More evidence ties insulin resistance to cognitive decline

(Reuters Health) – - Having reduced sensitivity to insulin may lead to more rapid decline in memory and other mental skills in old age even among people who don’t have diabetes, a recent study suggests.

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.

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: Journal of Alzheimer’s Disease, online March 10, 2017.