Having less muscle and more body fat may affect how flexible our thinking gets as we become older, new research suggests.
Researchers also found that changes in parts of the immune system could be responsible for the effect.
These findings could lead to new treatments that help maintain mental flexibility in aging adults with obesity, sedentary lifestyles, or muscle loss that naturally happens with aging.
Muscle, Aging, and Body Fat
The longitudinal study looked at data from more than 4,000 middle-aged to older UK Biobank participants, both men, and women. The researchers examined direct measurements of lean muscle mass, abdominal fat, and subcutaneous fat, and how they were related to changes in fluid intelligence over six years.
The researchers discovered people mostly in their 40s and 50s who had higher amounts of fat in their midsection had worse fluid intelligence as they got older. Higher muscle mass, by contrast, appeared to be a protective factor. These relationships stayed the same even after taking into account chronological age, level of education, and socioeconomic status.
“Chronological age doesn’t seem to be a factor in fluid intelligence decreasing over time. It appears to be biological age, which here is the amount of fat and muscle,”
says coauthor Auriel Willette, assistant professor of food science and human nutrition at Iowa State University.
Generally, people begin to gain fat and lose lean muscle once they hit middle age, a trend that continues as they get older. To overcome this, implementing exercise routines to maintain lean muscle becomes more critical.
Coauthor Brandon Klinedinst, a Ph.D. student in neuroscience, says exercising, especially resistance training, is essential for middle-aged women, who naturally tend to have less muscle mass than men.
Immune System Alterations
The study also looked at whether or not changes in immune system activity could explain links between fat or muscle and fluid intelligence.
Previous studies have shown that people with a higher body mass index (BMI) have more immune system activity in their blood, which activates the immune system in the brain and causes problems with cognition. BMI only takes into account total body mass, so it has not been clear whether fat, muscle or both jump-start the immune system.
In this study, in women, changes in two types of white blood cells, lymphocytes and eosinophils, explained the entire link between more abdominal fat and worse fluid intelligence. In men, a completely different kind of white blood cell, basophils, accounted for roughly half of the fat and fluid intelligence link.
While muscle mass was protective, the immune system did not seem to play a role.
The study found correlations between body fat and decreased fluid intelligence, but it is unknown at this time if it could increase the risk of Alzheimer’s disease.
“Further studies would be needed to see if people with less muscle mass and more fat mass are more likely to develop Alzheimer’s disease, and what the role of the immune system is,”
Starting a New Year’s resolution now to work out more and eat healthier may be a good idea, not only for your overall health but to maintain healthy brain function.
“If you eat alright and do at least brisk walking some of the time, it might help you with mentally staying quick on your feet,”
 Brandon S. Klinedinst, et al. Aging-related changes in fluid intelligence, muscle and adipose mass, and sex-specific immunologic mediation: A longitudinal UK Biobank study. Brain, Behavior, and Immunity; Volume 82, November 2019, Pages 396-405