The main aim of this paper was to examine the association between the different patterns of change in SRH and memory trajectories over eight years in samples of US and English older adults.
From a cross-sectional perspective, in both samples, individuals who rated their own health as excellent, very good and good in a stable pattern had the highest memory scores and those who rated their own health as fair or poor in a stable pattern over time had the lower scores in both memory tasks. Moreover, in general, no differences were found between the individuals who had an improving, declining or fluctuating pattern in SRH over time. These results are consistent with previous research which has found that there is a positive association between SRH and cognitive performance in older adults (Anstey & Christensen, 2000; Earles et al., 1997; Hultsch et al., 1999; Walker et al., 2004).
From a longitudinal perspective, in both samples, results showed that, for the immediate recall test, individuals whose SRH declined over time also showed a faster decline in their memory scores over time. For the delayed recall test, those individuals who rated their health as excellent, very good and good in a stable pattern over time declined slower than those who had a fair or poor stable pattern, declining or fluctuating SRH pattern over time. In general, the association between decline in SRH and decline in cognition provides some evidence that supports earlier studies suggestion of SRH as a useful predictor of cognitive decline (Carmelli et al., 1997; Earles et al., 1997; Hultsch et al., 1999; Sargent-Cox et al., 2011). However, some of these studies considered SRH only at baseline (Sargent-Cox et al., 2011) or only used two (Carmelli et al., 1997; Earles et al., 1997) or three measurement occasions (Hultsch et al., 1999). In addition, the present study focused on general population and not on clinical samples with cognitive impairment or dementia as some of the cited studies, and therefore, future research should examine whether changes in SRH are also predictive of cognitive impairment and all-cause dementia.
Perhaps one of the most interesting results is the distinction of speed of decline as a function of the pattern of SRH over time. Specifically, for the delayed recall task, there were significant differences in the rate of decline between those who had an excellent, very good and good stable pattern, fair or poor stable pattern, a declining or a fluctuating SRH pattern over time. Although several studies have identified these different patterns of change in SRH in older adults and associated them with health outcomes (Leinonen et al., 2002; Vogelsang, 2014), the present study is the first, to our knowledge, to have examined how these patterns can predict rate of decline in memory in older adults. Our results suggest that reporting changes in our own perception of health might be a good indicator of cognitive decline and, within this context, it would be interesting to examine if these changes could be an early symptom of preclinical dementia. This would be in the line with Jylhä (2009) conceptual model of SRH. She highlights that a unique source of information is provided by the body sensations which are directly available to the individual and these sensations might be a reflection of physiological dysregulations. The fact that decline in memory has been found to be directly associated with decline in SRH could be due to different reasons. A possible explanation could be that there are common underlying mechanisms which drive both declines, or maybe the decline in SRH could be a reflection of the own perception of change in general cognitive performance. Further research to detangle the possible causes that might be driving both changes in older adults is needed.
One of the strengths of this study was the use of SRH as a measure of general health as previous research has found SRH an adequate proxy to evaluate overall health in older adults (Galenkamp et al., 2013; Idler & Benyamini, 1997; Jylhä, 2009; Sargent-Cox, Anstey & Luszcz, 2010; Vogelsang, 2014) even when compared with objective measures of health (Lima-Costa et al., 2011; Wu et al., 2013). According to the SRH conceptual model proposed by Jylhä (2009), self-ratings of health involve a cognitive process that accounts for culturally and historically varying conceptions of health, reference groups, health expectations, cultural conventions in the use of the scale, and it does not only reflect diagnosed health conditions but very early signs of conditions which may not be yet diagnosed. Our findings suggest that SRH, in general, and SRH patterns of change, in particular, could be used as a marker of cognitive decline. However, it should be noted that as self-ratings of health involve a cognitive process, these ratings could be compromised in individuals who are already experiencing decline. Therefore, future studies should not only further examine the association between changes in SRH and cognitive impairment but also explore and compare the predictive value of SRH changes and changes in objective measures of health considering cognitively intact and cognitively impaired individuals. Some studies have further questioned the comparability of SRH across cultures (Jylhä, Guralnik, Ferruci, Jokela & Heikkinen, 1998) and that might be a reason of some of the slightly different results between the English and US sample. It would be extremely interesting to perform future studies with other countries to further our understanding of this kind of issues. However, the main aim of the study was to further our understanding of this association using a systematic approach, but the comparison of both samples was not intended.
Moreover, from a methodological point of view it is interesting that our cross-sectional findings are very similar in both samples but some differences arise when longitudinal results were examined. Replications of longitudinal studies are specially challenging as some characteristics of the studies cannot be exactly replicated (for example, mortality follow-ups or environmental and cultural influences); therefore, some authors have suggested the inclusion of a greater number of studies to minimize the effects of this variability between studies (Hofer & Piccinin, 2009). In this line, future replications of the present study with other large longitudinal studies are needed. Furthermore, the slight differences that were found between those respondents that had complete data and those that did not could be associated to the fact that the sample consists mainly of those individuals who are generally maintaining their cognitive function over time. This limitation has been previously addressed when exploring change in older adults’ studies and it makes the inferences of these studies conditional on the probability of surviving and remaining in the study (Hofer & Sliwinski, 2006). In addition, it has to be noted that the English sample was smaller than the US one, and therefore, some power issues might be affecting the English data. Finally, the present study focused in memory measures, immediate and delayed recall tasks, as they have been shown be good measures to identify cognitive decline even in young old adults (e.g., Schönknecht, Pantel, Kruse & Schröder, 2005). However, further research should replicate the present study with measures of processing speed or other cognitive measures.
To sum up, there is a significant association between the different patterns of change in SRH and memory trajectories in older adults. More specifically, different rates of decline in memory can be identified in the different patterns of SRH over time. Those who had an excellent, very good and good stable pattern of SRH had the slowest rate of decline compared to those who had a fair or poor stable pattern, declining or fluctuating SRH pattern over time. These results suggest that changes in SRH in older adults might be considered an indicator of cognitive decline and therefore, prevention and screening programs should be encouraged to use this easy, inexpensive and broadly used measure to identify possible pre-clinical dementia.