The world is getting older. The population of those who are at least 65 years old is projected to increase to 1.6 billion people worldwide by 2050. In the US, 11.3% of the population was over 65 in 1980; in 2010, it was 13%, and by 2040, it is projected to be 20%.
Yet in the US, the rate of dementia—a general cognitive decline that occurs with age, with symptoms like trouble recalling names and difficulty navigating maps—has dropped in recent years. Researchers from the University of Michigan analyzed survey results from over 21,000 adults and found that from 2000 to 2012, the rate of dementia in the US decreased from 11.6% to 8.8%. Their work was published in the Journal of the American Medical Association—Internal Medicine.
A team led by Kenneth Langa, an internist specializing in age-related illness at Michigan, analyzed survey results from the Health and Retirement Study, which collects data from phone interviews with adults who are on average 75 years old. Participants were given three cognitive tasks: first, they were read a list of words that they were supposed to remember, and then asked to repeat the list back. Second, they were asked to count backwards from 20. Third, they were told to try to recite descending multiples of seven, starting from 100.
Based on how well they performed, the participants were scored as having no cognitive problems, some cognitive problems, or dementia. Surveys can’t definitively diagnose dementia the way that an in-person doctor’s visit can, but have been shown to be 78% accurate, the authors explain. They asked responders questions about their general health. Their hope is that if they can find common factors in older healthy individuals—like diet, access to medicine, or other factors—they can figure out which ones are linked to preventing dementia.
Dementia risk has been tied genetics and brain structure, years spent in school (more school means a lower chance of dementia), and even gender. But because dementia is such a complicated condition, and doesn’t appear the same way in each patient, it’s hard to isolate which lifestyle factors directly impact the brain’s decline over the years, and which apparent correlations are simply coincidence.
Consider obesity: In the past, scientific literature correlated obesity with dementia risk. But this study adds to the growing body of literature that suggests rising rates of obesity might actually be helping drive dementia rates down. The reason, authors suspect, is likely to do with better treatment for conditions connected to obesity—like diabetes, high blood pressure, and heart disease. High blood pressure and other kinds of heart disease can strain the blood supply to the brain, causing cognitive decline over the years.
“It seems as though the most evidence that controlling blood pressure more aggressively seems to prevent cardiac problems and lower your risk for [cognitive problems],” says Langa.
The authors noted that there are so many other social and environmental factors that could be related to declining dementia rates. “The true causes for these trends are still a bit of a mystery that we haven’t really pinned down,” says Langa. “Things that put you at higher risk are happening across your entire life span, maybe even before you’re born.” But, noting connections between other kinds of treatment and cognitive decline are a good place to start looking for causes. They next plan to analyze the results of similar surveys conducted internationally to see if they can find other links to cardiovascular health or other lifestyle factors.