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 So-called insulin resistance, the body’s failure to respond normally
to the hormone insulin, is a hallmark of diabetes. Diabetes itself –
a disease in which the body can't properly use insulin to convert
blood sugar into energy - has been linked to cognitive decline and
dementia, but the exact nature of the connection isn’t as clear.
In the current study, researchers followed 489 older adults for more
than two decades. They found people with the highest levels of
insulin resistance had the worst cognitive performance and the
lowest scores on tests of memory and a mental skill known as
executive function.
“There is growing evidence that insulin carries out multiple
functions in the brain and thus poor regulation of insulin may
contribute to accelerated cognitive decline and potentially to
Alzheimer's disease,” said senior study author David Tanne of Tel
Aviv University in Israel.
“It is not just people with Type 2 diabetes,” Tanne said by email.
“Even people with mild or moderate insulin resistance who don't have
Type 2 diabetes are at increased risk over time.”
At the start of the study, patients were 58 years old on average and
all of them had cardiovascular disease. People were left out of the
analysis if they had diabetes at the beginning of the study or if
they developed the condition during the follow-up period.
Researchers did cognitive assessments once when participants were
around 72 years old and again when they were about 77.
Cognitive functions were assessed with a computerized battery of
tests that examined memory, executive function, visual spatial
processing and attention.
When researchers accounted for cardiovascular risk factors, they
still found higher levels of insulin resistance were associated with
greater cognitive decline. The association held up even when
researchers excluded people with a history of stroke, dementia or
diabetes.
One limitation of the study is that most participants were men, and
the results might be different in women, researchers note in the
Journal of Alzheimer’s Disease. A substantial proportion of
participants didn’t remain in the study through the second cognitive
assessment, potentially leaving only healthier individuals for the
final analysis.
The study also wasn’t a controlled experiment designed to prove that
insulin resistance directly causes cognitive decline.
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One leading theory about insulin resistance is that the high levels
of blood sugar that it can cause have a negative impact on blood
vessels in the brain, which increases the risk for dementia, said
Barbara Bendlin, a researcher at the University of
Wisconsin-Madison’s Alzheimer’s Disease Research Center. Some
studies suggest that high levels of blood insulin also have a
negative effect on the brain, even independently of high blood
sugar, she added.
“Insulin is involved in helping brain cells form connections, so it
plays a direct role in helping us form memories,” Bendlin, who
wasn’t involved in the study, said by email. “Some studies suggest
that abnormally high levels of blood insulin may actually cause
there to be less insulin in the brain, due to changes in how insulin
is transported into the brain.”
Because the exact reasons for the connection between insulin
resistance and cognitive decline are unclear, it’s difficult to give
patients specific medical advice based on the study results, said
Fernanda De Felice, a neurobiology researcher at the Federal
University of Rio de Janeiro in Brazil who wasn’t involved in the
study.
“However, there are some studies indicating that exercise and
healthy diet may prevent insulin resistance in the brain,” De Felice
said by email. “Having proper sleep and avoiding stress also helps.”
SOURCE: http://bit.ly/2nXUdPW Journal of Alzheimer’s Disease, online
March 10, 2017.
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