Abstract
AIMS: Cross-sectional studies demonstrate an association between low serum levels of vitamin D and slower gait speed in older individuals. However, longitudinal studies remain inconclusive. This study investigates whether vitamin D deficiency and insufficiency are risk factors for the incidence of slowness. MATERIALS AND METHODS: A total of 2815 participants from the English Longitudinal Study of Ageing (ELSA), aged ≥60 years and with a baseline gait speed >0.8 m/s, were followed for six years. Baseline serum levels of vitamin D [25(OH)D] were categorized as "sufficiency" (>50 nmol/L), "insufficiency" (>30 and ≤50 nmol/L) or "deficiency" (≤30 nmol/L). Gait speed was reassessed at four and six years of follow-up to identify incident cases of slowness (walking speed ≤0.8 m/s). A Poisson regression model, adjusted for sociodemographic, behavioural and clinical characteristics at baseline, was conducted to determine the association between serum 25(OH)D levels and the risk of slowness. RESULTS: The incidence densities of slowness per 1000 person-years were 67.4 (95% CI: 60.93-74.64) for sufficiency, 76.7 (95% CI: 68.30-86.22) for insufficiency and 90.7 (95% CI: 78.46-104.92) for deficiency. Serum 25(OH)D deficiency was associated with a 22% increase in the risk of slowness (IRR: 1.22; 95% CI: 1.01-1.49) compared with serum 25(OH)D sufficiency. No significant association was observed for serum 25(OH)D insufficiency. CONCLUSIONS: Serum 25(OH)D deficiency is a risk factor for the incidence of slowness in older individuals, suggesting that maintaining sufficient 25(OH)D levels could be a strategic approach to minimise long-term mobility impairment.