OH) D and bioavailable 25(OH) D in males with varying durations of sun-exposure between indoor and outdoor environments in Delhi during August–September. They demonstrated that serum 25(OH)D increases by 2.03 ng/mL per hour of sun-exposure (95% confidence interval 1.77–2.28; P < 0.001).…”