Bilingualism Is Associated with a Delayed Onset of Dementia but Not with a Lower Risk of Developing it: a Systematic Review with Meta-Analyses

Abstract

Some studies have linked bilingualism with a later onset of dementia, Alzheimer's disease (AD), and mild cognitive impairment (MCI). Not all studies have observed such relationships, however. Differences in study outcomes may be due to methodological limitations and the presence of confounding factors within studies such as immigration status and level of education. We conducted the first systematic review with meta-analysis combining cross-sectional studies to explore if bilingualism might delay symptom onset and diagnosis of dementia, AD, and MCI. Primary outcomes included the age of symptom onset, the age at diagnosis of MCI or dementia, and the risk of developing MCI or dementia. A secondary outcome included the degree of disease severity at dementia diagnosis. There was no difference in the age of MCI diagnosis between monolinguals and bilinguals [mean difference: 3.2; 95% confidence intervals (CI): −3.4, 9.7]. Bilinguals vs. monolinguals reported experiencing AD symptoms 4.7 years (95% CI: 3.3, 6.1) later. Bilinguals vs. monolinguals were diagnosed with dementia 3.3 years (95% CI: 1.7, 4.9) later. Here, 95% prediction intervals showed a large dispersion of effect sizes (−1.9 to 8.5). We investigated this dispersion with a subgroup meta-analysis comparing studies that had recruited participants with dementia to studies that had recruited participants with AD on the age of dementia and AD diagnosis between mono-and bilinguals. Results showed that bilinguals vs. monolinguals were 1.9 years (95% CI: −0.9, 4.7) and 4.2 (95% CI: 2.0, 6.4) older than monolinguals at the time of dementia and AD diagnosis, respectively. The mean difference between the two subgroups was not significant. There was no significant risk reduction (odds ratio: 0.89; 95% CI: 0.68-1.16) in developing dementia among bilinguals vs. monolinguals. Also, there was no significant difference (Hedges' g = 0.05; 95% CI: −0.13, 0.24) in disease severity at dementia diagnosis between bilinguals and monolinguals, despite bilinguals being significantly older. The majority of studies had adjusted for level of education suggesting that education might not have played a role in the observed delay in dementia among bilinguals vs. monolinguals. Although findings indicated that bilingualism was on average related to a delayed onset of dementia, the magnitude of this relationship varied across different settings. This variation may be due to unexplained heterogeneity and different sources of bias in the included studies. Registration: PROSPERO CRD42015019100.

Moderation-Mediation Effects in Bilingualism and Cognitive Reserve

“…Moreover, a more recent meta-analyses including eight studies found that on average, bilinguals exhibit AD symptoms 4.1 years and are diagnosed with AD 2.0 years later than monolinguals (Paulavicius et al, 2020). Another confirmed a 4.7-year delay in AD onset and a 3.3-year delay in the diagnosis of dementia, but no delay in the diagnosis of mild cognitive impairment (Brini et al, 2020). An additional meta-analysis of 21 studies (18 of which provided the age of AD onset and six of which provided AD incidence) found that the delay in manifestation of AD symptoms has a moderate effect size, although the effect size of rate of AD incidence was weaker (Anderson et al, 2020).…”

“…Finally, bilingualism, or the ability to speak two or more languages, is a skill that requires cognitive effort and interaction by many individuals on a regular basis. Similar to other mentally stimulating activities, bilingualism is linked to a delay in the onset and diagnosis of dementia (Bialystok et al, 2007), a robust finding that has been replicated multiple times (Anderson et al, 2020;Brini et al, 2020;Paulavicius et al, 2020). Although research on the relationship between the two is nascent, substantial and significant evidence has been found for bilingualism as a crucial factor that increases cognitive reserve.…”

“…An additional meta-analysis of 21 studies (18 of which provided the age of AD onset and six of which provided AD incidence) found that the delay in manifestation of AD symptoms has a moderate effect size, although the effect size of rate of AD incidence was weaker (Anderson et al, 2020). The overall conclusion from these studies was that the reported effects did not depend on education (Brini et al, 2020), SES, or publication bias (Anderson et al, 2020). Together, these results suggest that bilingualism has a low likelihood of preventing dementia but it is likely to delay dementia by roughly 4 years in manifestation of AD symptoms.…”

“…Clearly, this research domain would benefit from studies specifically looking at the moderating effects of bilingualism on the aging brain. Excellent literature reviews of the effects of bilingualism and cognitive reserve are provided by Calvo et al (2016) and Klimova et al (2017), and recent meta-analyses by Mukadam et al (2017b); Anderson et al (2020); Brini et al (2020); Paulavicius et al (2020). None of these articles make reference to the plausibility of bilingualism as a moderator, or equally if not more important, as a mediator, to which we now turn.…”

Bilingualism: A Global Public Health Strategy for Healthy Cognitive Aging

“…This study only included prospective studies and studies of different types of dementia (54). Overall, studies suggest that bilingualism is associated with a delayed onset of clinical presentation of dementia but not reduced risk of developing dementia or reduced incidence of dementia (54)(55)(56).…”