People with diabetes are at increased risk of having a heart attack or stroke at an early age, but that’s not the only worry. Diabetes appears to dramatically increase a person’s risk of developing Alzheimer’s disease or other types of dementia later in life, according to a new study conducted in Japan.

In the study, which included more than 1,000 men and women over age 60, researchers found that people with diabetes were twice as likely as the other study participants to develop Alzheimer’s disease within 15 years. They were also 1.75 times more likely to develop dementia of any kind.

“It’s really important for the [public’s] health to understand that diabetes is a significant risk factor for all of these types of dementia,” says Rachel Whitmer, Ph.D., an epidemiologist in the research division of Kaiser Permanente Northern California, a nonprofit health-care organization based in Oakland, California.

Whitmer, who studies risk factors for Alzheimer’s but wasn’t involved in the new research, stresses that many questions remain about the link between diabetes and dementia. The new study was “well done” and provides “really good evidence that people with diabetes are at greater risk,” she says, “but we really need to look at other studies to find out why.”

More studies are needed and it makes sense that the longer you live, the more of a chance you will develop some sort of dementia. But, it also makes sense that via diet and exercise, you should avoid diabetes in the first place.

Diabetes could contribute to dementia in several ways, which researchers are still sorting out. Insulin resistance, which causes high blood sugar and in some cases leads to type 2 diabetes, may interfere with the body’s ability to break down a protein (amyloid) that forms brain plaques that have been linked to Alzheimer’s. High blood sugar (glucose) also produces certain oxygen-containing molecules that can damage cells, in a process known as oxidative stress.

In addition, high blood sugar — along with high cholesterol — plays a role in the hardening and narrowing of arteries in the brain. This condition, known as atherosclerosis, can bring about vascular dementia, which occurs when artery blockages (including strokes) kill brain tissue.

“Having high glucose is a stressor to the nervous system and to the blood vessels,” says David Geldmacher, M.D., a professor of neurology at the University of Alabama at Birmingham. “The emerging information on Alzheimer’s disease and glucose shows us that we do need to remain vigilant on blood sugar levels as we get older.”