In the current study we examined the relationship of smoking status to cognitive decline (CD) in a sample of older Mexican American adults. The prevalence of smoking in our sample (18.4% of males and 8% of females were current smokers) was slightly higher than that reported from other national data. Others have found that among Hispanic Medicare recipients, 12.7% of males and 6.6% of females were current smokers (31
). Data also show that smoking prevalence differs by race and ethnicity. The prevalence of smoking among older Hispanic men is generally greater than that of older White men (11.9%), but less than that of older Black men (20.5%) (31
). For older Hispanic women, smoking prevalence is less than that of both White (10.4%) and Black women (11.3%).
Consistent with studies of non-Hispanics (3
), we found smoking predicted CD such that current smokers, compared to non-smokers, experienced a greater decline on a measure of cognitive functioning, the MMSE, over seven years. Importantly, this study involved several features that have been absent from other such studies, including a prospective design, a continuous measure of global cognitive functioning assessed at four occasions, a large sample size, a relatively long follow-up period, and the use of latent growth curve analysis. Moreover, this study focused on older adults of Mexican origin, one of the fastest growing populations in the US.
There are several hypotheses regarding the mechanisms by which smoking may affect CD. One hypothesis is that smoking causes oxidative stress, or cumulative damage caused by free radicals, to cells and organs including the brain (33
). Oxidative stress is evident in the pathogenesis of AD and may cause neuron degeneration (34
). Cigarette smoke contains free radicals (35
) and is involved in the generation of oxidative stress (36
). Furthermore, smokers tend to have both a lower dietary intake and circulation of antioxidants that neutralize free radicals (37
A second hypothesis is that long-term exposure to cigarettes may lead to atherosclerosis, resulting in stroke and subsequent vascular dementia. Tobacco smoke has been shown to increase risk of atherosclerosis (38
), which is caused by the formation of plaques within the arteries. Several ingredients in cigarettes and cigarette smoke, including nicotine monoxide, damage the endothelium and lead to the narrowing of blood vessels, increasing the likelihood of a blockage, and thus of a heart attack or stroke (38
Smoking may also affect cognition and the brain due to indirect effects on other conditions such as lung functioning (33
). For example, smoking has been shown to cause lung injury that leads to chronic obstructive pulmonary disease (38
). Poor lung functioning is associated with both poorer cognitive functioning and brain atrophy (39
). Finally, smoking may interact with other risk factors such as alcohol consumption and genetics (e.g., APOE gene) that are associated with increased CD (33
Although the mechanism by which smoking directly affects CD is as yet unknown, there is evidence to suggest that smoking does negatively affect brain structure (33
). In individuals with normal neurological and cognitive status at baseline, smoking has been shown to accelerate worsening white matter grade (40
), leuko-araiosis, cerebral atrophy, and cerebral perfusional declines, which are markers of depleted neuronal synaptic reserves that predispose individuals to CD and the onset of dementia (41
). On the other hand, it is important to note that others have not detected an effect of smoking on total brain atrophy (43
). However, some research has shown that reduction in total brain volume is independent of other degenerative changes, such as white matter hyperintensities, although this study found that smoking was related to both types of degeneration over time (44
While some factors known to influence CD (e.g., genetics) cannot be changed, smoking is a potentially modifiable behavior. Therefore, the benefits of smoking cessation among older Hispanics, in relation to CD in particular, should be explored. Some studies have suggested that quitting smoking may have benefits on cognition (3
). These findings point to the positive impact of smoking cessation on cognition even among older adults. In addition, there are other significant health benefits to quitting smoking even at an older age (32
Despite the many potential benefits of smoking cessation, there has been more focus on offering smoking cessation programs to young and middle-aged adults (45
) and to non-Hispanics (46
). Risk factors for smoking-related health conditions may not be addressed by clinicians because many assume that it is too late and too difficult for older adults to attempt to modify smoking behavior (47
). Additionally, older smokers may be unaware that there are significant health benefits of smoking cessation late in life (48
). Studies of community samples have found the cessation rate among older adults to be 10% (49
). Importantly, when offered the tools they need, older smokers quit smoking at rates comparable to those of younger smokers (48
). In particular, tailoring cessation programs in ways that are appropriate to age and ethnicity/culture has been effective in some studies for older adults (50
) and Hispanics (51
As in every study there are limitations that should be considered. One limitation of the present study was that there was an implicit assumption that the covariates were time-invariant. It was assumed, for example, that the demographic and health status variables remained invariant. Our model did not account for the likely change in health status over time.
Second, there was considerable attrition over time through the death of participants. Given the selective mortality of younger smokers (compared to non-smokers), we may have underestimated the influence of smoking on CD due to the premature death of smokers who could have experienced CD had they survived over the seven year follow-up period. In contrast, it should be noted that an additional latent growth curve analysis was conducted including all participants with missing data, that is, the data from participants who died between wave 1 and wave 4. With the inclusion of participants with missing data, there was only a trend towards smokers showing more CD than non-smokers (p = .08). This may have been the result of smokers dying prematurely of smoking-related illnesses before smoking affected cognitive functioning. That is, we may not have followed smoking participants, who died prematurely, long enough to document the changes in cognitive functioning related to smoking. Nonetheless, the current study’s results may not generalize to the population as a whole.
Third, there are other variables associated with smoking and CD which were not measured in the current study and which may have enhanced the apparent association between smoking and CD. Specifically, health and lifestyle factors associated with both smoking and CD may explain, in part, the observed association between smoking and CD. For example, smokers may have poorer nutrition (52
), be more likely to drink harmful levels of alcohol, or undertake less physical activity than nonsmokers (3
Future research could expand on the present investigation in several ways. First, there were no comparisons to other racial or ethnic groups to examine the possibility of a differential effect of smoking on CD. Second, the current study cannot identify the specific mechanisms by which smoking accelerates CD. Future investigations should employ more specific measures of smoking exposure that can quantify inhaled doses including smoking topography (e.g., puff volume, duration) or measures of cotinine (26
), rather than rely on self-reports of smoking behavior. In addition, biomarkers of oxidative stress or atherosclerosis could be included. Third, the benefits of smoking cessation on cognitive functioning should be explored perhaps through the inclusion of cognitive measures in large-scale studies of smoking cessation.
In sum, we found smoking to predict CD in older Mexican American adults. This finding is important because of the consequences for health care in Mexican Americans. Future research should focus on the specific needs of Hispanic elders in addressing smoking cessation.