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The study, presented at an international Alzheimer's conference in Paris, is among the first attempts to link risk factors with the degenerative brain disease, which causes memory loss, disability and eventually death.
Only a tiny percentage of cases -- about one percent -- are clearly caused by genetic factors.
Otherwise, while the process by which the disease attacks nerve cells in the brain is well known, its origins remain poorly understood.
Barnes and colleagues used a statistical method to measure the percentage of cases which might be attributable, at least in part, to each of the risk factors assessed.
Worldwide, they found that a low level of education was linked to 19 percent of cases, smoking to 14 percent, physical inactivity to 13 percent, depression to 11 percent, mid-life hypertension and obesity to five and two percent, respectively, and diabetes to two percent.
When combined, these seven modifiable risk factors contribute to as many as 17 million Alzheimer's cases worldwide, and about three million in the United States, the study found.
While eliminating harmful lifestyle habits entirely is likely to remain a theoretical exercise, the more realistic goal of reducing them by a quarter would cut the number of cases globally by three million, the researchers calculated.
"The next step is to perform large-scale studies with people to discover whether changing these lifestyle factors will actually lower Alzheimer's risk," Barnes said in a statement.
The number of people afflicted by Alzheimer's is expected to more than triple by 2050 as populations across the planet age.
The disease is characterised by unwanted proteins that form plaque in some areas of the brain, ultimately destroying neurons and leading to irreversible brain damage.
Typical symptoms are memory loss, erratic behaviour and extreme agitation.
Alzheimer's affects 13 percent of people over 65, and up to 50 percent of those over 85.
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Only a tiny percentage of cases -- about one percent -- are clearly caused by genetic factors.
Otherwise, while the process by which the disease attacks nerve cells in the brain is well known, its origins remain poorly understood.
Barnes and colleagues used a statistical method to measure the percentage of cases which might be attributable, at least in part, to each of the risk factors assessed.
Worldwide, they found that a low level of education was linked to 19 percent of cases, smoking to 14 percent, physical inactivity to 13 percent, depression to 11 percent, mid-life hypertension and obesity to five and two percent, respectively, and diabetes to two percent.
When combined, these seven modifiable risk factors contribute to as many as 17 million Alzheimer's cases worldwide, and about three million in the United States, the study found.
While eliminating harmful lifestyle habits entirely is likely to remain a theoretical exercise, the more realistic goal of reducing them by a quarter would cut the number of cases globally by three million, the researchers calculated.
"The next step is to perform large-scale studies with people to discover whether changing these lifestyle factors will actually lower Alzheimer's risk," Barnes said in a statement.
The number of people afflicted by Alzheimer's is expected to more than triple by 2050 as populations across the planet age.
The disease is characterised by unwanted proteins that form plaque in some areas of the brain, ultimately destroying neurons and leading to irreversible brain damage.
Typical symptoms are memory loss, erratic behaviour and extreme agitation.
Alzheimer's affects 13 percent of people over 65, and up to 50 percent of those over 85.
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