What's new: Obesity appears to increase a person's chances of cognitive decline in old age -- but so, paradoxically, does weighing too little for one's height.
The finding: People who maintain a healthy weight have a lower risk of dementia compared with those who are underweight or obese, according to a study by researchers at Johns Hopkins University and the University of Iowa, published in the journal Obesity Reviews last month.
Put another way, when body weight is plotted against risk of dementia on a graph, the result is a U-shaped curve, said lead study author Dr. Youfa Wang, associate professor of international health at the Johns Hopkins Bloomberg School of Public Health.
Obesity, or having a body mass index (BMI) of 30 or higher, increased a person's risk of any type of dementia more than 40%; being underweight (having a BMI of less than 18.5) did the same. Obesity also appeared to have a particularly strong effect on Alzheimer's disease, increasing the risk of the disease by 80%.
How the study was done: Wang and colleagues sifted through the medical literature going back to 1995 in a search of well-designed, long-term studies that had investigated the link between body weight and dementia in adults 40 years of age and older.
The team found 10 relevant studies -- including one that followed more than 10,000 U.S. adults for 27 years -- and in essence recrunched the data in those studies, conducting what epidemiologists call a "pooled" statistical analysis.
Why it matters: Past studies on the relationship between body weight and dementia have varied widely in size and design and have produced a mixed bag of results. And yet the nature of this relationship could be relevant to many Americans. More than two-thirds of adults are overweight or obese, and dementia may affect as many as 10% of those over age 65. According to Wang, his results suggest that preventing obesity could reduce the prevalence of dementia in the U.S. by as much as 20%.
What we still don't know: Wang emphasized that the current research was based on a limited number of studies, which, taken individually, yielded a variety of conclusions.
"I don't want people to overinterpret these findings," he said. To clarify the relationship between body weight and the risk of dementia, he said, researchers need to conduct more large-scale, long-term studies. Wang's work revealed that the studies that showed the strongest relationship between obesity and Alzheimer's disease were those that followed study participants for more than 10 years and began following them before they reached age 60.
Wang also said scientists don't yet understand how body weight could affect the risk of cognitive decline. In fact, some think that obesity, for instance, may simply be an indicator of the presence of other risk factors associated with dementia, such as decreased physical activity or symptoms of depression. "There is a lot of speculation" on the subject, Wang said, "but there is still no consensus."