These findings from Iowa State University in the US, 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.
The study, published in the journal "Brain, Behaviour, and Immunity", looked at the data from more than 4,000 middle-aged to older UK Biobank participants, both men and women.
"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," said study lead author Auriel Willette, Assistant Professor.
For the findings, the research team 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.
They discovered people mostly in their 40s and 50s who had higher amounts of fat in their mid-section had worse fluid intelligence as they got older.
Greater 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, the study said.
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 important.
Researchers said exercising, especially resistance training, is essential for middle-aged women, who naturally tend to have less muscle mass than men.
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, the entire link between more abdominal fat and worse fluid intelligence was explained by changes in two types of white blood cells: lymphocytes and eosinophils.
In men, a completely different type of white blood cell, basophils, explained roughly half of the fat and fluid intelligence link.
While muscle mass was protective, the immune system did not seem to play a role.
While the study found correlations between body fat and decreased fluid intelligence, it is unknown at this time if it could increase the risk of Alzheimer's disease.
( With inputs from IANS )