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The study found that people with type 2 diabetes -- particularly those who are overweight or obese -- have thinner gray matter in several areas of the brain.
These brain regions are related to memory, executive function, movement generation and visual information processing, said the study's senior author, Dr. In Kyoon Lyoo. He's director of the Ewha University Brain Institute in Seoul, South Korea.
"Obesity leads to increased risk of type 2 diabetes, metabolic dysfunction and is also associated with brain alterations independently," Lyoo said. "We aimed to investigate whether overweight/obesity influenced brain structure and cognitive function in individuals with early stage of type 2 diabetes."
The study included: 50 overweight or obese people with type 2 diabetes; 50 normal-weight people with type 2 diabetes, and 50 normal-weight people without diabetes.
The Korean study volunteers were between 30 and 60 years old. Those with diabetes had it for five years or less, and they were attempting lifestyle modifications and/or taking oral medication to lower blood sugar levels. No one was taking insulin.
The normal-weight group with type 2 diabetes had slightly better blood sugar control -- a hemoglobin A1C level of 7 percent. The overweight folks with type 2 diabetes had hemoglobin A1C levels of 7.3 percent.
Hemoglobin A1C is a two- to three- month estimate of average blood sugar levels. The American Diabetes Association generally recommends an A1C of 7 percent or less.
All study participants underwent MRI brain scans and tests to measure memory and thinking skills.
"Cortical thickness was decreased in several regions of the diabetic brains. Further thinning of the temporal lobes found in overweight/obese individuals with type 2 diabetes suggests that these regions are specifically vulnerable to combined effects of obesity and type 2 diabetes," Lyoo said.
He said this study alone cannot tease out whether the effect is from excess weight or diabetes or both. But the study did find that the longer someone had diabetes, the more likely they were to have brain changes.
Lyoo said factors such as insulin resistance, inflammation and poor blood sugar management might bring about the changes.
Memory and thinking skills were decreased in people with diabetes -- regardless of weight -- compared to the normal-weight people without type 2 diabetes, the study found.
Because the study only included an Asian population, Lyoo said it isn't clear if these effects would apply to other populations, such as Americans. He also said it isn't known if these effects occur in people with type 1 diabetes, the less common form of diabetes.
Dr. Sami Saba is an attending physician in neuromuscular medicine and electromyography at Lenox Hill Hospital in New York City.
"The regions most affected were the temporal lobes, which are also most prominently affected in people with Alzheimer's," he said of the research.
"While this was not proven on this study, it does suggest that those with diabetes who are also overweight are at higher risk for developing Alzheimer's-type cognitive impairment than those with diabetes who are not overweight," Saba said.
But, he also noted that a major limitation of this study was the lack of overweight/obese people without diabetes to serve as a comparison group.
The take-home message, said Saba, is that weight control is an "important factor in preserving brain health in these patients." He said it's one more reason to work to prevent weight gain.
Lyoo said good blood-sugar management would probably help slow down or prevent these diabetes- or obesity-related brain changes.
Dr. William Cefalu is the chief scientific, medical and mission officer for the American Diabetes Association.
"The presence of overweight and obesity has been shown in other studies to be associated with early structural changes in the brain, and may contribute to cognitive issues," he said.
But, he said that diabetes may also play a role. Both Lyoo and Cefalu said that more research is needed to figure out which factor is at the root of these changes.
The study was released April 27 in the journal Diabetologia.
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SOURCES: In Kyoon Lyoo, M.D., Ph.D., director, Ewha University Brain Institute, Seoul, South Korea; Sami Saba, M.D., attending physician in neuromuscular medicine and electromyography, department of neurology, Lenox Hill Hospital, New York City; William Cefalu, M.D., chief scientific, medical and mission officer, American Diabetes Association; April 27, 2017, Diabetologia