A new study suggests that in women, cognitive decline may start in midlife.
As people begin to live longer lives, it has become increasingly important to understand age-related cognitive decline.
By 2030, the population of people in the United States over 65 years old is expected to increase by twofold from 2000, reaching 72 million.
There is a lack of consensus as to the exact age when cognitive decline starts to happen. Some researchers argue that cognition starts to slow down as early as one's 30s, while others have pointed to the ages of 55, 60, or even 70 as marking the beginning of the process.
A team of researchers from the University of California, Los Angeles (UCLA), set out to examine the existing evidence on cognitive decline in midlife women. Their study suggests that, at least in women, cognitive decline may start sooner than previously thought.
The study authors are Arun Karlamangla, MeiHua Huang, WeiJuan Han, and Gail Greendale from UCLA, and Margie Lachman from Brandeis University in Waltham, MA.
They point out that previous studies in this area may not have accounted for the so-called practice effects. These occur when repeat testing in the same individuals affects the results, which may sometimes mask the effects of menopause transition.
The results of the new study were published in the journal PLoS One.
Analyzing cognitive decline in midlife women
Karlamangla and team looked at the data collected in the Study of Women's Health Across the Nation (SWAN) - a community-based, longitudinal, observational study of middle-aged women.
SWAN collected data on the cognitive abilities of 2,709 healthy women aged between 42 and 52.
Of these women, 80 percent had their cognitive abilities tested at three or more visits. The meta-analysis conducted by Karlamangla and team used the third cognitive test as their baseline.
Several women were then excluded due to health issues or location, leaving a sample of 2,124 women, who were clinically followed for 10 years after menopause.
The tests carried out within SWAN included annual processing speed tests, verbal episodic memory - both immediate and delayed - and working memory.
The meta-analysis aimed to reduce practice effects, as well as the effects of menopause transition, by using the third cognitive tests as baseline, where the average age was 54 years, and most women were post-menopausal.
In total, the study analyzed the results of 7,185 cognition tests with an average follow-up period of 6.5 years.
The meta-analysis adjusted for practice effects, memory retention, menopause symptoms, and other covariates.
Women's cognition declined by around 5 percent
The analysis revealed strong evidence of early cognitive decline in middle-aged women.
After adjusting for the aforementioned variables, the cognition scores previously gathered by the SWAN test declined in two of the four cognition tests.
Overall, during the 10-year period, the women's cognitive sharpness deteriorated by an average of 4.9 percent.
Cognitive speed declined by a mean of 0.28 percent per year.
Specifically, the speed of perception and reaction - which make up the cognitive processing speed - declined by approximately 1 percent every 2 years, and verbal memory deteriorated at an average rate of around 1 percent every 5 years.
The authors conclude:
"This study provides good new evidence of cognitive aging in women in midlife, with significant longitudinal declines in both processing speed and verbal memory. Unlike previous longitudinal studies in midlife that were based on 3 or fewer cognition assessments, and could not adequately account for practice effects, we analyzed up to 6 annual or biennial assessments, allowing us to minimize the impact of practice effects and unmask declines."
However, the authors concede that more research is needed to identify the factors that are driving these decline rates, as well as to develop interventions that may slow down cognitive aging.