September/October 2018

Research Review: Sedentary Habits Linked to Memory Loss
By Jamie Santa Cruz
Today's Geriatric Medicine
Vol. 11 No. 5 P. 10

Sedentary lifestyles have been linked with a myriad of ills in recent years, including cardiovascular disease, type 2 diabetes, multiple forms of cancer, poor mental health, and premature death.1 Now, new research from UCLA suggests that sedentary patterns may also be associated with changes in brain structures related to memory formation—an important finding for understanding dementia and developing interventions to support brain health.2

The findings should encourage physicians to focus more attention on how much time patients spend sitting, says Prabha Siddarth, PhD, a biostatistician at the UCLA Semel Institute for Neuroscience and Human Behavior and the lead author of the study. "A lot of clinicians do ask patients about physical activity, but they may not necessarily ask about their sitting habits," Siddarth says.

The new research, published in PLoS One, involved community-dwelling adults ranging in age from 45 to 75. Each participant filled out a questionnaire about their physical activity and the average number of hours they spend sitting per week. Siddarth and her colleagues then imaged each subject with high-resolution MRI scans to measure the thickness of each participant's medial temporal lobe. Finally, the researchers examined how the thickness of the medial temporal lobe varied with physical activity and time spent sitting.

As hypothesized, a greater number of hours spent sitting during the week was significantly associated with a thinner medial temporal lobe. Thinning in this region of the brain is associated with memory loss and Alzheimer's disease.3

Due to advances in technology and changes in society, time spent sitting is increasing, especially in the workplace, says Joe Nocera, PhD, an assistant professor in the department of neurology at the Emory University School of Medicine. Although Nocera was not involved in the UCLA research, he says the new study and others like it are "critical so that we can better understand the implications for health that sitting might have."

Physical Activity May Not Compensate for Time Spent Sitting
Significantly, though hours spent sitting was associated with a thinner medial temporal lobe, level of physical activity was not associated with thickness of the same region. In other words, even high amounts of physical activity did not appear to compensate for time spent sitting.

"Structured physical activity is great, but the other side of that coin is we have to decrease the amount of time that people engage in sitting behaviors," Nocera says.

The finding regarding the lack of impact of physical activity was not expected, Siddarth says. "We had originally hypothesized that both lower levels of physical activity and sitting time would be associated with less thickness," she says. "We were surprised that we didn't see that effect."

Prabha notes, however, that the new study was preliminary and had a sample size of only 35 participants; a larger sample might show a greater effect of physical activity.

Regardless, Siddarth is quick to note that the study doesn't undermine the value of exercise. The message is not that physical activity isn't important, she says, but rather that "sitting time may be a more significant predictor of thickness [of the medial temporal lobe], and physical activity, even at higher levels, may not be enough to offset the bad effects of sitting."

Research Still in Early Stages
Substantial previous research has examined the impact of physical activity on the brain, and these earlier studies have generally shown a positive impact of exercise. Specifically, physical activity has been shown to positively influence brain structure at both the micro and macro levels,4-11 and exercise interventions have shown promise for delaying the onset of dementia.12,13

But research on the other side of the equation—sedentary behavior and its impact on brain health—is still in its infancy. According to Nocera, the new UCLA study is one of the earlier published reports on sedentary behavior and the size of brain structures.

Previously, several studies have suggested that sedentary patterns increase the risk of cognitive impairment with age.14,15 Although there's little research on the connection between sedentary behavior and brain volumes, a 2016 study of healthy older adults showed a connection: Those who were more sedentary were at higher risk for a five-year decrease in white matter volume.16 And a 2014 study examined the connection between sedentary behavior and blood flow in the hippocampus, finding that a sedentary lifestyle may be a risk factor for dysregulation of cerebral blood flow among those at genetic risk of developing Alzheimer's disease.17

The Need for Future Research
Although the new UCLA study shows an association between sedentary behavior and a thinner medial temporal lobe, its cross-sectional design stops short of demonstrating a causal connection. As the authors note in the study, other variables besides sedentary behavior—such as alcohol use and hypertension, which the study did not consider—could also contribute to changes in brain structure. Longitudinal studies are needed to determine whether sedentary behavior is in fact a cause of thinning in the medial temporal lobe.

The UCLA findings are also limited by the fact that they relied on self-reports of physical activity and time spent sitting, which may not be accurate. "A lot of these individuals are older adults, and they could have difficulty recalling past events," Nocera says. Even among younger individuals, "recalling past events is always a limitation."

Finally, the study focused only on the total number of hours spent sitting, but the activity questionnaire did not ask participants whether they took breaks for movement during their sedentary periods and it did not ask what participants were doing when they were sitting. "There is a large literature showing that being mentally active is very important in brain health," Siddarth says. "So if you're doing Sudoku or if you're doing crossword puzzles when you're sitting, is that different than watching TV? All those need to be teased out."

Implications for Physicians
Despite the study's limitations, the preliminary findings have important implications for clinicians. Specifically, Siddarth says, clinicians should be asking patients about how long they spend sitting each day as part of the basic health information they collect from patients at each visit. Currently, she says, it's common for physicians to ask patients about their physical activity levels. But while the issue of low physical activity and sedentary behavior are both important, they are not the same, since individuals can engage in regular vigorous exercise but still sit for the majority of the day.

Patients who are relatively sedentary should be encouraged to sit less in general, Siddarth says. But what to do for individuals who don't have much choice about sitting for large portions of the day? It's not yet known whether breaking up sedentary periods with small spurts of movement would ultimately make a difference in terms of brain health, but in the absence of evidence, short activity breaks are worth a try.

"Obviously the thing to do is reduce the number of hours we spend sitting," Siddarth says. "But for some of us who have jobs that require us to sit in front of a computer, the next best thing is to get up and move around every hour or so for five minutes."

— Jamie Santa Cruz is a freelance writer based in Englewood, Colorado.

References
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