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. The research also explained why having more body fat than muscle mass can affect cognition from midlife onward.

The 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, Behaviour, and Immunity paper. 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. 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.  It also showed that biological, not chronological, age has effect. Commenting on the findings, Assistant professor of food science and human nutrition at ISU, Auriel A. Willette said:

“Chronological age doesn’t seem to be a factor in fluid intelligence decreasing over time,” says “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.


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