Does lifestyle impact on cognitive function in old age?

This prospective study aimed to identify an optimal lifestyle profile to protect against memory loss in older individuals from areas representative of the north, south, and west of China. Individuals aged 60 years or older who had normal cognition and underwent apolipoprotein E (APOE) genotyping at baseline in 2009 were included. Participants were followed up until death, discontinuation, or 26 December 2019.

Six lifestyle factors were assessed:

• a healthy diet (adherence to the recommended intake of at least 7 of 12 eligible food items),
• regular physical exercise (≥150 min of moderate intensity or ≥75 min of vigorous intensity, per week),
• active social contact (≥twice per week),
• active cognitive activity (≥twice per week),
• never or previously smoked,
• never drinking alcohol.

Participants were categorised into the favourable group if they had 4-6 healthy lifestyle factors, into the average group for two to three factors, and into the unfavourable group for zero to one factor.

Memory function was assessed using the World Health Organization/University of California-Los Angeles Auditory Verbal Learning Test, and global cognition was assessed via the Mini-Mental State Examination. Linear mixed models were used to explore the impact of lifestyle factors on memory in the study sample.

A total of 29 072 participants were included (mean age of 72.23 years; 48.54% (n=14 113) were women; and 20.43% (n=5939) were APOE ε4 carriers). Over the 10-year follow-up period (2009-19), participants in the favourable group had slower memory decline than those in the unfavourable group (by 0.028 points/year, 95% confidence interval 0.023 to 0.032, P<0.001). APOE ε4 carriers with favourable (0.027, 95% confidence interval 0.023 to 0.031) and average (0.014, 0.010 to 0.019) lifestyles exhibited a slower memory decline than those with unfavourable lifestyles. Among people who were not carriers of APOE ε4, similar results were observed among participants in the favourable (0.029 points/year, 95% confidence interval 0.019 to 0.039) and average (0.019, 0.011 to 0.027) groups compared with those in the unfavourable group. APOE ε4 status and lifestyle profiles did not show a significant interaction effect on memory decline (P=0.52).

The authors concluded that a healthy lifestyle is associated with slower memory decline, even in the presence of the APOE ε4 allele. This study might offer important information to protect older adults against memory decline.

This is an important and meticulously reported study. It is the first large-scale investigation that assesses the effects of different lifestyle profiles, APOE ε4 status, and their interactions on longitudinal memory trajectories over a 10-year follow-up period. The results show that lifestyle is associated with the rate of memory decline in cognitively normal older individuals, including in people who are genetically susceptible to memory decline. The authors are rightly careful to avoid causal inferences between lifestyle and memory decline. To demonstrate causality beyond doubt, we would need different study designs.

The authors also discuss several weaknesses of the study:

• Firstly, the assessments of lifestyle factors were based on self-reports and are, therefore, prone to measurement errors.
• Secondly, several participants were excluded due to missing data or not returning for follow-up evaluations, which might have led to selection bias.
• Thirdly, the proportion of individuals with an unhealthy lifestyle might have been underestimated in the study because people with poor health were less likely to have participated in the study.
• Fourthly, given the nature of the study design, it could not assess whether maintaining a healthy lifestyle had already started influencing memory by the time of enrolment in the study.
• Fifthly, the evaluation of memory using a single neuropsychological test that does not comprehensively reflect overall memory function. However, the Auditory Verbal Learning Test is an effective instrument for memory assessment, and a composite score was used based on four Auditory Verbal Learning Test subscales to represent memory conditions to the greatest extent possible.
• Sixthly, as participants might become familiar with repeated cognitive testing, a learning effect could have influenced the results.
• Finally, memory decline was studied solely among older adults; however, memory problems commonly affect young individuals as well.

The authors, therefore, state that further studies should be conducted to facilitate a more extensive investigation into the effects of a healthy lifestyle on memory decline across the lifespan. This approach would help to elucidate the crucial age window during which a healthy lifestyle can exert the most favourable effect.