A minimum vitamin D level of 50nmol/L should be the public health target in postmenopausal women to prevent fractures, according to findings from the Perth Longitudinal Study of Ageing in Women.

Risk of hip fracture was 40% lower in women with baseline 25-hydroxyvitamin D (25OHD) levels of 50-0.75nmol/L compared to those with levels under 50nmol/L, analysis of 15-year follow up data from 1348 women aged 70-85 years old found.

However, there was no additional benefit on fracture risk for vitamin D levels higher than 0.75nmol/L according to according to researchers from the Department of Endocrinology at Sir Charles Gairdner Hospital and the University of WA.

In the study of women recruited in 1998 into a calcium supplementation trial, the mean baseline plasma 25OHD was 66.9nmol/L, with  and 28.5%, 36.4%, and 35.1% of women having levels <50, 50 to 74.9, and ≥75 nmol/L, respectively.

During follow up total hip and femoral neck BMD and trabecular bone score, but not spine BMD, were higher with increasing plasma 25OHD up to 100 nmol/L.

Over the 14.5 year study period 27.6% of women had a fracture‐related hospitalisation and 10.6% had a hip fracture‐related hospitalisation.

Compared with women with 25OHD <50 nmol/L, those with levels 50-74.9 and ≥75 nmol/L had significantly lower risk for hip fracture [Hazard Ratio 0.60 and 0.61 respectively].

The Hazard Ratios for any fracture‐related hospitalisation were 0.77 and 0.70, respectively.

Writing in the Journal of Bone and Mineral Research, co-authors Dr Kun Zhu (PhD) and Professor Richard Prince said there was ongoing debate about the ideal level of vitamin D needed to maintain bone structure and prevent fractures in old age, with inconsistent results from three major population-based studies.

They noted that the US Preventive Services Taskforce does not recommend screening for low vitamin D levels and recommends against supplements for people without vitamin D deficiency.

But some of the evidence for the recommendations came from trials that did not take into account the fact that men have higher vitamin D thresholds for fracture risk, they said. There were also problems with the high doses of vitamin D used in some trials, which may have an adverse effect on falls risk, they added.

“Our data suggest that 25OHD levels below 50 nmol/L are related to increased risk for negative bone health outcomes, such as impaired bone structure and increased fracture risk,” they concluded.

“Consequently, women with 25OHD below this level should be the target population of vitamin D intervention,” they recommended.