Bigger brains suggest better brain health and cognitive function. And since the 1930s, our brains have been growing in size. Now, a new study shows how this is affecting our risk of developing dementia, particularly Alzheimer's disease.
In 2020, there were more than 55 million people worldwide living with dementia, and that number is expected to double every 20 years. However, the rise in the number of people with the condition is likely a reflection of our aging population and the fact that we’re living longer. A 2016 National Institutes of Health (NIH)-funded study found that there’s been a progressive decline in newly reported dementia cases since the 1970s, with an average reduction of 20% per decade.
But what’s contributing to this decline? The 2016 study examined the impact of education, among other factors, on dementia risk, finding that, by the 2000s, the incidence had declined by 44% in those with at least a high school diploma compared to the ’70s. While it noted the link between education and dementia, the study didn’t examine the potential cause(s).
A new study by UC Davis Health may have an explanation: Our brains are simply larger now.
“The decade someone is born appears to impact brain size and potentially long-term brain health,” said Charles De Carli, a neurology professor, director of the UC Davis Alzheimer’s Disease Research Center, and the study’s lead author. “Genetics plays a major role in determining brain size, but our findings indicate external influences – such as health, social, cultural, and educational factors – may also play a role.”
UC Davis researchers used the same dataset in the current study as the previous one, the Framingham Heart Study (FHS). Initiated in 1948, the community-based population study of more than 15,000 individuals from Framingham, Massachusetts, looked at three generations of participants to ascertain heart and brain health trends.
Between 1999 and 2019, brain MRIs were conducted on FHS participants, and the researchers examined the scans of 3,226 individuals (53% female, 47% male) born between 1930 and 1970. None of the participants had cognitive impairment or a history of stroke, which increases dementia risk.
When the researchers compared the MRIs of people born in the 1930s to the 1970s cohort, they found gradual but consistent increases in the size of several brain structures. For one, intracranial volume (ICV) or the volume inside the skull (cranium) increased decade by decade from an average of 1,234 mL/41.7 fl oz in the ’30s to 1,321 mL/44.7 fl oz in the ’70s – a 6.6% increase.
Although people were also taller in the 1970s than they were in the ’30s, after adjusting for height, the differences in ICV remained. Studies have suggested that a larger ICV indicates a larger ‘brain reserve,’ which could protect against dementia.
Size increases were also seen in white matter and cortical gray matter. Gray matter is the outermost surface of the brain (the cortex) and is important for mental functions, memory, emotions, and movement. Several conditions, including stroke, Parkinson’s disease, and Alzheimer’s disease, can affect gray matter. White matter lies beneath the gray matter and contains millions of bundles of nerve fibers (it’s white due to the electrical insulation, myelin, that coats the fibers). Between the 1930s and the 1970s, the researchers observed a 7.7% greater volume of white matter and a 2.2% greater volume of cortical gray matter. The volume of the hippocampus, whose biggest job is holding short-term memories and transferring them to long-term storage, was also increased by 5.7%. And the cortical surface area, that’s the wrinkly-looking visible gray matter layer, was up by 14.9%.
“Larger brain structures like those observed in our study may reflect improved brain development and improved brain health,” DeCarli said. “A larger brain structure represents a larger brain reserve and may buffer the late-life effects of age-related brain diseases like Alzheimer’s and related dementias.”
Acknowledging the substantial influence of genetics, the researchers believe their findings indicate that early-life environmental influences are more likely contributors to larger brain structures and reduced dementia risk. They say the increased brain size they observed likely reflects improvements in health, education, and sociocultural factors since the 1930s, as well as improvements in modifiable dementia risk factors like heart disease, high blood pressure, obesity, exercise, and diabetes.
However, a limitation of the study is that the FHS cohort is predominantly non-Hispanic White, healthy, and well-educated. Therefore, it’s not representative of the broader US population. Against this, though, is the design of the study, which followed a large number of participants throughout much of their lifespan, and spanned nearly 80 years of birth dates.
The study was published in the journal JAMA Neurology.
Source: UC Davis Health