Abstract

Background: Vitamin D deficiency in older adults may impair muscle function and physical performance. This prospective observational cohort study examined the 12-month outcomes of vitamin D supplementation, delivered as part of routine clinical care, in older adults with vitamin D deficiency compared with an insufficiency comparison group. Methods: A total of 194 participants aged ≥60 years were enrolled: 164 with vitamin D deficiency (25(OH)D <20 ng/mL; supplementation group) and 30 with insufficiency (20–29 ng/mL; comparison group). The supplementation group received supplementation targeting 25(OH)D >30 ng/mL; the comparison group received no supplementation. The Handgrip Strength Test (HGS), the Gait Speed Test (GST), the Short Physical Performance Battery Test (SPPB), and the Timed Up and Go Test (TUG) were assessed at baseline and 12 months. Results: Supplementation raised 25(OH)D by 142.9% versus 5.4% in the comparison group (p < 0.001). For the primary outcome, the supplementation group showed a 16.2% increase in the HGS versus no change in the comparison group (p < 0.001; ANCOVA partial η2 = 0.284). For the secondary outcomes, the comparison group was associated with a significant functional decline relative to the supplementation group: the median change on the SPPB in the comparison group was 0.0% (IQR: −8.7, 0.0; between-group p < 0.001) and the TUG worsened by +3.2% (between-group p < 0.001), while the supplementation group remained stable. The ANCOVA indicated between-group differences on the SPPB (p < 0.001) and TUG (p = 0.002). Gait speed showed no significant difference. Conclusions: Vitamin D supplementation was associated with more favourable functional outcomes across multiple musculoskeletal domains in vitamin D-deficient older adults. The comparison group’s deterioration on the SPPB and TUG is consistent with an association between correcting deficiency and preserved function.

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