A study of thousands of middle-aged and older people has linked having more body fat and less muscle mass to changes in mental flexibility with age. The research also suggests that changes to the immune system may play a role.
Researchers from Iowa State University (ISU) in Ames analyzed data on 4,431 males and females with an average age of 64.5 years and no cognitive impairments.
They report their findings in a recent Brain, Behavior, and Immunity paper.
The data came from the U.K. Biobank, which is tracking the health and well-being of 0.5 million volunteers around the United Kingdom. The volunteers were between 40 and 69 years of age when they enrolled during 2006–2010.
The researchers examined the relationship that variations in abdominal subcutaneous fat and lean muscle mass had with changes in fluid intelligence over a 6 year period.
Fluid intelligence refers to reasoning, thinking abstractly, and solving problems in novel situations, regardless of how much knowledge the person has acquired.
The analysis showed that fluid intelligence tended to reduce with age in those participants who carried more abdominal fat.
In contrast, having more muscle mass appeared to protect against this decline. The team also found that the effect of muscle mass was greater than that of having more body fat.
These links remained even after the researchers adjusted the results to remove the effects of potential influencers, such as chronological age, socioeconomic status, and educational level.
“Chronological age doesn’t seem to be a factor in fluid intelligence decreasing over time,” says Auriel A. Willette, Ph.D., assistant professor of food science and human nutrition at ISU. “It appears to be biological age, which, here, is the amount of fat and muscle.”
He and his colleagues also investigated the role of the immune system in the links between fluid intelligence, fat, and muscle.
Other studies have found that having a higher body mass index (BMI) is often associated with increased immune activity in the blood. This activity can trigger immune reactions in the brain that disrupt memory and thinking.
Those studies have not been able to pinpoint whether higher fat, muscle mass, or both trigger the immune activity because BMI does not distinguish between them.
When Willette and colleagues looked at what was happening in the immune systems of their U.K. Biobank participants, they found differences between males and females.
In the females, they found that changes in two types of white blood cell — lymphocytes and eosinophils — accounted for all of the link between increased abdominal fat and reduced fluid intelligence.
The explanation for males, however, was very different. For these participants, about half of the link between body fat and fluid intelligence involved basophils, another type of white blood cell.
The team found no involvement of the immune system in the protective effect of higher muscle mass.
With advancing middle age, there is a tendency for the body to reduce lean muscle and increase fat.
This trend continues into older age. First study author Brandon S. Klinedinst, a Ph.D. student in neuroscience at ISU, says that it is especially important for people as they approach middle age to continue to exercise to maintain muscle mass.
Resistance training, he suggests, is particularly important for females in their middle years because they have a greater tendency toward reduced muscle mass than males.
The team believes that the findings could pave the way to new treatments that help aging adults maintain mental flexibility, particularly if they have obesity, are not physically active, or experience the loss of lean muscle that usually accompanies aging.
“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.”
Auriel A. Willette, Ph.D.