A midlife diagnosis of diabetes or prediabetes may raise the risk of memory and thinking problems over the next 20 years, new research suggests.
Having diabetes in midlife was linked with a 19 percent greater decline in memory and thinking (cognitive) skills over 20 years, according to the new study.
"What we saw was, people with prediabetes, diabetes and poorly controlled diabetes had the higher risks of cognitive decline. The people with the worse cognitive decline were those with poorly controlled diabetes," said study researcher Elizabeth Selvin, an associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health in Baltimore.
However, the study authors acknowledged that this study was only able to find an association between diabetes and prediabetes and an increased risk of memory and thinking problems later in life. It wasn't able to determine if the blood sugar disorders were the actual cause of the memory and thinking issues.
Findings from the study are published in the Dec. 2 Annals of Internal Medicine. It was funded by the U.S. National Institutes of Health.
About 21 million U.S. adults have diabetes, according to background information in the study. In type 2 diabetes, the body doesn't use the hormone insulin effectively. Insulin helps get the sugars from foods into the body's cells to be used for energy. Type 2 diabetes is a risk factor for heart disease, stroke, high blood pressure, blindness and kidney disease, according to the study.
Diabetes has also been linked with dementia risk, but how diabetes relates to earlier declines in memory and thinking is less well known, the study authors wrote.
"We know that cognitive decline occurs five to seven years before dementia. Our goal was to look at how diabetes might be contributing," Selvin said.
The new research followed more than 13,000 middle-aged adults over 20 years. They came from four states: Maryland, Minnesota, Mississippi and North Carolina. At the start of the study -- 1990 to 1992 -- the study volunteers were 48 to 67 years old.
Selvin and her colleagues evaluated the study participants' memory and thinking abilities at three different visits over the years. The researchers also had data on whether the volunteers had diabetes or prediabetes, as well as their blood sugar levels at various times in the study.
The researchers measured declines in thinking and memory on a continuum, so it's difficult to give exact measures of the decline linked to the diabetes, Selvin said. But, on average, a 60-year-old who has diabetes has cognitive decline on par with a healthy 65-year-old who is aging normally, according to the researchers.
The study also found that memory and thinking decline was greater for people with prediabetes compared to people with normal blood sugar levels. And, people with diabetes who had higher blood sugar levels (measured as an HbA1C of more than 7 percent) had an even greater risk than those who had lower average blood sugar levels. (HbA1C is a measurement that estimates average blood sugar levels over two to three months, according to the American Diabetes Association.)
The researchers also noted that people who had diabetes for a longer time had more significant memory and thinking problems later in life.
Exactly why the two are linked is unclear, Selvin said. But it could be related to common effects on the blood vessels, she said. Diabetes-related damage to blood vessels may also trigger cognitive changes.
"The study is consistent with other literature we have seen," said Heather Snyder, director of medical and scientific operations for the Alzheimer's Association. Snyder reviewed the study's findings.
Those with diabetes appear to be at greater risk of cognitive problems, she said, but added, "not everyone with diabetes goes on to develop greater cognitive decline."