Obesity -- even a few extra pounds -- already is a known cause of cardiovascular disease and type 2 diabetes. But now there's evidence that excess weight shrinks the brains of elderly people, making them potentially more vulnerable to Alzheimer's disease, dementia and cognitive decline.
"The key thing is, good vascular health equals good brain health," said Cyrus Raji, who's in the M.D./Ph.D program at the University of Pittsburgh School of Medicine. "What's bad for the heart also is bad for the brain."
Dr. Raji was lead author of the study, "Brain Structure and Obesity," published online this week in Human Brain Mapping.
The study focused on 94 subjects, 70 and older, who were overweight (with a body mass index from 25 to 29.9) or obese (with a BMI 30 or higher). Normal BMI is 18.5 to 24.9.
Subjects did not have any clinical signs of Alzheimer's or dementia when their brains were scanned, with a clinical follow-up examination five years later. The idea was only to show impacts on brain volume attributable to obesity and weight gain, not Alzheimer's or brain disease.
Results proved dramatic.
Overweight patients lost 4 percent of the tissue in the frontal lobes of their brains when compared with peers of normal weight. But those who were obese had twice the brain loss -- 8 percent less tissue -- in the frontal lobes crucial to attention and planning.
The evidence indicates that the same disease processes affecting the heart, including cardiovascular disease and type 2 diabetes, also affect the brain. Dr. Raji said weight gain reduces blood flow, nutrients and oxygen to brain tissue, resulting in shrinkage.
The study was based on sophisticated brain images of Pittsburgh patients that the University of California Los Angeles then used to produce three-dimensional maps of the brain.
"Two things are special about this study," Dr. Raji said. "We have very high resolution brain scans of people, so you can see the structures well. We also have advanced computer techniques to map every part of the brain with a three-dimensional mapping technique that couldn't be done before."
Mapping techniques also made detailed brain-volume measurements possible.
Based on study results, elderly people defined as obese lost brain tissue in five areas of the brain: The frontal and temporal lobes (critical for planning and memory), the anterior cingulate gyrus (attention and executive functions), the hippocampus (long-term memory), and basal ganglia (movement). Overweight subjects showed brain loss in the basal ganglia, but also showed shrinkage in the corona radiata, white matter comprised of axons, and the parietal lobe (sensation and cognition).
The study concludes that obese people's brains look 16 years older than those of lean people, while the brains of overweight people look eight years older.
Dr. Raji said brain studies prove that age alone does not shrink the brain. Shrinkage more likely results from type 2 diabetes and heart disease that tend to occur in older patients.
In type 2 diabetes, people become insensitive to the insulin they naturally produce, prompting excessive insulin production. Excess insulin in the bloodstream, a condition known as hyperinsulinemia, appears to adversely affect blood vessels and neurons, possibly contributing to brain loss.
Blood containing too much insulin also fails to clear amyloid plaque from the brain as efficiently as healthy blood. In Alzheimer's disease, buildup of amyloid plaque causes brain damage.
"It did not surprise me that obesity has a bad relationship to brain volume," Dr. Raji said. "What surprised me was, the same brain areas attacked by Alzheimer's also are affected by obesity."
Both, for example, can affect the hippocampus, he said.
"It suggests that people have less brain reserve to fight future problems such as Alzheimer's," he said. "It's not the shrinkage, but where it's taking place."
That's not to say obesity causes Alzheimer's. But it could make a person more vulnerable to it.
But those effects, Dr. Raji said, can be countered by weight loss. Exercise also has a positive impact on brain function.
"If you watch your weight you can maintain your brain's health with aging and potentially reduce risk for Alzheimer's disease," Dr. Raji said.
David Templeton can be reached at email@example.com or 412-263-1578.