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updated 12/2/2014 10:45:55 AM ET 2014-12-02T15:45:55

People who have type 2 diabetes in midlife may be more likely to experience cognitive problems over the next two decades of their life, compared with people who don't have the condition, according to a new study.

In the study, researchers followed more than 13,000 adults for more than 20 years. The participants were from communities in Maryland, North Carolina, Minnesota and Mississippi, and were 57 years old, on average, at the study's start.

About 1,800 of the participants had diabetes, and almost all of these patients had type 2 diabetes, which is related to diet and lifestyle, and develops over time, as the body stops producing enough insulin or responding to the hormone.

The participants who had diabetes at the beginning of the study showed a cognitive decline over 20 years that was 19 percent more severe, on average, than that of those who did not have diabetes. [ Alzheimer's Vs. Normal Aging: How to Tell the Difference ]

"The association of diabetes and glucose control assessed in midlife were very strong risk factors for cognitive decline over the next 20 years," said study author Elizabeth Selvin, an associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

The people in the study were examined five times between 1987 and 2013. The researchers looked at whether the participants had diabetes, and also at other factors of their health, such as smoking, alcohol consumption, body mass index, blood pressure, and history of coronary heart disease or stroke. The researchers assessed the participants' cognitive function by testing their verbal learning ability, short-term memory and the speed at which they processed new information.

They found that the cognitive ability in people with diabetes declined more, compared with the cognitive ability of the people who did not have diabetes.

"This research shows that to protect the brain in old age, we need to tackle cardiovascular risk factors in midlife," Selvin told Live Science. "Diabetes in midlife is an important potentially modifiable risk factor for cognitive decline in old age."

More than 29 million people in the United States currently have diabetes, the majority of which are cases of type 2 diabetes, which is 3 million more than the previous estimate of 26 million in 2010, according to a report from the Centers for Disease Control and Prevention in June. Moreover, one in four people does not even know he or she has the disease, the agency said.

People can lower their risk of type 2 diabetes by losing weight — even a loss of 5 to 10 percent of body weight can play a significant role in diabetes prevention, Selvin said.

The exact mechanism behind the link between diabetes and cognitive decline is not clear, but it may be that high levels of blood sugar damage small vessels in the brain, she said.

Previous research has linked diabetes with subsequent cognitive decline, but the duration of those previous studies was short, with follow-ups ranging from about six to 12 years, according to the new study.

"A unique and important aspect of our study was that we had a large community-based population of persons followed for more than two decades," Selvin said. "It is only by studying people for 20 years that we can truly understand how risk factors in midlife may contribute to health problems decades later in old age."

The study was published today (Dec. 1) in the journal Annals of Internal Medicine.

© 2012 LiveScience.com. All rights reserved.

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