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Analysing dementia risk

A new study maps out the exact percentage risk of developing dementia posed by nine factors. John Windell looks at the methodology behind the mathematics.

The Lancet commission on dementia prevention, intervention and care made the headlines in July when it reported that one in three cases of dementia could potentially be prevented by tackling a series of risk factors that raise people’s chances of developing Alzheimer’s and other forms of dementia. 

For the first time, this new international study modelled the impact of nine common health and lifestyle factors throughout people’s lives, from childhood to old age, and calculated a precise figure for the level of dementia risk that is attached to each of these factors (see below). 

The percentage risk increase of factors:

less education 7.5% 
high blood pressure 2.0%
obesity 0.8%
hearing loss in mid-life 
(45-65 years) 9.1%
smoking 5.5% 
depression 4.0%
physical inactivity 2.6%
social isolation 2.3%
diabetes (65 years and over) 1.2%

The study argues that if all these risk factors were to be fully eliminated over the life course, the proportion of dementia cases that could potentially be avoided is 35%. That’s no small claim, particularly as the most recent estimates suggest that 47 million people around the world have dementia, and that the cost of care amounts to around £632bn. Further projections show that as the worldwide population ages, the number of people affected by dementia will hit 131 million by 2050.

In throwing a spotlight on the potential scale of the dementia problem, and the extent of the preventative work needed to reduce the impact, the report makes sobering reading. Given the precision of the figures, and the implications, it is worth looking at how the authors have come up with the level of risk associated with each of the factors.

They began by identifying the factors associated with dementia at different stages of life that are potentially reversible and could have a positive effect on the future incidence of the condition. They looked at the risk factors listed in guidelines published by the National Institute for Health and Care Excellence and US National Institutes of Health, which included the well-reported cardiovascular-related risks, such as diabetes, hypertension, obesity, physical inactivity, smoking and depression. They also identified other factors, such as having no education beyond the primary stage, depression and social isolation. 
The final factor was hearing loss in middle age – this was added because it can impair the cognitive environment, though it might equally be related to the depression and social isolation that is linked to dementia.
Bringing together all these risk factors from across the lifespan, the researchers then devised a novel model of risk, and it is from this that they developed the population attributable fractions (PAF) for those risks, showing the potential impact that eliminating each of them might have.

In calculating the PAFs, the researchers drew on previous studies and models of risk, using the data they found in them on the relative risk and prevalence for each factor. They also looked at other systematic reviews, sought out further papers, and conducted their own analyses. For example, for social isolation and hearing loss, the researchers did their own review.

As far as possible, they used the prevalence and relative risk data from international studies, so their figures would reflect the global risk of dementia. They also concentrated on all-cause rather than cause-specific dementia, as this is where the most data existed, with the exception of smoking, where the most reliable figures came from studies on Alzheimer’s.

Another key part of the PAF formula is the communality of risk factors – the variance in observed variables accounted for by common factors. Here, they used data from the 2014 Health Survey for England. This is a representative sample of more than 10,000 adults in the UK that covers all the risk factors, except social contact – for this, they used cohabitation as a proxy, assuming that people who live with others have more social contact than those who live alone.

Feeding all these data into their PAF formula, the authors came up with the risk attributed to each factor. There are limitations to the estimates. For example, they did not take into account diet and alcohol intake, and global figures were not available for some factors. 

Regardless, their results can be seen as an important step forward in combating a condition that has proved resistant to treatment and prevention. As lead author Professor Gill Livingston at University College London says: “Although dementia is diagnosed in later life, the brain changes usually begin to develop years before, with risk factors for developing the disease occurring throughout life, not just in old age. We believe that a broader approach to prevention of dementia which reflects these changing risk factors will benefit our ageing societies and help to prevent the rising number of dementia cases globally.”   

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