During the interviews, participants were asked about difficulties performing six Activities of Daily Living (ADL), including bathing, dressing, eating, grooming, toileting, and continence. InCHIANTI participants were classified as having difficulty only if they required help or were unable to perform the task. This requirement identified a relatively small proportion of participants who were very disabled. The Short Physical Performance Battery score (SPPB) (23) was based on the results of balance tests, times to complete two 4-meter walks, and times to stand up from the seated position five times. The SPPB has been validated as a predictive marker for mortality and disability incidence (23). A 7-meter usual pace walk was also used. In addition, we used the Mini-Mental State Examination score (24) to assess cognitive functioning. Details of the InCHIANTI participants are given in Table 1. Fasting IL-18 serum concentrations were measured as described by Ferrucci and colleagues (12).

Note: For Activities of Daily Living (ADL), poor functioning is defined as needing help from another person or unable to do one or more of the activities (dressing, bathing, eating, grooming, toileting, or continence). Poor functioning for the performance scores, walking times, and cognitive levels are defined as the lowest 20%, slowest 20%, and lowest 20%, respectively, of the measured variable within the 65- to 80-year age group.

EPESE¼Established Populations for Epidemiological Study of the Elderly; SD ¼ standard deviation; MMSE ¼ Mini-Mental State Examination; SPMSQ ¼ Short Portable Mental Status Questionnaire.

In a recent study of four genes in the IL-18 system (IL- 18, IL-18 receptor, IL-18 receptor accessory protein, and IL- 18 binding protein) using 1288 patients with coronary artery disease (15), one single nucleotide polymorphism (SNP) was found to alter IL-18 serum levels. This SNP was also predictive of cardiovascular mortality over 6 years, but further studies will be needed to confirm this association.

Iowa-EPESE

The study design of the EPESE is described elsewhere (25). Briefly, between 1981 and 1983 the entire population aged 65 years or older, living in two Iowa counties was surveyed, and follow-up data were collected yearly for 7 years. Blood specimens were obtained from those partic- ipants reinterviewed for the sixth annual follow-up in 1988. To avoid confounding by ethnicity, we limited analyses to participants reporting that they were ‘‘White’’ to a question about race. Of these participants, 11% did not know the specific national or geographic origin of their maternal or paternal families, so we adjusted for this in models including Iowa-EPESE data by including two different values for ethnic origin: ‘‘known white’’ and ‘‘white but parents of unknown country of origin.’’

The timing of onset of physical and cognitive functional limitations is a core marker of biological age (16), and functioning is often a more useful measure of older people’s health than are individual diseases, as multiple diseases are common and severity varies widely. Many aspects of physical functioning—including muscle strength (17), gait speed, and time to complete five chair stands (18)—show substantial heritability in older people. Cognitive function is also highly heritable (19,20).

ADL items in this study included bathing, grooming, dressing, eating without aid, transferring from bed to chair, and using the toilet. The number of ADL problems was counted as tasks for which the participant reported difficulty irrespective of any need for help—this was a lower disability threshold than that used in the InCHIANTI study. The three components of the SPPB were tested in a way similar to those in the InCHIANTI study, except that the usual pace walk test distance was short (8 feet) and timings were carried out by stopwatch rather than by electronic sensors. The Short Portable Mental Status Questionnaire (26) was used to measure cognition. IL-18 concentrations were not available in this study.

In this study, we tested three related hypotheses: (i) that common variation in the IL-18 gene alters IL-18 serum concentrations, (ii) that increased IL-18 serum concentra- tions are associated with impaired functioning in old age, and (iii) that IL-18 gene alleles that reduce IL-18 concen- trations also improve physical functioning in old age. To do this, we used IL-18 serum concentrations and functioning data from the InCHIANTI study (21). We used a Mendelian randomization (22) approach to test the strength of asso- ciation between IL-18 concentrations and measures of func- tioning free from reverse causality and confounding. We used a second study, the Iowa- Established Populations for Epidemiological Study of the Elderly (EPESE) study pop-

  1L18 genotype   IL18 serumconcentration   Physical and Cognitive Functioning

 =       8&8 8 8 & 8 & &“Jwlowd sjeAe] Wndes UeOW 8171Rare Homozygotes (CC)Common Homozygotes (TT) Hetrozygotes (TC)185744256 Genotype

Physical and Cognitive Functioning

Figure 2. Mean interleukin-18 (IL-18) concentrations by alleles of the haplotype tagging IL-18 single nucleotide polymorphism rs5744256 (In- CHIANTI study, ages 65–80 years). Age-adjusted and sex-adjusted linear regression p 1 3 10(cid:3)5.

Figure 1. Triangle of associations of interleukin-18 (IL-18) genotypes, IL-18 serum concentrations, and physical functioning in old age. Possible causal directions of associations are shown by direction of arrows. Physical functioning and IL-18 concentrations cannot alter genotypes, but causal associations between physical functioning and IL-18 concentrations could occur in both directions.