ciated with a reduction in walking time (p ¼ .016, Table 3). We observed a similar trend with improved functioning measured by SPPB, although this did not reach nominal significance (p ¼ .065).

In the Iowa-EPESE study, we saw a similar trend with the C allele of rs5744256 associated with reduction in walking time (p ¼ .026, Table 3). The SPPB score trended in the same direction but did not reach nominal significance (p ¼ .092).

Table 3. Association of the IL-18 Serum Level Altering SNP rs5744256 With Short Physical Performance Battery Scores and Walking Times in Participants Aged 65–80 Years in the InCHIANTI and Iowa-EPESE Studies

TT Genotype TC CC Total Tested Physical Mean Mean Mean Linear Regression Study Performance N (95% CI) N (95% CI) N (95% CI) N Coefficient (95% CI)* p Value InCHIANTI Summary 445 10.48 201 10.87 28 10.96 674 0.283 .065 Physical (10.24–10.72) (10.59–11.15) (10.37–11.56) (–0.02 to 0.58) Performance Battery 7-m walk 436 6.42 198 5.93 28 6.01 662 (cid:3)0.038 .016 times (6.17–6.79) (5.721–6.13) (5.46–6.57) ((cid:3)0.07 to (cid:3)0.01) Iowa-EPESE Summary 557 8.45 376 8.45 64 9.22 997 0.222 .092 Physical (8.24–8.67) (8.18–8.73) (8.63–9.81) ((cid:3)0.04 to 0.48) Performance Battery 8-ft walk 556 3.99 375 3.94 64 3.49 995 (cid:3)0.040 .026 times (3.84–4.14) (3.75–4.12) (3.15–3.83) ((cid:3)0.08 to (cid:3)0.005)

Notes: *All models were adjusted for age and sex, and Iowa models (which included only participants reporting being white) were adjusted for origin of parental families (known vs unknown): dependent variable logged. IL-18 ¼ Interleukin-18; SNP ¼ single nucleotide polymorphism; EPESE ¼ Established Populations for Epidemiological Study of the Elderly; CI ¼ confidence interval.

Table 4. Association of IL-18 Serum Levels (Log Transformed) With 7-m Walking Times (InCHIANTI Study Age Group 65–80 Years), in Simple Linear Regression and Instrumental Variable Models

Regression Change With Doubling Outcome Coefficient* 95% CI p Value of IL-18 Serum Levely Linear regression 0.082 0.029–0.135 .002 1.058 Instrumental variable model (instrument ¼ SNP rs5744256) 0.324 0.050–0.598 .021 1.252

Notes: IL-18 ¼ Interleukin-18; CI ¼ confidence interval; SNP ¼ single nucleotide *Test of equality of linear regression and instrumental variable estimates: p ¼ .048. yRatios of geometric means by a doubling in IL-18 serum level.

associated with a 0.25 SD reduction in serum concentrations of IL-18 per allele and with better performance, based on walk tests in 65- to 80-year-olds. We showed that the effect on walk speed is consistent with the expected effect given the association between the SNP and IL-18 concentrations and the association between IL-18 concentrations and functioning. As reverse association between genotypes and functioning is very unlikely, this evidence suggests that IL-18 may be an active or causal factor in inflammation and age-related poor physical function. Our results are consistent across two studies. However, the association between the genotype and functioning does not reach genome-wide thresholds for significance, so further studies are needed to confirm this association.

When data from the two studies were combined (using ranked data) the associations remained, even after adjust- ment for age, sex, study, and known family origin, in ordinal regression models. For the SPPB, the ordinal regression coefficient for the rs5744256 SNP was b ¼ 0.1740 (95% CI, 0.0280–0.3200; p ¼ .019). For ranked walk speeds in the two studies together, the adjusted association with rs5744256 had an ordinal regression coefficient of b ¼ (95% CI, (cid:3)0.3821 to (cid:3)0.0947; p ¼ .001).

Loss to follow-up over the first 6 years of the Iowa- EPESE study was strongly associated with disability at baseline, which could result in differential attrition of gene variants detrimental to functioning. We therefore did additional analyses restricted to those participants who reported being free of functional impairments at baseline. In this incident cohort (n ¼ 510), the regression coefficient for the SNP walking time association was in the same direction but reduced, and not nominally significant (coefficient (cid:3)0.015; p ¼ .505).

The details of measurement of the SPPB and walking times differed between the InCHIANTI and Iowa-EPESE participants. To deal with this in pooled analyses we have ranked all scores within study, before combining them in ordinal regression models. This process should have removed distributional differences between the measures. The minor allele frequency for rs5744256 was higher in the Iowa-EPESE study than in the InCHIANTI study (44% vs 33%, respectively), and we therefore corrected for study in the pooled models. In the Iowa-EPESE study, all partic- ipants included in our analyses reported being ‘‘white’’ to a question on race, and the majority identified maternal and paternal family origins in Europe, although a limited group did not know these origins. Although the entire included sample is likely to have been of Caucasian origin, we have adjusted for this possible source of admixture. The finding of broadly consistent associations within two quite separate population studies also makes it unlikely that admixture could have biased the results.

Mendelian Randomization Using rs5744256