Population-specific recommendations on sunlight exposure could reduce risk of vitamin D deficiency: results of a dose-response study in UK S. Asians

Vitamin D insufficiency and deficiency remain prevalent at northerly latitude, particularly in individuals of South Asian ethnicity. Health agencies advise that low vitamin D status can be avoided through short casual summer sunlight exposures but this advice is geared towards white skinned people. A UV exposure regime simulating this can produce vitamin D sufficiency (25[OH]D ≥20 ng/mL) in 90% of UK white Caucasians but none of the S. Asians attained this level. As skin cancer risk is low in S. Asians, we examined whether higher UV exposure levels could produce vitamin D sufficiency and avoid deficiency (25[OH]D <10 ng/mL). In a dose-response study, 60 healthy S. Asians (20-60y) received one of 6 UVR exposures ranging from 0.65-3.9 SED, equivalent to 15-90 minutes midday summer sunlight at 53.5°N (Manchester, UK), 3 x weekly for 6 weeks. Exposures were performed in a whole body cabinet (95% 320-400 nm, 5% 290-320 nm), with subjects wearing casual clothes revealing ~35% skin area. Weekly blood samples were taken for serum 25(OH)D analysis. All 51 subjects completing the UV course were vitamin D insufficient at baseline (mean±SD 25[OH]D 6.5±2.8 ng/mL) with 90% deficient (<10 ng/mL). Serum 25(OH)D was significant higher in all dose groups post-course (P≤0.01) but only 6/51 subjects reached ≥20 ng/mL. The 3.25 SED group attained the highest mean±SD 25(OH)D level (18.0±6.2 ng/mL) and greatest rise (12.7±7.8 ng/mL), with the highest dose of 3.9 SED failing to produce higher levels. A 25(OH)D level ≥10 ng/mL was achieved by 31/33 (94%) subjects receiving ≥1.95 SED (equivalent to ≥45 minutes unshaded midday sunlight exposure at UK latitude). Initial rise in 25(OH)D appeared linear in all dose groups then started to plateau as the course continued. Consistent with the above findings, a non-linear one-phase association model predicted none of the dose groups to reach a mean 25(OH)D ≥20 ng/mL, but mean 25(OH)D levels in those receiving ≥1.95 SED would plateau at >15 ng/mL. Current sunlight exposure guidelines are inappropriate for S. Asians living at northerly latitude. Vitamin D status of this population sector could be enhanced by targeted guidance on increased sunlight exposure to achieve a level that avoids deficiency (25[OH]D <10 ng/mL), thus avoiding risk of osteomalacia and rickets, and may assist dietary strategies in attempts to reach ≥20 ng/mL.