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.
Biological, not chronological, age has effect
“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 importance of resistance training
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.