Where fat is distributed throughout the body (and not just its total amount) has a large and independent impact on brain structure, brain function, and cognitive performance in middle-aged and older adults. This new research natural mental health.
Body mass index (BMI) has long served as a standard clinical and research measure of obesity, but it does not capture where fat is stored in the body. Fat stored in different anatomical regions behaves biologically very differently. Visceral fat (fat accumulated around internal organs) releases inflammatory chemicals associated with neuroinflammation and neurodegeneration. In contrast, limb fat is thought to have protective effects in some situations.
Despite this growing awareness, most previous studies on obesity and brain health have relied solely on BMI, and the distinct contributions of individual fat stores to brain aging and cognitive decline are largely unknown.
The team, led by Anqi Qiu from Hong Kong Polytechnic University, analyzed data from more than 18,000 UK Biobank participants, with an average age of around 62.5 years and around 45% men.
Deep fat (visceral fat) in the arms, legs, trunk, and abdomen was precisely measured using a special scanning technique (dual-energy X-ray absorptiometry, DXA). Brain health was assessed using multiple types of MRI scans (structural MRI, resting-state functional MRI, and diffusion weighted imaging), and participants completed a battery of cognitive tests including reasoning, memory, executive function, and processing speed. Using statistical methods, we were able to mathematically remove the effect of overall BMI and examine the unique effects of each fat region independently of general obesity.
The results demonstrated that each fat depot is associated with a distinct pattern of neurological changes. Fat in the upper arms and trunk was particularly associated with thinning of the sensorimotor cortex, an area of the brain involved in movement and touch. Arm fat was also consistently associated with decreased volume in the hippocampus, an important area for memory. All four fat types were associated with decreased volume in deep brain structures, and all were associated with weaker connections between brain regions involved in movement and coordination.
Visceral fat (fat that accumulates deeply around abdominal organs) stood out as particularly harmful. This was most strongly associated with deterioration of the brain’s white matter (the cables that connect different parts of the brain), with signs of decreased nerve fiber density, increased fluid accumulation within brain tissue, and disorganized nerve fibers.
Interestingly, leg fat was uniquely associated with weakened connections within the brain’s limbic system, which controls emotion, memory, and reward. The authors suggest that this may be related to the hormone leptin, which lower body fat secretes in high levels and acts on brain areas associated with memory.
The study used computer modeling to calculate the “brain age” of participants’ neural networks. Researchers found that accelerated brain aging, particularly in the sensorimotor, limbic, and default mode networks, is an important pathway by which local fat negatively impacts cognitive performance.
Across all measures of thinking ability, visceral fat consistently showed the strongest negative indirect effect on cognitive function. As the authors note, these findings “highlight the importance of considering regional obesity beyond BMI when characterizing its associations with brain and cognitive aging.”
Some important limitations should be recognized. For example, cross-sectional designs cannot determine causality (does fat cause brain changes or vice versa), and a predominantly white British sample limits generalizability to more diverse populations. Furthermore, DXA scans lack the resolution to distinguish between subcutaneous fat (just under the skin) and visceral fat, especially within the trunk region, which may limit insight into its distinct effects.
The study, “Regional Obesity Shapes Adult Brain and Cognition,” was authored by Die Zhang, Yingji Fu, Chenye Shen, Chaoqiang Liu, Nanguang Chen, Hua Cao, Kui Kai Lau, and Anqi Qiu.
