A secular trends analysis using brain imaging data from the long-running Framingham Heart Study revealed an increase in intracranial volume (ICV), cortical gray matter, white matter, and hippocampal volumes, as well as cortical surface area in people born in the 1970s versus those born in the 1930s.
“We hypothesize that the increased size of the brain will lead to increased ‘reserve’ against the diseases of aging, consequently reducing overall risk of dementia,” said Charles DeCarli, MD, director of the Alzheimer’s Disease Research Center and Imaging of Dementia and Aging Laboratory, Department of Neurology and Center for Neuroscience, University of California at Davis.
The study was published online in JAMA Neurology.
Dementia Protection?
An earlier report from the Framingham Heart Study suggested that dementia incidence is declining.
“This difference occurred among persons with at least a high school education and was not affected by differences in vascular risk. Our work was stimulated by this finding and the possibility that differences in brain size might be occurring over the three generations of the Framingham Heart Study which might explain an increased resilience to dementia,” said Dr. DeCarli.
The cross-sectional study used data from 3226 Framingham participants (53% women) born in the decades 1930–1970. None had dementia or a history of stroke. At a mean age of 57.7 years, they underwent brain MRI.
Compared with the 1930s birth decade, the 1970s birth decade had a 6.6% greater ICV (1321 mL vs 1234 mL), 7.7% greater white matter volume (476.3 mL vs 441.9 mL), 5.7% greater hippocampal volume (6.69 mL vs 6.51 mL), and 14.9% greater cortical surface area (2222 cm2 vs 1933 cm2).
Cortical thickness was thinner by 21% over the same period, coinciding with larger intracranial volume, cerebral white matter volume, and cortical surface area.
“We were surprised to find that the brain is getting larger, but the cortex is thinning very slightly. The apparent thinning of the cortex is related to the increased need for expansion of the cortical ribbon. This is based on hypotheses related to the effects of evolution and cortical development designed to make neuronal integration most efficient,” said Dr. DeCarli.
Repeat analysis applied to a subgroup of 1145 individuals of similar age range born in the 1940s (mean age, 60 years) and 1950s (mean age, 59 years) resulted in similar findings.
“These findings likely reflect both secular improvements in early life environmental influences through health, social-cultural, and educational factors, as well as secular improvements in modifiable dementia risk factors leading to better brain health and reserve,” the authors wrote.
While the effects observed are “likely to be small at the level of the individual, they are likely to be substantial at the population level, adding to growing literature that suggests optimized brain development and ideal health through modification of risk factors could substantially modify the effect of common neurodegenerative diseases such as stroke and Alzheiemer’s disease on dementia incidence,” they added.
Limitations included the predominately non-Hispanic White, healthy, and well-educated population that is the Framingham cohort, which is not representative of the broader US population. The cross-sectional nature of the study also limited causal inference.