Effects of Physical Activity on Cognitive Functioning in Middle Age: Evidence From the Whitehall II Prospective Cohort Study

Archana Singh-Manoux, PhD; Melvyn Hillsdon, PhD; Eric Brunner, PhD; Michael Marmot, PhD, MBBS, FFPHM, FRCP

Disclosures

Am J Public Health. 2005;95(12):2252-2258. 

In This Article

Discussion

Our results, derived from a large prospective cohort study of British civil servants, indicate that physical activity has a beneficial impact on cognitive functioning in middle age. The association between physical activity and cognitive performance previously has been examined in the elderly; in that age group there is considerable evidence that lack of physical activity is a risk factor for poor cognitive functioning. Our results showed a small but significant association between physical activity and cognitive functioning in middle age. The age- and gender-adjusted relationship between physical activity and cognitive performance was greatly attenuated by the addition of education and socioeconomic position to the model (results not shown), both of which have been shown to be critically important confounders of this relationship.[27,28] However, in analyses adjusted for all of the covariates assessed (age, gender, education, employment grade, self-rated health, blood pressure level, cholesterol level, smoking status, mental health status, social network index score), including a proxy for baseline cognitive functioning (Mill Hill Vocabulary Scale), physical activity remained significantly associated with fluid intelligence (AH 4-I score) and phonemic fluency.

We examined prospective, cross-sectional, and cumulative effects, because these effects signify different aspects of the relationship between physical activity and cognitive functioning. The prospective analyses showed that low levels of physical activity at baseline were significantly associated with lower scores on all cognitive tests 11 years later. After adjustment for the covariates, a significant association remained between low levels of physical activity and poor performance on the AH 4-I, a measure of fluid intelligence. The cross- sectional associations were more extensive, particularly before adjustment for baseline cognitive functioning. However, after adjustment for covariates, the cross-sectional associations were significant only for AH 4-I score and phonemic fluency. Dustman et al. suggested that such cross-sectional associations in fact represent cumulative effects of physical activity.[29] Our data allowed us to explore this hypothesis. The cumulative effect of physical activity was evident in a linear dose-response relationship, particularly for AH 4-I score. A significant linear effect also was observed for the measures of verbal fluency, accounted for mostly by the adverse effects of low levels of physical activity throughout the 3 measurement phases.

We examined different aspects of cognitive functioning rather than assessing general impairment with an instrument such as the Mini Mental State Examination, which is widely used in research on the elderly but is inappropriate for use among younger populations owing to ceiling effects (minor variation in scores—all participants score highly). A battery of cognitive tests similar to that used in this study better captures variability in cognitive scores. The disadvantage is the lack of standard criteria with which to judge poor cognitive performance. In keeping with other research in this area, we used lowest-quintile rankings to denote poor performance.[30] Our results indicate that fluid intelligence, as assessed with the AH 4-I, is particularly vulnerable to the negative effects associated with low levels of physical activity. These findings support the suggestion from earlier research that physical activity moderates the decline in cognitive functioning typically associated with aging.[31,32] The aspect of cognitive functioning that declines most with age is fluid intelligence,[33] and our results show that low levels of physical exercise are already a risk factor in middle age. Fluid intelligence is seen to be intrinsically associated with information processing and involves short-term memory, abstract thinking, creativity, ability to solve novel problems, and reaction time.

Our results concerning the association between physical activity and cognitive functioning in middle age add to the body of research showing the protective effect of physical activity on cognitive decline and dementia among the elderly. Identification of risk factors associated with cognitive decline, particularly earlier in the life course, is critical in formulating prevention or intervention strategies. It is highly likely that the relationship between physical activity and cognitive performance is reciprocal; that is, increased physical activity leads to better cognitive functioning and brighter people exercise more. However, it is the former pathway that has public health significance. In our data, causal direction is inferred rather than shown directly, in that a proxy for baseline cognitive functioning was controlled. Crystallized intelligence does not decline in middle age,[33] and we used this type of intelligence as a proxy for baseline level of cognitive functioning. Further support for the protective effect of physical activity on cognitive functioning has been provided by the results of fitness trials[15] and a study showing that this association holds even after adjustment for childhood cognitive performance.[34]

Although several viable hypotheses have been proposed, the mechanisms underlying the association between physical activity and cognitive functioning are poorly understood. Physical activity has been shown to sustain cerebral blood flow,[10,35] and it may improve aerobic capacity and cerebral nutrient supply.[36,37] Physical activity also is believed to facilitate neurotransmitter metabolism.[29,37] Cotman and Berchtold suggested that physical activity triggers molecular and cellular changes that support and maintain brain plasticity.[38] For example, 1 investigation revealed greater neuronal plasticity in the larger posterior hippocampi of London taxi drivers, who undertake intensive navigational study of the city as part of their training.[39] Another possibility is that physical activity reduces the risk of conditions or diseases (e.g., hypertension, diabetes, cardiovascular disease) that impair cognitive functioning.[40,41] Evidence from animal studies has shown that physical activity is associated with cellular, molecular, and neurochemical changes.[42,43,44,45] The effects observed have been related to increases in cerebral vascularization, increased dopamine levels, and molecular changes in neurotrophin factors that serve neuroprotective functions.[46]

A number of limitations of our study should be noted. First, although the sample covered a wide socioeconomic range, the data were derived from white-collar civil servants and cannot be assumed to represent general populations. Second, physical activity levels were self-reported and may have been underestimated or overestimated. This possibility is mitigated by the fact that comparisons between self-reported physical activity levels and measures of physical fitness have shown little discrepancy in other studies,[47] and the former self-reports of physical activity are more suitable for assessing activity over time.[48,49] Another problem is that the measure of physical activity used at phase 5 was different from that used at phases 1 and 3. However, because we employed broad physical activity categories, this discrepancy is unlikely to have influenced our results.

Third, our findings were probably affected by higher rates of missing data among the older- and lower-grade participants. This pattern of missing data is relatively common in longitudinal studies. Because both older- and lower-grade categories were related to low levels of physical activity and poor cognitive performance, it is likely that the effects reported here have been underestimated. Finally, use of the Mill Hill Vocabulary Scale as a proxy for baseline cognitive functioning is problematic in that it probably led to overadjustment and incomplete resolution of the issue of reverse causation.

In conclusion, our results indicate that physical activity is an important factor in cognitive functioning in middle age and that its effects appear earlier than previously reported. We showed that fluid intelligence is particularly at risk from lack of activity. Physical activity has long been linked to survival[50,51,52] and well-being among the elderly,[53] and there is evidence suggesting that the effects of physical activity on cognitive functioning are stronger in this age group than in others.[7,8,11,12,15] Further research is required to examine whether fluid intelligence remains at risk as individuals age and whether other aspects of cognitive ability, such as verbal fluency, also become increasingly associated with levels of physical activity.

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