In a first, researchers have found that less muscle and more body fat may affect how flexible human thinking gets with age, likely due to changes in parts of the immune system. The researchers, including those from Iowa State University in the US, looked at data from more than 4,000 middle-aged to older participants from the UK Biobank -- a large long-term study in the UK investigating the contributions of genetic predisposition, and environmental exposure to the development of disease.Â
In these participants, they 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 study, published in the journal Brain, Behavior, and Immunity, noted that people mostly in their 40s and 50s -- who had higher amounts of fat in their mid-section -- had relatively worse fluid intelligence as they got older.
On the contrary, it said a greater muscle mass, may be a protective factor. According the researchers, these relationships stayed the same even after taking into account 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," said study co-author Auriel Willette from Iowa State University.
People begin to gain fat and lose lean muscle once they hit middle age, a trend that continues as one gets older, the researchers said. They suggested that implementing exercise routines can help maintain lean muscle.
Resistance training, according to the researchers, is essential for middle-aged women, who naturally tend to have less muscle mass than men. The scientists said the link between more abdominal fat and worse fluid intelligence is explained by changes in two types of white blood cells -- the lymphocytes and eosinophils.
In men, they added, a completely different type of white blood cell, basophils, explained roughly half of the fat and fluid intelligence link. According to the scientists, while muscle mass was found to be protective, the immune system did not seem to play a role.
"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," said study co-author Brandon Klinedinst.