Nine risk factors are responsible for up to two-thirds of the world's Alzheimer's cases, according to a new study, but there are also factors that seem to protect against the disease.
Dementia affects about five to seven percent of the population, and Alzheimer's cases represent about 60 percent of that total. While estimates vary, roughly 5 million Americans are thought to have the disease, according to the National Institute on Aging.
There is no cure for Alzheimer's, and it is the sixth-leading cause of death in the United States, according to the Centers for Disease Control and Prevention. But there may be steps you can take to reduce your risk.
A team of researchers from the University of California, San Francisco, examined more than 300 studies identifying Azheimer's risk factors, looking for behaviors patients may be able to change in order to lower their chances of contracting the disease.
The study showed that heavy smoking is the most significant risk factor for developing Alzheimer's, while the most significant protective factor was a healthy diet, such as the often-praised Mediterranean diet, said Jin-Tai Yu, a researcher at the University of California, San Francisco, and one of the study's coauthors, in an email to CNBC.
Narrowing of the arteries and obesity are also risk factors, as are type-2 diabetes (in the Asian population), low educational attainment, depression, high blood pressure and frailty.
Having a high level of the amino acid homocysteine was also a risk factor. Homocysteine is made in the body, and high levels may be the result of vitamin-B deficiency or excessive levels of methionine in the body, though the exact cause is not known.
Apart from maintaining a healthy diet, other steps to best protect yourself from the disease include staying physically active, keeping alcohol use light or moderate and taking in sufficient amounts of certain vitamins like B9 (folate), C, and E.
The team published its results late Thursday in the Journal of Neurology, Neurosurgery and Psychiatry.
Alzheimer's is a highly complex disease "with a highly genetic underpinning," and the study's results should be interpreted conservatively, Yu said.
"In other words, we cannot expect to completely avoid [Alzheimer's] occurrence if we controlled all the modifiable risk factors," Yu said in an email to CNBC.
There were some factors that seemed to have contrary effects, depending on the circumstances. Smoking and type-2 diabetes accompanied greater Alzheimer's risk to Asian populations than other "Western" populations, where smoking actually accompanied a lower risk. Having a high body-mass index in mid-life put some at greater risk, while a bit of excessive weight later in life seemed to decrease the risk for others.
Readers should "consider these results from a more comprehensive perspective given that the cross-impacts of these factors to the overall health are complex," Yu said.
The team cautioned that it analyzed only observational studies, not clinical trials, so the study cannot say that these various risk factors are causing Alzheimer's, only that they accompany a higher likelihood of suffering it.
Nevertheless, Yu said he hopes the research will give doctors useful information in helping their patients avoid the onset of the disease.