Among middle-aged and older people, prolonged sitting may harm the brain.
Researchers from the University of California, Los Angeles (UCLA) discovered that among 35 adults aged 45–75 without dementia, those who spent more time sitting in the day had greater thinning of the medial temporal lobe.
This is an area of the brain that is important for making new memories.
Even high levels of physical activity did not make a difference, the authors conclude, in a report on their findings that is now published in the journal PLOS ONE.
The study adds to a growing body of evidence that suggests that too much sitting can increase the risk of heart disease, diabetes, and other diseases, even among those who are physically active.
Senior study author David Merrill, an assistant clinical professor of psychiatry and biobehavioral sciences at UCLA, and colleagues propose that further research should now be done to see whether reducing sedentary behavior reverses the effect that they found.
Few studies about sitting and dementia risk
In their study background, the authors refer to the increasing amount of literature that suggests that physical exercise might delay the development of Alzheimer's disease and other dementias and can benefit brain structure.
One explanation that has been suggested for this effect is that physical activity raises blood flow in the brain, which, in turn, helps the growth of new nerve cells and offsets decline in structure and function.
But in comparison with the volume of literature on the impact of exercise, "there is a paucity of research on the relationship between sedentary behavior and dementia risk," and only few studies have examined the "mechanistic" effect on the brain, note the authors.
This should be a cause for concern, they argue — especially as it has been suggested that around 13 percent of the global burden of Alzheimer's disease may be due to spending too much time sitting.
Based on such an estimate, they calculate that reducing sedentary behavior by 25 percent "could potentially prevent more than 1 million" cases of Alzheimer's disease worldwide.
The team decided to focus on the medial temporal lobe because it is known that this area of the brain declines with age and that this leads to memory impairment.
Also, they note, greater "aerobic fitness" has been tied to greater volume of the hippocampus, an area of the medial temporal lobe that has been "heavily studied" and is important for memory.
Sitting may impair brain despite exercise
For their study, the researchers explored links between medial temporal lobe thickness, exercise, and sitting time in 25 women and 10 men aged 45–75 who did not have symptoms of dementia.
The data on average hours spent sitting every day and physical activity levels came from detailed questionnaires that the men and women filled in. Medial temporal lobe thickness was measured from MRI scans of their brains.
When they analyzed the data, the researchers found "[n]o significant correlations" between levels of physical activity and medial temporal lobe thickness.
However, they did find that the more sedentary people had less medial temporal lobe thickness.
While they did not investigate the mechanisms through which prolonged sitting might be bad for the brain, the authors refer to a suggestion that "sedentary behavior may have deleterious effects on glycemic control."
They speculate that this could result in increased variability of blood sugar and lead to reduced blood flow in the brain, which, in turn, impairs brain health.
They also point out that their findings are "preliminary" and do not prove that prolonged sitting actually causes the medial temporal lobe to become thinner. They propose that:
"Future studies should include longitudinal analyses and explore mechanisms, as well as the efficacy of decreasing sedentary behaviors to reverse this association."