A synthesis of the entire array of risk and protective factors for fluid intelligence across adulthood is beyond the scope of this article. Thus, we focused our literature review on the four modifiable factors examined in our analyses: leisure-time physical activity, leisure-time cognitive activity, self-directed work, and hypertension.
Leisure-time physical activity is a potential protective factor for cognition and is gaining attention because of its possible relationship to physical fitness. Leisure-time physical activity includes all muscular activities and might or might not include regular exercise training that is aimed at improving physical fitness. There is growing evidence that high levels of physical fitness are related to improved function of the pre-frontal lobe, where inductive reasoning is thought to be located, and of the hippocampal region, where verbal memory is thought to be located (Lambert, Fernandez, & Frick, 2005
). Physical activity of low intensity was associated with greater cognitive decline 10 years later in 295 older men compared to physical activity of high intensity (van Gelder et al., 2004
). In a meta-analysis aerobic exercise training was shown to improve executive control, control processing, speed, and visualspatial function in older adults without cognitive impairment (Colcombe & Kramer, 2003
). In another meta-analysis (Heyn, Abreu, & Ottenbacher, 2004
), structured exercise training was found to enhance cognitive performance as measured by standard instruments such as the Mini-Mental State Examination (MMSE), even in persons with dementia. Another meta-analysis of nine observational studies (in which the sample size ranged from 469 to 4,615) showed that leisure-time physical activity earlier in life is associated with fewer occurrences of dementia and Alzheimer’s disease (AD; Fratiglioni, Paillard-Borg, & Winblad, 2004
). Leisure-time physical activity of at least twice a week in midlife was related to reduced risk for dementia (odds ratio = .48) and AD (odds ratio = .38) 21 years later in a sample of 1,449 persons aged 65–79 years (Rovio et al., 2005
). Similar findings were reported in a study indicating that of the 1,740 participants, those who exercised three or more times per week in 1994 had significantly less dementia in 2003 (13/1,000 person years developed dementia) than those who did not engage in as much exercise (19.7/1,000 persons years developed dementia; Larson et al., 2006
). However, others have failed to demonstrate a relationship between leisure-time physical activity and reduced AD risk (Verghese, Lipton, & Katz, 2003
). It is also unclear how leisure-time physical activity relates to verbal memory and inductive reasoning, because few researchers have directly measured these variables. It is possible that individuals with higher fluid intelligence are more likely to be more physically active and to participate in a range of activities that enhance brain health.
Leisure-time cognitive activity (e.g., puzzles) is another potential protective factor for cognition. A longitudinal study (N
= 801) found that a 1-point increase in leisure-time cognitive activity score was associated with a 64% reduction in the odds of developing AD, controlling for education and occupation (Wilson et al., 2002
). Leisure-time cognitive activity was associated with reduced AD risk even after controlling for subgroups of early AD manifestation, apolipoprotein E alleles 4, medical condition, and depression (Wilson et al., 2002
). A meta-analysis of 22 studies with than 29,000 individuals showed robust evidence that leisure-time cognitive activity is associated with a decreased risk of dementia (Valenzuela & Sachdev, 2006
). In six of these studies, the relationship was longitudinally examined. The researchers found a 50% lower incidence of dementia. Frequent participation in leisure-time cognitive activity is thought to place a high demand on the brain and improve brain reserve by stimulating neuronal activation and by buffering against neural degeneration (Valenzuela & Sachdev). Despite the encouraging findings, the association of leisure-time cognitive activity with verbal memory and inductive reasoning is not well understood.
Self-directed work at employment/occupation, including work complexity (complexity with data processing and people interaction), work control (autonomous, low supervisor control), and work routine (lack of repetitive job tasks), is posited to improve cognition (Andel, Kareholt, Parker, Thorslund, & Gatz, 2007
; Schooler, 1984
; Schooler, Mulatu, & Oates, 2004
). Similar to the mechanism of action for leisure-time cognitive activity, these work conditions stimulate an individual to perform cognitively demanding tasks on a daily basis. In a sample of 233 individuals (including married couples), more complex work was associated with better cognition 20 years later (Schooler, Mulatu, & Oates, 1999
). In contrast, lower work complexity -- characterized by lower mental and higher physical occupational demands -- was associated with increased risk for AD after controlling for race, sex, age, and education (N
= 357; Smyth et al., 2004
). However, the effect of self-directed work on cognition might be more complex and mediated by confounding variables such as higher occupational status (Andel et al.). For example, after controlling for age and education, higher occupational status (professional, white collar jobs) was associated with better fluid intelligence 10 years later in a sample of 6,073 British workers from the Whitehall II study, accounting for 27% and 52% of the variance in inductive reasoning in men and women, respectively (Brunner, 2005
). After controlling for occupational status, age, sex, and childhood socioeconomic status, work complexity remained significantly and positively correlated with MMSE scores in a cross-sectional analysis of 386 participants from a nationally representative Swedish sample (Andel et al.). Whether the association of self-directed work with verbal memory and inductive reasoning found in this cross-sectional study will hold in longitudinal designs is unknown.
An inverse relationship has been found between blood pressure and cognition (Elias, Robbins, Schultz, & Pierce, 1990
; Elias, Schultz, Robbins, & Elias, 1989
; Hertzog, Schaie, & Gribbin, 1978
). Uncontrolled hypertension was related to deficits in fluid intelligence above and beyond those attributable to age alone (N
= 357; Brady, Spiro, & Gaziano, 2005
). In a 5-year longitudinal study of 46 adults, systolic blood pressure at follow-up correlated with white matter hyperintensities in those with hypertension and with a decline in fluid intelligence such as reasoning (Raz, Rodrigue, Kennedy, & Acker, 2007
In the current literature, cognitive impairment is frequently operationalized as a diagnosis of AD or dementia. Research findings support a possible association between reduced dementia risk and the modifiable factors of leisure-time physical activity, leisure-time cognitive activity, self-directed work, and normal blood pressure. There is, however, a lack of research that examines the associations of these same factors with verbal memory and inductive reasoning by using longitudinal designs in healthy populations. Moreover, little work has been done to discern the interactive effects among these factors. Understanding these relationships over time has the potential to promote successful cognitive aging by facilitating early recognition and diagnosis of cognitive impairment and by informing the investigation and application of preventive interventions. Thus, this study was to examine the association of four modifiable factors -- leisure-time physical activity, leisure-time cognitive activity, self-directed work, and hypertension -- with changes in verbal memory and inductive reasoning in a sample of adults followed for 14 years.