The paper reviewed here is ‘Physical activity and risk of neurodegenerative disease: a systematic review of prospective evidence’ by Hamer and Chida. This is an important subject given the impending epidemic of dementia that is being predicted. The underlying question is obvious – can exercise reduce our risk of dementia?
The researchers undertake a meta-analysis of epidemiological studies investigating the relationship between risk of dementia and physical activity according to a previously identified protocol for ‘reporting meta-analysis of observational studies in epidemiology’ by Stroup and colleagues in 2000. They identify three databases (1990-2007) for their initial search
(2) The Cochrane Database of Systematic Reviews
(3) The Web of Science Databases
They used the terms ‘physical activity’ or ‘exercise’ and ‘Alzheimer’s Disease’, ‘Parkinson’s Disease’ and ‘Cognition’. I thought it might have been useful to add ‘Vascular Dementia’ to the search particularly as this is a relatively common form of dementia and can occur comorbidly with Alzheimer’s Disease. It would have been interesting to see other forms of dementia including Frontotemporal Dementia. However the terms can be justified on the basis that the results are presented according to the subtypes as well as the broader umbrella of ‘dementia’.
After the initial filtering of papers according to the inclusion criteria (e.g english-language papers, prospective cohort studies), the researchers rated papers independently according to quality (e.g quality of outcome measures used) and validity. The studies of interest included the Hazards Ratio and/or Relative Risk ratios for dementia according to level of physical activity.
They identified 1866 studies and included 16 in the final analysis. I wasn’t clear on whether the physical activity levels were comparable between the subjects in the studies. The papers included organised physical activity into groupings and the researchers then selected out a comparison of the highest physical activity group against the lowest physical activity group.
The pooled relative risk ratio for Alzheimer’s Disease in the highest v lowest physical activity groups was 0.55 with a 95% Confidence Interval of 0.36-0.84 (p=0.006) although they found significant heterogeneity in these results. They were unable to find publication biases (using the Begg’s asymmetry method) in the analyses. They identified more ‘robust’ effects in the ‘higher quality’ studies compared to the ‘lower quality’ studies. They concluded that high levels of physical activity could reduce the risk of Alzheimer’s Disease by45% on the basis of these findings.
With an analysis of the epidemiological data here, there remains the question of whether there could be confounders. One confounder might be the comorbidity of Vascular Dementia with Alzheimer’s Disease particularly as the physical activity will be expected to impact on cardiovascular health. A more recent confounder however is quite surprising – sitting! This study has only just been covered in the media (e.g here and here) so I haven’t had a chance to see the original paper (which I couldn’t find on the site yet). Perhaps this would be an area for further study.
Hamer M and Chida Y. Physical activity and risk of neurodegenerative disease: a systematic review of prospective evidence. Psychological Medicine. 2009. 39. 3-11.
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