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Half of Alzheimer's cases worldwide can be traced to seven common risk factors, a new study shows.

The findings also suggest that by tackling those underlying issues – which range from lack of education to obesity – the rising tide of dementia could be slowed considerably.

"We are assuming that when you change the risk factor, then you change the risk," said Deborah Barnes, an associate professor of psychiatry at the University of California, San Francisco, and lead author of the study. "What we need to do now is figure out whether that assumption is correct."

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Worldwide, an estimated 33.6 million people are living with Alzheimer's and related dementias, including more than 500,000 in Canada. There are more than six million new cases diagnosed each year, including 110,000 in Canada.

The research, published online Tuesday in the medical journal Lancet Neurology, is a systematic review of earlier studies examining risk factors for Alzheimer's disease.

Dr. Barnes and her colleagues used mathematical modelling to estimate the global impact of the modifiable risk factors. They calculated that 17.2 million of the current cases may have been prevented.

The seven risk factors, in descending order of magnitude, were identified as:

* Low education: "Use it or lose it" is an important credo in Alzheimer's prevention. Schooling is key because stimulating the brain builds neural networks and the more education a person has the more likely they are to engage in stimulating brain activity. Yet, worldwide, 40 per cent of the population has a primary school education or less. The researchers estimated that low education was associated with 19.1 per cent of Alzheimer's cases, or 6.4 million cases globally.

* Smoking: Among other things, smoking weakens blood vessels and it affects blood flow to the brain. But almost one-third of adults in the world still smoke. The research estimates 13.9 per cent of Alzheimer's cases are linked to smoking.

* Physical inactivity: Studies show that people who are physically active have better cognitive abilities and are less likely to develop dementia. Worldwide about one in six people are inactive. The new study found that 12.7 per cent of Alzheimer's cases were likely due to inactivity.

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* Depression: People who suffer from depression have more than double the risk of developing dementia. About one in seven people in the world will suffer from serious depression. Researchers calculated that about 10.6 per cent of Alzheimer's cases could be traced to depression.

* High blood pressure: About one in nine people in the world have hypertension in middle age. The study estimated that poorly-controlled blood pressure accounts for 5.1 per cent of Alzheimer's cases.

* Diabetes: Research shows patients with type 2 diabetes have a significantly higher risk of developing Alzheimer's. Globally, almost seven per cent of adults have diabetes. The research team found it could be responsible for about 2.4 per cent of Alzheimer's cases.

*Obesity: Women and men who are obese at middle age have an increased risk of dementia later in life. Worldwide about 3.5 per cent of the population is both obese and middle-aged. The study found that obesity is associated with about two per cent of Alzheimer's cases.

The findings were also presented Tuesday at the Alzheimer's Association International Conference in Paris.

Dr. Barnes said if these risk factors were eliminated entirely it is not realistic to think these risk factors could be eliminated entirely, but even small reductions could have a significant impact.

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The researcher estimated that reducing the seven key risk factors by 10 per cent would translate to 1.1 million fewer cases of Alzheimer's, while a 25 per cent reduction would translate into 3 million fewer cases a year.

"It gives us a little bit of hope about things we could do now about the epidemic that is coming our way," Dr. Barnes said. She said that is particularly true given the lack of progress in developing effective treatments for Alzheimer's and other forms of dementia.

Currently, the number of people living with dementia is doubling every 20 years and, without significant advances in prevention, the number is expected to reach 115 million in 2050.

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