At baseline, 4399 participants aged 65-85 were seen at the study centre. We excluded from the analyses 722 who had one or more prevalent medical conditions strongly associated with decreased walking speed (477 with coronary artery disease, 211 with stroke, 53 with Parkinson’s disease, 38 with dementia, 12 with hip fracture). Walking speed at baseline was missing for 469 participants, who tended to be older (P<0.001) and with a higher BMI (P<0.001) than the other participants; there was no difference for other characteristics. All cause mortality during follow-up was not significantly higher (P=0.12) in participants without a measure of walking speed compared with those with a measure available. The mean (SD) age of the 3208 participants included in the study was 73.2 (4.6); 2085 (65%) were women. Vital status was determined for 3200 (99.8%) and missing for 8 (0.2%) at the end of the third follow-up (2005-6). During a mean (SD) follow-up of 5.1 (1.0) years, corresponding to 16
414 person years, 209 participants died (99 from cancer, 59 from cardiovascular disease, 51 from other causes).
Table 1 summarises participants’ baseline characteristics by vital status and thirds of walking speed. Those who died during follow-up were older, taller, more often men, and had a higher prevalence of cardiovascular risk factors (diabetes mellitus, hypertension, smoking) compared with survivors. About half of those who died were in the lowest third of walking speed, compared with one third of those who remained alive. Those in the lowest third of walking speed were older, smaller, had higher BMI, and were more likely to have depressive symptoms, less education, lower mini-mental state examination scores, and lower physical activity levels than those in the upper thirds of walking speed. They were also more likely to have hypertension or diabetes, have exertional dyspnoea or peripheral artery disease, and die during follow-up (table 1). There were no significant differences in baseline characteristics according to causes of mortality (data not shown), except for chronic use of non-steroidal anti-inflammatory drugs, which was associated with an increased cardiovascular mortality (P=0.03).
Table 1 Baseline characteristics of study sample by vital status and thirds of walking speed. Figures are numbers (percentages) unless stated otherwise
Figure 1 shows the cumulative risk of death (all cause, cancer, cardiovascular, other causes) by thirds of walking speed. There was a significant difference in all cause mortality across thirds of walking speed (log rank, P<0.001). Kaplan-Meier curves began to separate between 20 and 30 months after baseline and continued to diverge afterwards. The difference in the cumulative risk of death was significant for cardiovascular deaths (P<0.001) and other causes of death (P=0.004), while there was no significant difference for cancer deaths (P=0.26). In each of these analyses, there was no significant difference in the cumulative incidence of death between the middle and highest thirds of walking speed, and the increased risk of death mainly concerned those in the lowest third of walking speed. Our subsequent results are for the lowest third of walking speed compared with the two upper thirds.
Fig 1 Kaplan-Meier estimates of cumulative incidence of death according to thirds of walking speed, overall and by cause of death. Cut offs (tertiles) for definition of thirds of walking speed were ≤1.50 m/s, 1.51-1.84 m/s, and ≥1.85 m/s (more ...)
Table 2 shows multivariable analyses of the relation between walking speed and mortality. In analyses adjusted for age and sex (table 2, model 2), participants in the lowest third of walking speed had an increased risk of dying during follow-up (hazard ratio 1.64, 95% confidence interval 1.24 to 2.17) compared with those in the upper thirds; the difference in mortality was significant for cardiovascular mortality (2.85, 1.64 to 4.94) and other causes of death (1.79, 1.00 to 3.19), while there was no association with deaths from cancer (1.13, 0.74 to 1.71). After adjustment for other covariates associated with walking speed, the association remained significant overall and for cardiovascular mortality (table 2, model 3). Further adjustment for baseline vascular risk factors, psychotropic drug use, exertional dyspnoea, peripheral artery disease, being dependent in at least one instrumental activity of daily living, depressive symptoms, and use of non-steroidal anti-inflammatory drugs for joint pain led to similar results (table 2, model 4). Inclusion of walking speed as a continuous variable or using other cut points (quartiles or quintiles) led to similar conclusions (data not shown).
Table 2 Relation between walking speed and risk of death, overall and from specific causes
Figure 2 shows the results of stratified analyses; in each strata, hazard ratios are adjusted for age, sex, BMI, height, education level, mini-mental state examination, physical activity, diabetes mellitus, hypertension, use of non-steroidal anti-inflammatory drugs for joint pain, psychotropic drug use, alcohol, smoking, dyslipidaemia, exertional dyspnoea, peripheral artery disease, dependence in instrumental activities of daily living, depressive symptoms, and homocysteine level (model 4). In all strata, cardiovascular mortality was higher in the lowest third of walking speed than in the upper thirds (model 4). Interaction tests comparing the hazard ratios across the strata were not significant.
Fig 2 Walking speed in lowest third and risk of cardiovascular death: stratified analyses. Hazard ratios estimate relative risk of cardiovascular death in those in lowest third of walking speed (≤1.50 m/s for men; ≤1.35 m/s for women) (more ...)
In analyses in which we excluded those who were dependent in at least one instrumental activity of daily living at baseline (171, 5.3%, table 1), the hazard ratio (model 4) was 1.50 (1.08 to 2.08) for overall mortality and 3.13 (1.64 to 5.99) for cardiovascular mortality, while there was no association with other causes of death (1.21, 0.62 to 2.38) and cancer mortality (1.10, 0.68 to 1.79). In sensitivity analyses, we included participants who reported coronary artery disease at baseline and had a measurement of walking speed; our findings remained unchanged. Hazard ratios (model 4) were 1.41 (1.05 to 1.89) for overall mortality, 2.48 (1.33 to 4.62) for cardiovascular mortality, 1.11 (0.72 to 1.72) for cancer mortality, and 1.34 (0.77 to 2.32) for other causes of death.