The number of older Americans with dementia and Alzheimer's disease will likely increase from more than 5 million in 2010 to up to 6.5 million over the next 10 years, straining the U.S. health-care system and family caregivers alike, said Margaret Gatz, PhD, of the University of Southern California, at APA's 2011 Annual Convention.

That picture could improve if there are significant advances in treatment and prevention, said Gatz. But at the same time, "the picture could look worse if modifiable risk factors like diabetes or obesity continue to rise among those now middle aged."

Through her more than 25 years of research on cognitive decline through the Swedish Twin Registry—a sample of nearly 12,000 twins now age 65 or older—Gatz has found that diabetes and obesity are among the most significant non-genetic risk factors for Alzheimer's disease and dementia.

Genes still appear to play the biggest role in Alzheimer's risk—her twin data indicate that about 70 percent of risk for Alzheimer's is likely genetic. But the findings on diabetes and obesity strengthen the argument for Americans to embrace healthier lifestyles, particularly underserved populations, which are more likely to have these conditions, said Gatz.

"These are the kind of health disparities we are very concerned about in the U.S. today and now here's one more potential implication—a possible increase in rates of Alzheimer's," she said.

Her twin study findings also indicate that many Alzheimer's risk factors exert their influence at different points in the life span. For example, diabetes appears to be particularly potent as a risk factor when its onset is in midlife rather than late life, Gatz said. Tapping into the sample's data on tooth loss before age 35, Gatz has also found that developing periodontal disease early in life is associated with a particular risk for developing Alzheimer's as older adults.

"Over three times more often, the twin with more tooth loss is the twin who develops dementia, and we find the same for Alzheimer's disease," said Gatz.

Cardiovascular disease, chronic stress and high cholesterol are other significant risk factors for cognitive decline that have emerged from her twin study. Her data also indicate that depression may be an early symptom of dementia and Alzheimer's disease.

The news isn't all grim, Gatz pointed out. Her data have also revealed several protective factors that appear to safeguard the brain against Alzheimer's, one of which is higher education. In the twin study, she found that the twin with the higher level of education is three times more likely not to develop dementia or Alzheimer's disease, even when controlling for genetic influences and other life experiences, said Gatz.

Other protective factors include participating in leisure activities, such as reading and cultural activities, said Gatz, but mainly for women. Light or regular exercise such as walking or playing sports is a strong protective factor for both men and women, added Gatz. Heavy exercise, such hard physical labor or training, was not found to be significantly protective.

People's work and career choices may also buffer against cognitive decline. Gatz's research shows that having a job that involves complex work with people—careers that involve persuasion, mentoring, instruction and supervision—relate to a lower risk of Alzheimer's. That said, people with those jobs also have a steeper rate of decline once Alzheimer's is diagnosed.

"This is consistent with the idea that education and [job] complexity push off the onset of Alzheimer's, but once there is a diagnosis, the decline is faster," she said.

While such protective factors offer no guarantee against developing the disease, they offer hope and a guide for precluding cognitive decline for all ages, Gatz said.

"Protection for good brain health is something that needs to be thought about as a lifetime commitment," she said.