Osteoporosis and metabolic bone disease: 73. Do Low Vitamin D Levels Predict Osteoporosis?

BACKGROUND: Although the role of vitamin D deficiency in causing osteomalacia is well recognised, there is limited evidence to suggest that vitamin D deficiency is associated with osteoporosis. One community based study has suggested a reduction in bone density in females with low vitamin D levels, while another has shown no effect of isolated vitamin D deficiency on population fracture rates. We aimed to assess bone density in a cohort of women with low vitamin D levels but no other identified risk factors for osteoporosis. We wished to see if there was any relationship between the two, in order to clarify whether measurement of vitamin D was justified in aiding selection of patients for DXA, and whether DXA was justified in patients found to have isolated vitamin D deficiency. METHODS: We identified 70 women referred to an open access DXA scanning service by their GP with low vitamin D levels (under 48 nmol/l) as their only risk factor. We performed DXA of hip and spine and recorded the absolute bone density and Z scores for each patient, together with their vitamin D levels. We calculated the mean (SD) values for Z score at hip and spine, and compared these to a control group of women referred for baseline DXA prior to commencing aromatase inhibitors for breast cancer. We also compared the results with those obtained from the rest of the GP database for women referred up with a 10 year risk of osteoporotic fractures calculated at over 10% using FRAX. These statistical comparisons were made using unpaired Students t test. Mean values for Z scores at hip and spine were calculated and compared within the index group for those with vitamin D levels above and below 20 nmol/l using Students t test. Correlation coefficients between vitamin D levels and Z scores at hip and spine were also calculated for the index group. RESULTS: Mean (SD) values for Z scores in the hip and spine for patients with isolated vitamin D deficiency were +0.30 (0.87) and +0.46 (0.92) respectively. These were not significantly different from controls whose corresponding values were +0.17 (0.34) and +0.49 (0.45). By contrast, patients with a 10 year risk of fracture of over 10% had lower values at hip -0.01 (0.07) and spine -0.13 (0.09) [p = 0.01]. Mean Z scores for those with very low vitamin D levels (under 20 nmol/l) were not significantly different at either site when compared to those with less severe deficiency (20-48 nmol/l) [p = 0.26]. There was no correlation between vitamin D