How your belly affects your brain: As we age, excess body fat worsens ‘fluid intelligence’

AMES, Iowa — There are plenty of reasons to strive for a healthy and fit body in old age, but just in case you needed one more, look no further than a recent study conducted at Iowa State University. Less muscle and excess body weight in older adults, specifically extra belly fat, may negatively influence one’s ability to think and react to new situations on the fly, technically referred to as “fluid intelligence.”

According to the research team, this deterioration in thinking capacity is caused by extra work the body’s immune system must put in for individuals with a particularly high BMI (body mass index). The study’s authors believe their findings may prove invaluable in the development of new treatments for maintaining and protecting mental flexibility among older adults dealing with obesity, an inactive lifestyle, or age-related muscle loss.

For the study, six years’ worth of data on over 4,000 middle-aged to elderly U.K. citizens (both men and women) was analyzed. Among the data sample chosen for this study, researchers analyzed each participant’s lean muscle mass, abdominal fat, and subcutaneous fat measurements, as well as how fluctuations in these body fat percentages related to changes in their fluid intelligence over a period of six years.

They discovered that adults in their 40s and 50s with high levels of mid-section fat exhibited increasingly worse fluid intelligence as years passed. However, muscle mass actually appeared to protect cognitive abilities and foster healthy fluid intelligence in old age. Even after the researchers accounted for other outside factors, such as socioeconomic status, age, and education level, the findings remained consistent.

“Chronological age doesn’t seem to be a factor in fluid intelligence decreasing over time,” comments study leader and assistant professor of food science and human nutrition Auriel Willette in a release. “It appears to be biological age, which here is the amount of fat and muscle.”

It’s very common, almost unavoidable, for people to lose some muscle and put on extra weight as they hit middle-age and enter their elderly years. That’s why, the research team say, it is important for middle-aged adults to stick to a workout regimen that promotes lean muscle growth. This is especially true for women, who naturally start out with less muscle mass to begin with than men.

Additionally, researchers wanted to investigate what role the immune system plays in this relationship. Previous research had already shown that immune system activity in the bloodstream is elevated among people with a higher BMI, leading to immune system activation in the brain, subsequently causing cognition issues. However, BMI as a measurement only really accounts for total overall body mass, meaning it has been unclear what actually kickstarts the immune system (muscle, fat, or both).

In pursuit of these answers, some differences in how male and female bodies react to excess weight were discovered. Among women, the connection between belly fat and poor thinking skills was fully associated with changes in two variations of white blood cells: lymphocytes and eosinophils. Among men, excess stomach fat initiated changes in a different type of white blood cell: basophils. This white blood cell variation in men appeared to account for about half of the connection between fat and fluid intelligence in males. Across both genders, muscle mass did in fact protect cognition, but it didn’t appear to influence the immune system.

Regarding the onset of Alzheimer’s, the study’s authors say they are unsure at this point if extra weight increases one’s odds of developing the degenerative brain disorder.

“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,” adds study author and PhD student Brandon Klinedinst.

“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,” professor Willette concludes.

The study is published in the scientific journal Brain, Behavior, and Immunity.