There is ample evidence for an association between depression and cognitive impairment, both in younger (Rock et al., Reference Rock, Roiser, Riedel and Blackwell2014) and older adults (Alexopoulos, Reference Alexopoulos2005), as well as in inpatients with a clinical diagnosis of major depression (Reppermund et al., Reference Reppermund, Ising, Lucae and Zihl2009) and in community-dwelling individuals with subclinical depressive symptoms (Krell-Roesch, Reference Krell-Roesch2021). The study by Cano-López et al. (Reference Cano-López2021) showed that depressive symptoms were associated with cognitive impairment in community-dwelling older adults 5 years later. The majority of participants in Cano-López et al. study had subclinical depression. Depressive symptoms at a subclinical level are common in older people, and the consequences can be similar to those of major depression. The findings confirm existing evidence on a relationship between depression and subsequent cognitive decline (Byers and Yaffe, Reference Byers and Yaffe2011) and emphasize the importance of closely monitoring cognitive functions of patients with both subclinical and clinical depression. There is, however, an ongoing debate on whether depression is a risk factor for dementia, a prodrome, or simply a reaction of receiving a diagnosis of dementia. The recent Lancet commission on dementia prevention report confirms that depression is not only associated with dementia incidence but can also be part of the prodrome and early stages of dementia (Livingston, Reference Livingston2020). The underlying mechanisms linking depression and cognitive decline are still, to a large extend, unknown. Future life-course studies examining potential mechanisms linking depression to cognitive decline are needed so that interventions, prevention, and specific health-care needs can be targeted.

Interestingly, the study by Cano-López et al. showed that when considering education levels, the association between depressive symptoms and cognitive impairment is only seen in individuals with low educational levels. This confirms the importance of cognitive reserve as a protective factor for cognitive decline. Cognitive reserve can explain variability in late-life cognitive performance. It is unlikely, however, that the level of education is the only contributor to a person’s cognitive reserve. Other factors, including but not limited to, lifestyle, exercise, cognitively stimulating behaviors, social interactions, and personality are likely to form parts of cognitive reserve. Thus, rather than being a fixed entity, the contributors to cognitive reserve can change across the lifespan (Stern, Reference Stern2012). This more fluid concept suggests that changes in lifestyle even later in life might contribute to cognitive reserve to protect against age-related cognitive decline or dementia.

Cano-López et al. also confirmed the relationship between cognitive function and decreased functional ability over 5 and even 10 years. Independence in performing activities of daily living (ADL) is an important aspect of aging. The loss of independence in these activities is a key factor affecting the quality of life in older people. The vast majority of participants (95%) in Cano-López et al’.s study showed no change in cognitive function over time. Given that basic ADL are typically not impaired in people with normal cognition or mild cognitive impairment (MCI), it would be interesting to look at more complex instrumental ADL (IADL) instead. In contrast to ADL, which refer to basic everyday activities such as toileting and feeding, IADL require integration of multiple steps and actions, and sequencing for goal completion. IADL performance relies on cognitive function, in particular on memory and executive functions, as remembering, planning, organizing and cognitive flexibility underly most complex everyday activities. The ability to perform IADL declines with progression of cognitive impairment and can already be compromised in individuals with MCI, especially for activities with a high cognitive demand (Reppermund, Reference Reppermund2013). These activities require more cognitive resources than simple IADL or ADL and are, therefore, more vulnerable to early cognitive changes.

In addition to depressive symptoms being a risk factor for cognitive decline, there is evidence that depressive symptoms can also be a risk factor for subsequent IADL impairments (de Paula, Reference de Paula2015) and that improvement in depressive symptoms is associated with an improvement in ADL (Nyunt et al., Reference Nyunt, Lim, Yap and Ng2012) and IADL (Li and Conwell, Reference Li and Conwell2009). These findings further suggest that improvement of depressive symptoms buffers the adverse effects of cognitive decline on functional ability. Early detection and treatment of depression, including subclinical depression, are critical to increase independent living and improve the quality of life of older people. Future studies assessing the longitudinal relationship between cognitive function, cognitive reserve, depressive symptoms and IADL in older people with normal cognition and those with MCI may shed more light on the interplay of these factors and their contribution to healthy aging and dementia, respectively.

New research suggests that there is a functional analogue to cognitive reserve, i.e. functional reserve, that can account for individual differences in functional ability (Kraal, Reference Kraal2021). Functional reserve may explain variability in functional performance, for example why some older people perform everyday activities well despite having cognitive impairment and vice versa. Functional reserve has been initially defined by Berezuk et al. (Reference Berezuk2017) as “the protective influence of participating in IADLs throughout life, which may then guard against neurodegenerative functional decline”. Berezuk et al. (2017) showed that male gender is associated with lower functional ability, based on less experience with certain IADL (Berezuk, Reference Berezuk2017). Consequently, individuals who perform more IADL across their lifespan may develop a greater functional reserve, which in turn may delay or slow functional decline. The hypothesis is that more experience in performing IADL throughout life may lead to neural and physical changes, resulting in a greater capacity to withstand neurodegeneration. A more recent, extended concept of functional reserve includes brain volume measures, a range of cognitive functions and selected demographics (Kraal, Reference Kraal2021). Greater functional reserve based on this definition was associated with lower dementia severity and slower clinical progression. However, similar to cognitive reserve, it is likely that other factors such as lifestyle and personality also contribute to functional reserve. This relatively new concept needs to be validated in longitudinal studies including an assessment of interactions between cognitive reserve and functional reserve.

It is important to note that most studies use self- or informant-reports to assess functional ability. While these subjective measures are most widely used due to their brevity, low-costs, and ease of administration, they have a number of limitations. Self-reports are prone to overestimate IADL ability due to cognitive impairment and lack of insight or awareness (Farias et al., Reference Farias, Mungas and Jagust2005). They can also be influenced by individual factors such as current mood. Informant reports are prone to underestimate IADL ability because of caregiver burden, depressive symptoms, relationship issues or limited time spent with the individual (Hackett et al., Reference Hackett, Mis, Drabick and Giovannetti2020). Another challenge is finding appropriate informants who are able to provide a reliable estimate of the individual’s functional ability. Spouses, who are often the informants, may themselves have cognitive impairment which can invalidate the report. Performance-based IADL assessments offer an objective alternative, where individuals perform a range of activities under direct observation of the assessor (Luttenberger et al., Reference Luttenberger, Reppermund, Schmiedeberg-Sohn, Book and Graessel2016; Reppermund, Reference Reppermund2017; Goldberg, Reference Goldberg2010). Studies using performed-based IADL assessments are warranted to further examine the relationship between depressive symptoms, cognitive function, cognitive reserve and IADL ability in older people across the cognitive range. The concept of functional reserve has also not been assessed or validated using objective, performance-based assessments of IADL.

Given the continuing growth of the older population and the potential link between depression, functional ability, and cognitive decline, it is important to better understand the underlying mechanisms to reveal targets for intervention and prevention. Future research is needed to address this.