Over 55 million people are living with dementia across the globe (WHO, 2023). Depending on the subtype of dementia, people experience different symptoms which continue to deteriorate, including difficulties with cognition, behavioural changes, motor and speech problems. In the absence of suitable pharmacological treatments that stop disease progression, it is important to focus both on developing and implementing the right care for those with dementia and their unpaid carers, and to investigate how to prevent the condition.

The 2024 report of the Lancet standing Commission, highlighted 14 modifiable risk factors for dementia (Livingston et al., 2024). These include:

• high cholesterol,
• vision and hearing loss,
• lower educational attainment
• social isolation,
• air pollution,
• traumatic brain injury,
• hypertension,
• diabetes,
• depression, and
• lifestyle factors, such as excessive alcohol consumption, obesity, smoking, and physical inactivity.

Whilst official reports cite growing numbers of dementia cases worldwide (Alzheimer’s Diseasse International, 2023), recent cohort studies identify emerging contradictory evidence, indicating a need for a systematic review of such cohort studies.

Therefore, Mukadam et al. (2024) reviewed cohort studies in 2024 and also investigated the contribution of modifiable risk factors to dementia prevalence (number of cases of dementia at a specific time point) and incidence (number of new cases of dementia over time).

Dementia is a disease with many moving parts – studying modifiable risk factors is important.

Methods

The authors conducted a two-step search for finding eligible cohort studies that have looked at prevalence and incidence of dementia and how modifiable risk factors have been linked to this. First, they searched for systematic reviews of cohort studies on the topic area (searches re-run in March 2024), and from 1,925 records, five reviews were considered relevant.

There were no restrictions on languages or date of publication. Studies from reviews were included if they were cohort studies on age-standardised dementia prevalence or incidence in the same geographical location, with at least two time points of data collection. Studies were excluded if they included dementia diagnosis based on electronic health record data.

Of these, 71 potentially eligible primary studies were found with 27 included in this cohort analysis. The authors extracted summary-level data from all included studies and calculated population attributable factors for all 14 modifiable risk factors where available in the data and at each time point of data collection.

Results

Of the included 27 studies, 13 reported trends in prevalence, 10 reported changes in incidence, and four reported both prevalence and incidence.

One of the key findings is that prevalence/incidence results are variable by country cohort. Ten studies from Europe and the US showed declining prevalence/incidence of dementia, whilst some studies from Japan, France, and Sweden showed increased prevalence rates over time. No significant changes were noted in incidence for dementia in the Nigerian study, whilst the four studies reporting on both prevalence and incidence painted a varied picture with no clear trend in either reduction or increase.

Looking at the role of modifiable risk factors and how these may have contributed to changes in prevalence and/or incidence of dementia, the authors had to follow up with study authors to receive further detail on potentially not reported risk factors. For the included studies, a maximum of seven risk factors were reported in a study, whilst one study had included 10 risk factors, the data of which was provided by original study authors after contact.

Focusing specifically on some cohort studies included in the review, including the Cognitive Function and Ageing studies in the UK, the Rotterdam study in the Netherlands, the H70 cohort in Sweden and the Framingham study in the US, educational attainment and smoking status appeared to contribute less to being risk factors for dementia over time whilst hypertension and obesity in particular were linked to greater increases in rates of prevalence and incidence of dementia.

Time to rethink? Smoking and education status may have less of an impact than we think…

Conclusions

Among the cohort studies reviewed in this paper, there is evidence for a reduction in the rates of prevalence and incidence of dementia over time, albeit findings are too varied to showcase a clear trend.

Most studies except one are from high-income countries, which biases the findings and highlights the need for cohort studies on dementia prevalence and incidence in lower- and middle-income countries, where the majority of people with dementia live. This creates some difficulty in making results comparable between geographical locations.

Interestingly, education and smoking have been found to contribute less as modifiable risk factors in the cohort studies (where data were available), whereas hypertension and obesity are on the rise and contribute to a greater extent to dementia numbers.

We need to understand more about lifelong modifiable risk factors, across the globe.

Strengths and limitations

This review was founded on a thorough scoping of the existing literature, including a deep search of systematic reviews. It set clear inclusion criteria, focusing on cohort studies and the availability of data on modifiable risk factors.

A key limitation of data gathered, arises more from the available existing evidence base on a thorough search, rather than the authors’ search methodology. The fact that only one cohort study was based in a lower- and middle-income country, or LMIC, specifically in Nigeria, raises questions about the representativeness and applicability of the findings of this research on a global scale.

This disparity in research infrastructure to conduct cohort studies, broadly affects studies on non-communicable diseases, such as dementia, in LMICs, Considering that the majority of people with dementia reside in LMICs, more resources are needed to support the gathering of high-quality evidence from these countries, to create a more global view of whether dementia prevalence and incidence are decreasing, and the effect to which risk factors may contribute to this.

Secondly, whilst the focus of this review was clearly set on modifiable risk factors, it would have been interesting to explore the impact of non-modifiable risk factors on dementia prevalence/incidence, including ethnic background. Given the link of non-modifiable factors to other biopsychosocial inequalities e.g.educational opportunities and healthy food, it is important to study their follow-on impacts on neurodegeneration, including dementia.

More global research collaboration and resourcing is required in dementia.

Implications for practice

Dementia affects millions of people worldwide, many of whom fail to receive a specific and accurate diagnosis, and often struggle accessing the care and support they need. This has further implications for unpaid carers who find it difficult to access and use support, and also for paid caring services who are under-resourced.

Notwithstanding the trend of reduction in prevalence and incidence of dementia over time, this review identifies interesting trends in the effects of modifiable risk factors on dementia, that could have significant implications for prevention and reducing disease progression in practice.

The outcome that educational attainment and smoking contribute less as risk factors, whereas obesity and hypertension appear to contribute to a greater degree to developing dementia, is of special importance. Given the rise of obesity world-wide, the findings from this review underline the growing impact of obesity as a public health challenge with longitudinal physical and neuropsychiatric effects, which is an important topic warranting further investigation.

Whilst obesity can be managed with healthy lifestyle choices, there is often a class and global divide, with nutritionally inferior processed ‘fast food’ choices cheaper and more readily available than fresh, balanced nutrition. This disparity may be particularly pronounced in certain LMICs, hence it is vital that there is a stronger focus in future studies on dementia prevalence and incidence in relation to different modifiable risk factors in LMICs.

Levelling access to nutritional opportunities may unlock potential to tackle dementia in low and middle income countries.

Statement of interests

None.

Links

Primary paper

Other references

Alzheimer’s Disease International, World Alzheimer’s Report, 2023.

Livingston, Gill et al., Dementia prevention, intervention, and care: 2024 report of the Lancet standing Commission, The Lancet, Volume 404, Issue 10452, 572 – 628

WHO, Dementia Factsheet, 15 Mar 2023

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