The team, from Cambridge University in the UK, points out the new estimate is lower than that made in a previous analysis that suggested over half of Alzheimer's cases were preventable. The researchers say the earlier study - which involved some of the same authors - overestimated the number of preventable Alzheimer's cases because it treated the risk factors independently, whereas the new analysis takes interactions into account.
For instance, the team explains that three of the risk factors that the previous study treated as independent of each other - diabetes, high blood pressure and obesity - are all linked to physical inactivity, and all four are also linked to educational level.
According to the World Health Organization (WHO), globally there are around 35 million people with dementia - a syndrome that affects memory, thinking, behavior and autonomy. This number is expected to almost double by 2030 to nearly 66 million, and triple by 2050 to 115 million.
About 70% of dementia cases are Alzheimer's disease, where brain cells die after faulty proteins accumulate inside and around them.
Study examined the effect of seven key lifestyle risk factors
While we do not know exactly what triggers the process of Alzheimer's, we know a complex interplay of genes and lifestyle is involved, says the team, which was led by Carol Brayne, a professor in the Cambridge Institute of Public Health at Cambridge University.
The researchers estimate that reducing the risk posed by seven lifestyle factors by only 10% would be enough to reduce the rate of Alzheimer's in 2050 by 8.5% - meaning that 9 million fewer people would develop the disease.
For the study, the team reviewed existing studies that had already pooled and analyzed the effect of seven key lifestyle risk factors for which there is consistent evidence of a link with Alzheimer's disease.These were:
- High blood pressure (hypertension) in midlife
- Obesity in midlife
- Lack of physical activity
- Poor educational attainment.
The researchers estimate that reducing the risk posed by each of these factors by only 10% would be enough to reduce the rate of Alzheimer's in 2050 by 8.5% - meaning that 9 million fewer people would develop the disease.
Poor education highest risk factor for Alzheimer's worldwide
For example, they found that overall, the risk factor with the highest impact on Alzheimer's worldwide was poor education, while in the US, Europe and the UK it was physical inactivity.
They also found that when they treated the seven risk factors as independent from each other, their combined worldwide effect brought the estimated percentage of preventable Alzheimer's cases to 49.9%, which equates to 16.8 million attributable cases. This is more or less what the 2011 study found.
However, after adjusting for the fact the risk factors are not independent - they overlap somewhat - the team found the estimated percentage of Alzheimer's cases that are preventable reduces to 28.2%, which equates to 9.6 million attributable cases.
For the US, Europe and the UK, the combined percentage was found to be a little higher, at about 30%, note the authors.
Physical activity reduces obesity, hypertension, diabetes and risk of dementia
Prof. Brayne says while there is no single way to prevent dementia, we can reduce our risk of developing it in old age - we know what many of the factors are and that they are often linked:
"Simply tackling physical inactivity, for example, will reduce levels of obesity, hypertension and diabetes, and prevent some people from developing dementia as well as allowing a healthier old age in general - it's a win-win situation."
Co-author Deborah Barnes, associate professor in the department of psychiatry at the School of Medicine in the University of California, San Francisco, led the 2011 study. She explains it is important to get these projections right, and also to accurately assess the impact that lifestyle changes can make:
"Alzheimer's disease is placing an ever increasing burden on health services worldwide as well as on both patients and their carers. Our hope is that these estimates will help public health professionals and health policy makers design effective strategies to prevent and manage this disease."
Funds from the National Institute for Health Research Collaboration for Leadership in Applied Health Research and Care (NIHR CLAHRC) for Cambridgeshire and Peterborough helped finance the study.