Obesity and dementia have a well-established connection in the medical literature. But a new study shows that just being overweight—with a BMI of 25 or above—in middle age might also significantly increase the odds that a person develops dementia later in life.

In a study of 8,534 both identical and fraternal twins aged 65 and older in Sweden, researchers found that individuals diagnosed with Alzheimer's disease or vascular dementia were 70 percent more likely to have been overweight when they were middle-aged.

Not all—or even most—of those who reported being overweight in midlife had dementia at the time of the study. About 30 percent of individuals were overweight or obese and only about 4 percent of participants were diagnosed with Alzheimer's disease or vascular dementia.

Nevertheless, the new findings, which will be published in the May 3 issue of Neurology, add support for the connection between extra pounds and cognitive decline. "Our results contribute to the growing evidence that controlling body weight or losing weight in middle age could reduce your risk of dementia," Weili Xu, of the Karolinska Institute in Stockholm and co-author of the new study, said in a prepared statement.

And with some 1.6 billion adults currently overweight or obese worldwide, the link is no small one. Even after adjusting for co-founding factors, such as age, gender and education, "midlife BMI as a continuous variable was significantly associated with an increased risk of dementia," the researchers reported. A 2010 study in the Journal of the American Geriatrics Society found that extra weight around the hips in postmenopausal women was also correlated with poorer cognitive functioning.

The mechanisms that link the two conditions are still unclear. Because the trend held with Alzheimer's disease as well as vascular dementia, the researchers pointed out that nonvascular pathways are likely involved. One possibility, the researchers explained in their study, is that being overweight in midlife "may reflect a lifetime exposure to an altered metabolic and inflammatory state," which has been linked to cognitive faltering. This makes sense, as they point out, because "adipose tissue is the largest endocrine organ and secretes inflammatory cytokines and growth hormones; some of them (such as leptin, interleukin-6 and C-reactive protein) may affect cognitive functioning."

And after tracing some of the patterns in the twin data, Xu said, "early life environmental factors and genetic factors may contribute to the link between midlife obesity and dementia." Even so, the findings "highlight the need to control body weight as early as midlife for prevention of dementia in late life," Xu and colleagues concluded in their study.

Image courtesy of iStockphoto/mathieukor