Being frail could tip older people towards dementia, according to a new study.
Scientists have found that those over the age of 59 who were weak were more likely to have brain changes linked to Alzheimer's and symptoms of dementia than those who in more robust health.
They say fewer symptoms were discovered among people of the same age group who had substantial brain changes but were not frail.
The findings, published in The Lancet Neurology journal, suggest that frailty should be considered in clinical care and management of Alzheimer's dementia.
Professor Kenneth Rockwood, of Canada's Nova Scotia Health Authority and Dalhousie University, who led the study, described it as "an enormous step in the right direction" for Alzheimer's research.
He said: "Our findings suggest that the expression of dementia symptoms results from several causes, and Alzheimer's disease-related brain changes are likely to be only one factor in a whole cascade of events that lead to clinical symptoms.
"Understanding how individual risk factors work together to give rise to late-life dementia is likely to offer a new way to develop targeted treatment options."
A team of Canada and US-based researchers looked at the links between frailty, Alzheimer's disease-related brain changes and Alzheimer's dementia among 456 people who are part of the Rush Memory and Ageing Project (MAP) in Illinois, USA.
Detailed cognitive testing and neurological examinations were among a range of evaluations carried out annually.
The study found that 242 of the participants had a diagnosis of possible or probable Alzheimer's dementia at their last clinical assessment.
The researchers also developed a frailty index using information about various health factors such as fatigue, joint and heart problems, osteoporosis, mobility and meal preparation.
They found that 35 people had substantial Alzheimer's disease-related brain changes without having been diagnosed with dementia, and that 50 had Alzheimer's dementia but had little disease-related brain changes.
The researchers also said there was a significant link between frailty and Alzheimer's disease-related brain changes when the frailty index was tweaked to take into account risk factors such as stroke, heart failure, high blood pressure, and diabetes and to strip out daily living activities.
Professor Rockwood said: "While frailty is likely to reduce the threshold for Alzheimer's disease-related brain changes to cause cognitive decline, it probably also contributes to other mechanisms in the body that give rise to dementia, weakening the direct link between Alzheimer's disease-related brain changes and dementia.
"While more research is needed, given that frailty is potentially reversible, it is possible that helping people to maintain function and independence in later life could reduce both dementia risk and the severity of debilitating symptoms common in this disease."
The researchers hope that future analysis could include observation studies into frailty, cognition and the biomarkers of Alzheimer's dementia to try and find a deeper explanation of what is happening.
