Fluid intelligence is described as the ability to solve novel reasoning problems; high fluid intelligence tends to imply a greater capacity to learn, ability to comprehend, and so forth. Fluid intelligence declines with age, but researchers here suggest that has more to do with the effects of visceral fat tissue and loss of muscle mass than it does with an inexorable aging process in the brain - at least into middle age, if not later in life. Both visceral fat and skeletal muscle are metabolically active tissues, though more is understood about the harms caused by visceral fat than about the protective effects that are lost as muscle declines with age. It is reasonable to think, based on weight of evidence, that chronic inflammation and other forms of immune dysfunction with age are strongly influenced by visceral fat, and that this in turn has an effect on brain function. All of the common neurodegenerative conditions of late life have a strong inflammatory component to their progression.
Researches looked at data from more than 4,000 middle-aged to older UK Biobank participants, both men and women. The researchers 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 researches discovered that 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. "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."
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. 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.