Population vitamin D stores are increasing in Tasmania and this is associated with less BMD loss over 10 years.

INTRODUCTION Vitamin D deficiency is a common, modifiable determinant of musculoskeletal health. There are limited data that examine the longitudinal change in population 25-hydroxyvitamin D (25(OH)D) and none that evaluate the long-term skeletal outcomes of longitudinal vitamin D status. METHODS Prospective cohort analysis of community-dwelling adults aged 50-80 years who had 25(OH)D assessed by radioimmunoassay and BMD by DXA at baseline (n=1096), 2.5 (n=870) and 10 (n=565) years. Sun exposure was quantified by questionnaire and supplement use at clinic review. 25(OH)D <50nmol/L was considered deficient. Participants were provided with their 25(OH)D results. RESULTS Over 10 years 25(OH)D increased (52.2 ± 17.0 to 63.5 ± 23.6 nmol/L, p<0.001). Participants with baseline deficiency had larger 25(OH)D increases than baseline sufficient participants (19.2 ± 25.3 vs 1.6 ± 23.3 nmol/L, p<0.001). Longitudinal change in 25(OH)D was associated with baseline summer (β=1.46, p<0.001) and winter (β=1.29, p=0.003) sun exposure, change in summer (β=1.27, p=0.002) and winter (β=1.47, p<0.001) sun exposure and vitamin D supplement use (β=25.0 - 33.0, p<0.001). Persistent vitamin D sufficiency was associated with less BMD loss at the femoral neck (β=0.020, p=0.027), lumbar spine (β=0.033, p=0.003) and total hip (β=0.023, p=0.021) compared to persistent vitamin D deficiency. Achieving vitamin D sufficiency was associated with less BMD loss at the lumbar spine (β=0.045, p<0.001) compared to persistent vitamin D deficiency. CONCLUSIONS Population 25(OH)D concentration increased due to a combination of increased sun exposure and supplement use. Maintaining or achieving vitamin D sufficiency was associated with less BMD loss over 10 years.