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Linking C-Reactive Protein to Late-Life Disability in the National Health and Nutrition Examination Survey (NHANES) 1999–2002

¹ Department of Internal Medicine, National Taiwan University Hospital, Taipei, Taiwan.
² Department of Physical Medicine and Rehabilitation, Harvard Medical School, and Spaulding Rehabilitation Hospital, Boston, Massachusetts.
³ Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts.

Address correspondence to Chung-Jen Yen, MD, 7 Chung-Shan S. Road, Department of Internal Medicine, National Taiwan University Hospital, Taipei, Taiwan. E-mail: ycj{at}ha.mc.ntu.edu.tw

Background. Chronic inflammation, measured by interleukin-6, predicts incident disability among elderly people. However, little is known about the relation of C-reactive protein (CRP) to disability.

Method. Participants (>60 years old, N = 1680) were from the National Health and Nutrition Examination Survey 1999–2002. Disability in activities of daily living (ADL), instrumental activities of daily living (IADL), leisure and social activities (LSA), lower extremity mobility (LEM), and general physical activities (GPA) was obtained by self-report. Peak muscle power was the product of isokinetic peak leg torque and peak force velocity. Functional limitations were evaluated via habitual walking speed, which was obtained from a 20-foot timed walk. CRP levels were quantified by using latex-enhanced nephelometry.

Results. Elevated CRP levels were associated with disability in IADL, LSA, LEM, and GPA, independent of basic demographics, chronic medical diseases, health behaviors, as well as nutritional markers. The corresponding odds ratios of disability for each standard-deviation increase in natural-log-transformed CRP were 1.18 (95% confidence interval [CI], 1.02–1.35), 1.18 (95% CI, 1.00–1.39), 1.17 (95% CI, 1.03–1.33), and 1.17 (95% CI, 1.05–1.31), respectively. The relationship diminished after additional adjustment of leg power and/or walking speed, meaning that impairment in leg power and limitations in gait speed likely mediate the association between CRP and disability. CRP had an inverse relationship to leg power and walking speed. Likewise, additional adjustment for leg power substantially diminished the association between CRP and walking speed, suggesting a mediating effect of power between CRP and gait speed.

Conclusions. Independent of chronic diseases, elevated CRP is associated with multiple domains of disability through mediation of muscle power, habitual gait speed, or both. Future research is needed to understand CRP as a risk factor for disability in older populations.