BACKGROUND AND OBJECTIVE: There is little epidemiologic evidence considering the combined effect of dynapenia and low 25-hydroxyvitamin D [25 (OH) D] on incident disability. Our aim was to investigate whether the combination of dynapenia and low 25 (OH) D serum levels increases the risk of activities of daily living (ADL) incident disability. DESIGN: Prospective cohort study. SETTINGS: English Longitudinal Study of Aging. PARTICIPANTS: A total of 4630 community-dwelling adults aged 50 years and older without ADL disability at baseline. MEASUREMENTS: The baseline sample was categorized into 4 groups (ie, nondynapenic/normal 25 (OH) D, low 25 (OH) D only, dynapenic only, and dynapenic/low 25 (OH) D according to their handgrip strength (<26 kg for men and <16 kg for women) and 25 (OH) D (≤50 nmol/L). The outcome was the presence of any ADL disability 2 years after baseline according to the modified Katz Index. Incidence rate ratios (IRRs) adjusted by sociodemographic, behavioral, and clinical characteristics were estimated using Poisson regression. RESULTS: The fully adjusted model showed that older adults with dynapenia only and those with lower serum levels of 25 (OH) D combined with dynapenia had higher incident ADL disability risk compared with nondynapenic and those with normal serum levels of 25 (OH) D. The IRRs for lower 25 (OH) D serum levels combined with dynapenia were higher than for dynapenia only, however, the confidence intervals (CIs) showed similar effect for these 2 groups. The IRRs were 1.31 for low 25(OH) D only (95% CI 0.99-1.74), 1.77 for dynapenia only (95% CI 1.08-2.88), and 1.94 for combined dynapenia and low 25(OH)D (95% CI 1.28-2.94). CONCLUSIONS: Dynapenia only and dynapenia combined with low 25 (OH) D serum levels were important risk factors for ADL disability in middle-aged individuals and older adults in 2 years of follow-up
