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.