Walking Speed, Cognitive Function, and Dementia Risk in the English Longitudinal Study of Ageing

Ruth A. Hackett, PhD; Hilary Davies-Kershaw, PhD; Dorina Cadar, PhD; Martin Orrell, PhD; Andrew Steptoe, DSc, DPhil

Disclosures

J Am Geriatr Soc. 2018;66(9):1670-1675. 

In This Article

Discussion

In this sample of 3,932 older adults, those with a slower walking speed were at greater risk of developing dementia. Individuals who experienced a greater decline in walking speed were also at greater risk. Participants with poorer cognition at baseline and those who experienced greater decline in cognition were also more likely to be diagnosed with dementia, although change in walking speed and change in cognition did not have an interactive effect on development of dementia.

Our finding that slower walking speed is a predictor of subsequent dementia is in line with previous research.[4,9] In a meta-analysis of 10 prospective studies in this area, 9 were conducted in the United States and one in Sweden.[9] Our study adds to this literature by demonstrating an association between walking speed and dementia in an English sample. To the best of our knowledge, this is also the largest sample used to address this question. The association remained significant in our sensitivity analyses, demonstrating the robustness of this finding.

The association between change in walking speed and dementia risk has not been investigated as much. In the current study, those with greater decline in walking speed over 2 measurement periods were more likely to develop dementia. This finding is in agreement with research in Swedish, French, and Japanese cohorts.[10–12] The relationship between change in walking speed and dementia remained after controlling for numerous covariates, including baseline walking speed. This suggests that, regardless of initial walking speed, a marked decrease over a relatively short period (2 years) may be an indicator of greater dementia risk.

We were interested in how cognition and walking speed might influence one another in affecting dementia risk. First, we found that individuals who had poorer baseline cognition were more likely to develop dementia. Second, those with greater decline in cognitive function between Waves 1 and 2 were at higher risk of dementia over follow-up. Finally, we investigated whether changes in cognition and walking speed interacted to affect risk of development of dementia, but we found no evidence to support this.

Initial cognitive function and decline in cognitive ability are associated with greater risk of subsequent dementia,[13] but understanding of the interplay between changes in walking speed and cognition is limited. Previous research attempting to ascertain whether these functions decline in parallel has been inconsistent, with one study suggesting that these functions decline in tandem,[18] another finding an association but only for one component of cognition (executive function),[19] and another finding no association at all.[20]

The present study extends this research by investigating whether changes in cognition and in walking speed interact to predict future dementia. Our findings suggest that declining walking speed and declining cognitive function are independent predictors of dementia but that these factors do not work synergistically. Cross-sectional evidence suggests that gait speed maps onto the stage of cognitive impairment; with the fastest speeds reported in individuals with mild cognitive impairment and the slowest speeds reported in those with advanced dementia.[28] It may be that this interplay between cognition and walking speed emerges only at the stage of clinically significant impairment rather than when individuals are still cognitively healthy.

This observational study provides information on the chronological, but not the causal, relationship between walking speed and dementia. Furthermore, reverse causality might be a factor. There is weak evidence from some[15,16] but not all studies[17] that cognitive function may predict walking speed and that walking speed might influence later cognitive status. In our sensitivity analysis excluding cases that occurred in the first 6 years after the walking speed assessment, the association between walking speed and subsequent dementia remained. This addresses the notion that undetected dementia cases affected walking speed and adds weight to the temporal sequence that gait slowing precedes dementia onset.

With regard to the mechanisms linking walking speed with future dementia, several possibilities could help explain our findings. It is thought that walking and cognition rely on similar brain regions, predominately in the prefrontal cortex.[29,30] Gait is a complex process in which the locomotor systems receive input from the basal ganglia, motor cortex, and cerebellum.[30] Although this process is largely automatic, walking relies on sensory feedback and high-order cognitive control.[29,30]

Neurodegeneration is a possible underlying mechanism linking declines in physical and cognitive function, with changes in subcortical white matter[31] and cortical gray matter volumes[31] associated with slower gait speeds. Vascular risk factors may also contribute to the link between gait speed decline and dementia through a similar pathway, because micro damage to the vessels of the prefrontal cortex and lesions (e.g., from stroke) is associated with white matter changes,[32] but our analyses were robust to adjustment for vascular risk factors and history of cardiometabolic disease.

Another potential mechanism is low-grade systemic inflammation. High concentrations of inflammatory markers are predictive of new-onset dementia[33] and have also been implicated in mobility impairment.[34] Neuroinflammation is thought to lead to impaired neuroplasticity in the brain areas controlling motor and cognitive function.[30] Furthermore, walking speed relies on muscle strength,[35] and muscle loss has been linked with deleterious inflammatory processes.[36]

These findings may have implications for efforts to delay dementia onset. The effect of exercise interventions on cognitive function and dementia is unclear.[37,38] Nonetheless, a meta-analysis of 42 studies examining the effects of 3 exercise interventions on walking speed[39] suggested that exercise can lead to increases in walking speed of up to 9.3%. Exercise may also improve cognitive function. A 2017 meta-analysis of 36 randomized control trials found that various types of exercise had a beneficial effect on cognition, regardless of baseline cognitive status.[40]

We analyzed data from a large representative sample of English adults using an objective test of walking speed and were able to confirm associations between walking speed and dementia risk after adjusting for multiple confounders, such as mobility impairment and cardiometabolic disease. Dementia was primarily identified according to self-reported physician diagnoses, which resulted in fewer cases than would be expected based on population estimates.[41] When this was supplemented by including diagnoses based on the IQCODE, the incidence of dementia was in line with U.K. estimates.[41] Although it is possible that we missed cases, given the consistency of our findings with earlier studies,[4,9–12] it is unlikely that misclassification bias accounted for our results. Furthermore, self-reported diagnoses of other conditions are noted to correspond closely with physician diagnoses, even in individuals with cognitive impairment.[42]

The average decrease in walking speed between Waves 1 and 2 was small and may not be clinically significant. Although the findings of our study suggest that individuals with larger declines in speed are more likely to develop dementia. Some aspects of cognition are thought to be more strongly associated with physical functioning than others.[11,15] However, this study did not investigate associations between walking speed and different aspects of cognition, meaning that potential interactions before dementia onset may have been missed.

Overall, this study suggests that individuals with slower walking speeds and those who have a greater decline in walking speed over time are at greater risk of dementia. Changes in walking speed and in cognition did not interact to affect development of dementia, indicating that they operate through independent pathways. Further research is required to understand the causal mechanisms underlying these associations and to determine whether increases in walking speed reduce dementia risk.

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