The Link Between Brain Size Growth and Decreasing Dementia Rates

In the last century, health in the United States has seen remarkable improvements, though these benefits have not been evenly distributed across the population. With life expectancy on the rise, there's an increased risk for Alzheimer's and other dementias. Yet, research such as the Framingham Heart Study suggests a decline in dementia rates, potentially due to better education and management of health conditions that can impact brain health, like cardiovascular disease. Additionally, being raised in a healthier environment may play a role. To explore this, researchers at the University of California, Davis, examined how the size and thickness of the brain have changed in individuals born from 1930 to 1970, investigating if birth decade influences brain health.

Methodology

The study included 3,266 participants from the Framingham Heart Study, born within the specified decades. Their brains were scanned via MRI from 1999 to 2019, allowing researchers to measure the brain's overall size (intracranial volume), the volumes of gray and white matter, the hippocampus (crucial for memory), and the thickness and surface area of the cortex.

Results

Findings indicated that those born in later years had progressively larger brains. There was a notable increase in the brain's internal volume, hippocampus size, white matter volume, and cortical surface area in later-born groups. For instance, the brain's internal volume grew by 6.6% (from 1234 mL to 1321 mL), and cortical surface area by 14.9% (from 1933 cm² to 2222 cm²), when comparing individuals born in the 1930s to those in the 1970s. However, the cortex's thickness decreased over the decades. 

These results suggest improved brain development over time, possibly due to better early life conditions. This trend may also help explain the decrease in dementia cases observed in the study's cohort.

Conclusion

Despite an aging American population and a potential rise in Alzheimer's cases, the disease's incidence rate is falling, with dementia cases decreasing by 20% each decade since the 1970s. Enhanced brain health and increased size are likely factors. Researchers believe that larger brains indicate better development and a larger "brain reserve," possibly mitigating the impact of aging-related brain diseases.

How could these changes in brain volume over time make it less likely for someone to develop dementia later in life?

A bigger brain may result from improved developmental conditions, healthcare, education, and social factors, alongside better management of dementia risk factors. These advancements, though seemingly minor at an individual level, can significantly affect population health. This suggests that altering risk factors could greatly reduce the incidence of neurodegenerative diseases like Alzheimer's. Additionally, this increase in brain size, alongside rising IQ levels over the 20th century, might bolster cognitive resilience, helping individuals maintain mental function into older age.

Reference:

DeCarli C, Maillard P, Pase MP, et al. Trends in Intracranial and Cerebral Volumes of Framingham Heart Study Participants Born 1930 to 1970. JAMA Neurol. Published online March 25, 2024. doi:10.1001/jamaneurol.2024.0469

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