Where you carry body fat affects your brain in different ways

Not all body fat acts the same on the brain. New research links fat in the arms, legs, torso, and deep around the organs to distinct patterns in brain structure and function.

The strongest red flag shows up with visceral fat, which tracks with deterioration in the brain’s wiring – one hallmark of Alzheimer’s disease. Other depots are connected to changes in movement, memory, and emotion circuits.

A team led by Anqi Qiu at The Hong Kong Polytechnic University analyzed body-composition scans and brain imaging from more than 18,000 adults in the UK Biobank. The average participant was 62.

After adjusting for age and other factors, the researchers mapped how fat stored in four regions – arms, legs, torso, and around internal organs – related to brain measures.

Arm and torso fat effects

Participants underwent whole-body composition scanning and multimodal brain imaging. The experts looked for region-specific links between fat and the cortex, the brain’s outer layer.

The team also examined deep gray matter structures, the integrity of white matter – the long cables that connect brain regions – and functional connectivity among key networks.

Above-average arm and torso fat was linked with thinning in the sensorimotor cortex, the strip of brain tissue that helps control movement and sensation. Arm fat showed another tie: lower volume in the hippocampus.

That structure is central to making new memories and is one of the first affected in Alzheimer’s disease. The arm-hippocampus link may help explain prior reports connecting arm fat to higher neurodegenerative risk, even when overall weight is similar.

“It was surprising that it goes in two different ways,” said Michal Schnaider Beeri from Rutgers University in New Jersey. The arm depot’s mixed associations – some seemingly protective, some harmful – underscore how complicated the fat–brain story is.

Leg fat alters emotion circuits

Higher-than-average leg fat was associated with decreased functional connectivity in the limbic network, which helps regulate emotion and reward. One possible reason is hormonal.

Lower-body fat secretes leptin, a hormone involved in appetite control. Researchers have linked elevated leptin levels to reduced limbic connectivity, suggesting that biochemical signals from leg fat may modulate how emotion circuits communicate.

Visceral fat harms brain wiring

Fat packed around internal organs carried the starkest associations. Unlike the other depots, visceral fat was the only one not linked with preservation of white matter.

Instead, it lined up with white-matter deterioration – another change often seen as Alzheimer’s advances.

“This may be because visceral fat produces more inflammatory molecules than fat in other areas,” said Sonia Anand at McMaster University in Canada, which could spur damaging inflammation in the brain.

Limitations of the study

This was an observational study. It can reveal patterns, not causes. According to Beeri, you cannot conclude causality of any kind, and some brain changes might even influence where the body stores fat.

The sample also involved mainly white participants, a common limitation in large biobanks, so the findings need testing in more diverse populations and across different ages and health histories.

Even with those caveats, the results point to a practical idea: fat depots are not interchangeable. Visceral fat appears especially important for brain health. This suggests targeting visceral fat reduction may provide greater benefits than generic, one-size-fits-all weight loss approaches.

“This may be because visceral fat produces more inflammatory molecules than fat in other areas,” Anand said, so dialing down that inflammatory load – through diet, activity, sleep, stress management, or future medicines – may help protect white matter and cognition.

Future research directions

Why did arm fat relate to both thinning in movement areas and smaller hippocampi, yet in other analyses show links that looked protective? “It was surprising that it goes in two different ways,” Beeri said.

Different fat depots release different cocktails of hormones, fatty acids, and inflammatory signals. Genetics, sex, and life stage may change the mix.

The next steps are longitudinal studies that track where fat accumulates over time, how brain networks change alongside it, and which lifestyle changes shift both in a healthier direction.

Where you carry fat matters for the brain – and the effects vary by depot. Arm and torso fat aligned with thinning in movement areas, and arm fat with smaller hippocampi. Leg fat aligned with weaker emotion network connectivity, and visceral fat with declining white matter integrity. The message is not alarm, but nuance.

Different fats send different signals. Targeting the most harmful depot – visceral fat – may offer a clearer path to protecting brain health than chasing the number on a scale alone.

The study is published in the journal Nature Mental Health.

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