Some adults’ brains look older than their age, and gut biology may be part of the picture

A growing body of research indicates that some middle-aged and young adults may have brain activity patterns that appear older than their chronological age. This phenomenon, captured by a "brain aging index," is linked to cognitive abilities, mood, and gut health. The brain aging index is calculated by comparing a computer-generated estimate of a person's age derived from brain scans to their actual chronological age.

A positive index suggests an older-looking brain, while a negative score indicates a younger-appearing brain.

Most prior studies on brain aging have focused on older adults already at risk for cognitive decline. However, emerging research suggests middle age may be a critical period when biological changes set the stage for later brain health. Gut biology has also gained attention for its potential connection to brain function. Studies have shown shifts in gut bacteria and stool chemicals across different neurological conditions and depression, with implications for brain shrinkage and cognitive performance.

To explore whether gut biology influences early brain aging in young and middle-aged adults, a study led by researcher Kanhao Zhao analyzed brain imaging and biological data from 1,462 healthy adults aged 18 to 65. The data was divided into three groups for analysis: a discovery group of 674 participants, a replication group of 444 participants, and an independent group of 344 participants.

All participants underwent resting-state functional magnetic resonance imaging, which measures brain activity patterns. They also completed cognitive and emotional tests, including assessments of intelligence, working memory, executive function, and self-reported depressive symptoms.

In the independent group, researchers collected stool samples for analysis using metagenomics and metabolomics. Metagenomics sequences the genetic material of gut bacteria, while metabolomics identifies specific metabolites produced by gut bacteria and their metabolic activities. Across all three groups, the brain aging index predictions aligned reasonably well with participants' actual ages, typically off by 6 to 9 years.

People with brains that appeared older than expected showed consistent patterns. Higher index scores were associated with stronger connections between the default mode network—active during inward-focused thought—and the frontoparietal network, which controls goal-directed thinking. These connections were particularly strong in the precuneus and posterior cingulate cortex, regions involved in internal thought and memory retrieval.

Conversely, individuals with older-appearing brains exhibited weaker connections in the ventral attention network, which helps orient to sudden environmental changes. This network imbalance was linked to poorer cognitive performance.

The brain aging index was modestly associated with lower scores on cognitive tests measuring performance IQ, working memory, and executive function. Individuals with higher brain aging index scores also reported more depressive symptoms. However, these associations were modest, accounting for at most about 14 percent of the variation in any single measure.

Notably, verbal IQ did not show a reliable link in either group. The study's results suggest that while brain aging may be detectable in young and middle-aged adults, the associations with cognition, mood, and gut biology are modest and should be viewed as population-level associations rather than definitive diagnostic tests.