This risk also appears to be heightened among people with prediabetes -- people who are on the verge of developing diabetes.
Exactly how diabetes and dementia are linked is not fully understood. But the new findings add to growing evidence that what is good for our hearts may also be good for our brains.
The study is published in Neurology.
In the study, 1,017 people 60 and older were given a glucose tolerance test to see if they had diabetes or prediabetes. Researchers from Kyushu University in Fukuoka, Japan, followed the participants for around 11 years and then tested them for dementia.
In that time, 232 people developed Alzheimer's disease and other forms of dementia.
Forty-one of 150 people with diabetes developed dementia. By contrast, 115 of the 559 people without diabetes developed dementia. An increased risk of dementia was also found in people with prediabetes.
The Diabetes-Dementia Connection
Zoe Arvanitakis, MD, says many questions remain regarding the "intriguing" relationship between diabetes and dementia. Arvanitakis is a neurologist at the Alzheimer's Disease Center at Rush University Medical Center in Chicago.
For starters, how are the two conditions linked? There are some plausible explanations, she says. Diabetes is known to increase stroke risk, and strokes can lead to mental problems and dementia.
If you lower your risk for diabetes, might you prevent the onset of Alzheimer's disease or other types of age-related dementia?
It is too early to say that, but "food that is good for the heart is also good for the brain," Arvanitakis says.
Some of the same heart-healthy habits that help lower diabetes risk, such as getting regular exercise and not smoking, may also improve the health of your brain. "It's too premature to say if you prevent diabetes, you would not develop dementia," she says.
There are other things beside diabetes and prediabetes that may increase risk for dementia, such as family history. "The mechanism linking diabetes and dementia still needs to be sorted out," Arvanitakis tells WebMD. "It is important to stay healthy and prevent vascular risk factors from getting out of hand. If you have diabetes, get your blood sugar under control."
Exercise regularly and eat a balanced diet, too. "Hopefully, this will help in the long run," she says.
Role of Cholesterol
Rachel Whitmer, PhD, says that it's not just diabetes and blood sugar abnormalities that may increase dementia risk either. She is a research scientist and epidemiologist at Kaiser Permanente Northern California Division of Research in Oakland, Calif.
Previous studies have shown that high blood cholesterol levels also raise a person's risk for developing Alzheimer's disease and other types of dementia.
"The silver lining to the cloud is that yes, vascular risk factors are also associated with dementia, but they are modifiable," she says. "You can change your cholesterol levels with exercise and diet."
The same holds true for diabetes and prediabetes. "Understand that what is good for the heart is good for the brain and even though dementia shows up late in life, you need to start thinking about it sooner," Whitmer tells WebMD.