In a recent review published in the journal Trends in Neurosciences, researchers examined the current evidence that emphasizes midlife as a critical period in brain aging, influencing cognitive trajectories and brain health. They recommend using models that account for non-linear changes across a wide age range to distinguish between processes specific to midlife and those that occur uniformly throughout life.
Review: The 'middle-aging' brain. Image Credit: Monkey Business Images / Shutterstock
Background
Middle age, spanning roughly 40 to 60–65 years, marks a transition to old age and predicts future health outcomes, including dementia risk. However, it is understudied compared to older age groups. Recent research reveals complex, non-linear biological aging processes, especially in the brain, during middle age. Further, gene expression and structural changes may predict cognitive decline accelerated by menopause in women. Understanding these processes could uncover new biomarkers and interventions for cognitive decline. In the present review, researchers examined the evidence from human and animal studies at multiple levels of analysis. They discussed middle age as an important period in brain aging, which could potentially be prognostic of future cognitive health.
Cognitive changes in the middle-aged brain
The Baltimore Longitudinal Study of Aging suggests that cognitive decline in middle age follows diverse and non-linear patterns, particularly affecting memory, reasoning, and reaction time. Notably, episodic memory displays instability during the transition from middle to old age, possibly influenced by changing social dynamics and career trajectories. Declines in processing speed also intensify during this period, with practice effects diminishing around the age of 60 years, potentially indicating early cognitive impairment. Genetic factors, like the apolipoprotein E (APOE) ε4 allele, may exacerbate memory decline in middle age. Neuroimaging studies reveal non-linear changes in hippocampal structure and function, suggesting a transition point for cognitive decline emergence. These trends are mirrored in mouse studies, confirming the significance of middle age in cognitive trajectories.
Structural and functional changes
Middle age brings both linear and non-linear changes to the brain, with significant alterations in structures like the hippocampus and white matter tracts occurring around the fifth to sixth decades. These changes affect cognitive functions, especially episodic memory, and are associated with overall cognitive status. Functional connectivity of brain networks also undergoes non-linear trends, with declines in system segregation and loss of functional specialization. These alterations potentially explain individual differences in cognitive aging trajectories, highlighting the complex biological dynamics at play during middle age.
Cellular and molecular changes
Research into organismal aging has increasingly employed large-scale "omics" assays, revealing both linear and non-linear trajectories across various molecular processes. Biological "clocks" constructed from features like deoxyribonucleic acid (DNA) methylation predict chronological age, with some clocks exhibiting non-linear patterns. Molecular processes, including gene expression and non-coding ribonucleic acid (RNA) expression, also demonstrate non-linear changes, particularly during middle age. These changes, observed in pathways related to mTOR (short for mammalian target of rapamycin), mitochondria, synapses, and inflammation, may underlie individual aging trajectories, highlighting the complex dynamics of aging. Additionally, brain changes during middle age may also be influenced by systemic factors and other organs, suggesting a holistic approach to understanding aging processes.
Peripheral regulators of middle-aging of the brain
During middle age, significant changes occur outside the brain, particularly in the systemic circulation, involving inflammatory pathways. These changes influence cognitive aging trajectories and are predictive of cognitive decline and incident dementia later in life. Studies indicate that markers of inflammation and immune response in midlife predict cognitive decline and dementia onset up to two decades later. Additionally, shifts in peripheral metabolite levels during midlife, possibly influenced by gut microbiota composition, may also impact neuroinflammation and cognition. Importantly, the biological age of peripheral organs can influence brain aging, highlighting the interconnectedness of systemic and cognitive aging processes during middle age.
Menopause and female middle age
Menopause, occurring around the age of 50 years in females, accelerates epigenetic aging and affects cognition, with variations among individuals. The transition to menopause is associated with cognitive declines, particularly in verbal episodic memory, along with changes in hippocampal volume, brain metabolism, and white matter integrity. Early menopause correlates with faster brain aging, possibly influenced by hormonal shifts. Hormonal fluctuations during the menstrual cycle also impact hippocampal volume. Studies in rats suggest menopause influences hippocampal pathways and immune response. Menopause likely serves as a breakpoint in various brain aging processes, although its effects vary across species. Overall, menopause is a crucial aspect of midlife, deserving further exploration in brain aging research.
Conclusion
In conclusion, middle-age is characterized by unique biological processes influencing the future brain and cognitive health. Complex trajectories in brain connectivity, gene expression, and systemic factors highlight the importance of longitudinal studies for improving our understanding of these processes. Promising interventions, such as exercise, offer avenues for mitigating cognitive decline. Sex differences observed in aging processes highlight the need for inclusive research approaches. Non-linear analysis methods, alongside broad age range studies, could help differentiate middle-age-specific changes from lifelong aging markers, offering insights into brain aging and cognitive health in an aging population.
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