Milk fortified with the current adequate intake for vitamin D (5μg) increases serum 25-hydroxyvitamin D compared to control milk but is not sufficient to prevent a seasonal decline in young women

Low vitamin D status in women of childbearing age may have implications for health. Vitamin D status of New Zealanders (NZ), based on low serum 25-hydroxyvitamin D (25OHD) is suboptimal. Vitamin D status may be improved with supplements and/or fortified foods. Recently an Adequate Intake (AI) for Australia and NZ was set at 5 μg/d vitamin D. We aimed to determine the effect of daily consumption of milk powder fortified with 5 μg vitamin D₃ on serum 25OHD concentration over 12 wks. 73 non-pregnant women (18 - 47 y) living in Dunedin, NZ (46°S) were randomised to receive either unfortified (control) or fortified (5 μg vitamin D₃) milk for 12 wks from January to April. Mean 25OHD was similar between groups at week 0 (control 74 vs 76 nmol/L) and fell significantly in both groups over the 12 weeks (control 53 nmol/L, fortified 65 nmol/L; p < 0.001). After 12 wks the fortified milk group had a serum 25OHD 19% (95% CI; 7, 32%) higher (10 nmol/L) than the control group after adjusting for baseline levels (p < 0.001). Daily consumption of fortified milk providing the current AI of 5 μg per day vitamin D₃ for 12 weeks resulted in higher 25OHD concentrations than control milk. This dose was not sufficient to prevent the seasonal decline in 25OHD. This study suggests an AI of 5 μg may be inadequate for New Zealanders to allow for seasonal changes in sunlight exposure, and is unlikely sufficient for other populations with low sunlight exposure.