Scientists have found that in some middle-aged and young people the pattern of brain activity looks "older" than their actual age. The study was published in the journal eBioMedicine. It covered more than 1,400 people aged 18 to 65, divided into three independent groups: 674, 444 and 344 participants. In some of them (182 people), stool samples were additionally studied.
The work is based on the so-called brain aging index. This is the difference between the age that a computer model predicts from brain scans and a person's actual age. A positive index means the brain looks older than expected, a negative one means younger. The authors emphasize: this is a statistical measure, not a measurement of the brain's biological age.
Participants underwent resting-state functional MRI. This method records changes in blood oxygen levels associated with the activity of different brain regions while a person lies without a task. This is how scientists assessed functional connectivity - how consistently distant brain regions work together. Volunteers also took tests of intelligence, working memory (the ability to hold information for current tasks) and executive functions (the ability to plan and control behavior), and also filled out questionnaires about symptoms of depression.
It turned out that in people with an "older" brain pattern, areas of the so-called default mode network (which is active when a person daydreams or recalls the past) and the frontoparietal network, which helps hold a goal, were more strongly connected. At the same time, the ventral attention network, which helps notice unexpected changes around, worked more weakly. This imbalance was combined with somewhat worse results on tests of working memory and executive functions, as well as with more pronounced symptoms of depression.
The associations were modest: correlation coefficients - a statistical measure of association that by itself does not prove causation - ranged from 0.12 to 0.38. This means that the aging index explained no more than 14% of the variance in results on individual tests, and usually much less. Not all measures matched: verbal intelligence showed no reliable association, and some results were confirmed in only one of the groups.
Analysis of gut bacteria and chemicals in stool also gave a weak signal. The combined profile of microbiota and metabolites (metabolites are small molecules formed in the process of metabolism and the vital activity of bacteria) explained only about 2% of the differences in the aging index. Some species of Clostridia and Collinsella, as well as fat-like molecules ceramides and 24-hydroxycholesterol, associated with inflammation and cholesterol metabolism, were more often associated with an "older" brain. People with a brain that looked "younger" had higher levels of substances related to neurotransmitter function and amino acid metabolism, including estetrol, a form of estrogen.
Lead author Kanhao Zhao, who carried out the work at the University of California, Los Angeles, notes: "The effects should be viewed as significant associations at the population level, not as a diagnostic test for an individual person." The study was cross-sectional, so it cannot establish that bacteria or metabolites cause accelerated brain aging. Reverse influence is also possible, as is a role for other factors, such as diet. In addition, microbiota analysis was performed in only 182 participants, predominantly women, and the data were collected in the United States and Japan.
Nevertheless, the authors believe that differences in brain function associated with age can be noticed long before old age. According to Zhao, the next step is long-term observations to understand whether such features predict future memory status and whether they change under the influence of lifestyle or treatment.