Individuals with depression are often found to perform worse on cognitive tests and to have an increased risk of dementia. The causes and the direction of these associations are however not well understood. We looked at two specific hypotheses, the aetiological risk factor hypothesis and the reverse causality hypothesis.

We analysed observational data from two cohorts, English Longitudinal Study of Ageing (ELSA) and Health and Retirement Study (HRS), using cross-lagged panel models with unit fixed effects. Each model was run once with depression and repeated with cognition as the dependent variable and the other variable as the main explanatory variable. All models were estimated separately for contemporaneous effects and lagged effects up to 8 years in the past. We contrasted the results with models making the random effects assumption.

Evidence from the fixed effects models is mixed. We find no evidence for the reverse causality hypothesis in ELSA and HRS. While there is no evidence for the aetiological risk factors hypothesis in ELSA, results from HRS indicate some effects.

Our findings suggest that current levels of cognitive function do not influence future levels of depression. Results in HRS provide some evidence that current levels of depressive symptoms influence future cognition.

Little is known about the effect of persistent depressive symptoms on the trajectory of cognitive decline.

We aimed to investigate the longitudinal association between the duration of depressive symptoms and subsequent cognitive decline over a 10-year follow-up period.

The English Longitudinal Study of Ageing cohort is a prospective and nationally representative cohort of men and women living in England aged ≥50 years. We examined 7610 participants with two assessments of depressive symptoms at wave 1 (2002–2003) and wave 2 (2004–2005), cognitive data at wave 2 and at least one reassessment of cognitive function (wave 3 to wave 7, 2006–2007 to 2014–2015).

The mean age of the 7610 participants was 65.2 ± 10.1 years, and 57.0% were women. Of these, 1157 (15.2%) participants had episodic depressive symptoms and 525 participants (6.9%) had persistent depressive symptoms. Compared with participants without depressive symptoms at wave 1 and wave 2, the multivariable-adjusted rates of global cognitive decline associated with episodic depressive symptoms and persistent depressive symptoms were faster by –0.065 points/year (95% CI –0.129 to –0.000) and –0.141 points/year (95% CI –0.236 to –0.046), respectively (P for trend < 0.001). Similarly, memory, executive and orientation function also declined faster with increasing duration of depressive symptoms (all P for trend < 0.05).

Our results demonstrated that depressive symptoms were significantly associated with subsequent cognitive decline over a 10-year follow-up period. Cumulative exposure of long-term depressive symptoms in elderly individuals could predict accelerated subsequent cognitive decline in a dose-response pattern.

None.

High-sensitivity C-reactive protein (hs-CRP) has been suggested to be involved in the process of cognitive decline. However, the results from previous studies exploring the relationship between hs-CRP concentration and cognitive decline are inconsistent.

We employed data from wave 2 (2004–2005) to wave 7 (2014–2015) of the English Longitudinal Study of Ageing. Cognitive function was assessed at baseline (wave 2) and reassessed biennially at waves 3–7.

A total of 5257 participants (54.9% women, mean age 65.4 ± 9.4 years) with baseline hs-CRP levels ranged from 0.2 to 210.0 mg/L (median: 2.0 mg/L, interquartile range: 0.9–4.1 mg/L) were studied. The mean follow-up duration was 8.1 ± 2.8 years, and the mean number of cognitive assessment was 4.9 ± 1.5. Linear mixed models show that a one-unit increment in natural log-transformed hs-CRP was associated with faster declines in global cognitive scores [−0.048 points/year, 95% confidence interval (CI) −0.072 to −0.023], memory scores (−0.022 points/year, 95% CI −0.031 to −0.013), and executive function scores (−0.025 points/year, 95% CI −0.043 to −0.006), after multivariable adjustment. Compared with the lowest quartile of hs-CRP, the multivariable-adjusted rate of global cognitive decline associated with the second, third, and highest quartile was faster by −0.043 points/year (95% CI −0.116 to 0.029), −0.090 points/year (95% CI −0.166 to −0.015), −0.145 (95% CI −0.221 to −0.069), respectively (p for trend <0.001). Similarly, memory and executive function also declined faster with increasing quartiles of hs-CRP.

A significant association between hs-CRP concentration and long-term cognitive decline was observed in this study. Hs-CRP might serve as a biomarker for cognitive decline.