4 year period participation in physical activity by

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question of whether, over a 6.4-year period, participation in physical activity by older adults reduces the rate of cognitive decline after accounting for participation in cognitively stimu- lating activities. After statistical adjustment for cognitive ac- tivities, each additional physical activity hour per week was associated with a slower rate of cognitive decline. However, this relationship was no longer significant after adjusting for cogni- tive activities (but see Richards et al., 2003, for a report of a relationship between physical but not intellectual activities and later-life cognition). At present, it is difficult to know which factors are most im- portant in moderating the influence of physical activity on later- life cognition and dementia. However, some possibilities that merit further study include the distinction between aerobic and nonaerobic physical activities; the utility of self-report versus more objectively measured physical activities; the role of physical-activity duration, intensity, and frequency; the nature of the components of cognition that serve as the criterion vari- ables; the age of participants at initial and final assessment; and genetic factors. A 6-year prospective observational study by Barnes, Yaffe, Satariano, and Tager (2003) included both self-report of physi- cal activity and objective (i.e., volume of expiratory oxygen [VO 2 ] at peak exertion) measures of cardiorespiratory fitness in 349 individuals over the age of 55. Interestingly, whereas a significant inverse relationship was observed for the objective fitness measures and cognitive decline, this was not the case for the self-report activity measures. Although the explanation for this dissociation cannot be unequivocally discerned in these data, it is conceivable that it is the aerobic aspects of the physical activities—which are more reliably indexed by the objective than self-report measures—that is more strongly re- lated to spared cognition than is those activities’ nonaerobic aspects. A number of investigators have also examined whether possession of e4 alleles on the apoe gene moderates the rela- tionship between physical activity and cognition. Unfortunately, however, results obtained thus far are equivocal with regard to whether possession of an e4 allele has a positive effect, negative effect, or no effect on the relationship between physical activity and cognition (Abbott et al., 2004; Larson et al., 2006; Podewils et al., 2005; Rovio et al., 2005; Schuit, Feskens, Launer, & Kromhout, 2001). This ambiguity could be the result of a number of different factors that may interact with genotype (such as those factors described above) in determining the relation- ship between physical activity and cognition. As will be dis- cussed below, a number of other genes also produce proteins relevant to neurotransmitter systems and nerve-growth factors that appear to play an important role in the relationship between physical activity and cognition. Such genes will be important to study in the future.

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