The longer a person has type 2 diabetes, the more substantial and steeper his cognitive decline later in life, a new report in the Journal of the American Geriatric Association said.

Researchers at Harvard Medical School tested more than 12,000 men and women with an average age of 70 for diabetes and mental acuity over the course of the four-year study. Diabetic participants performed more poorly on initial cognitive tests than non-diabetics, and showed a markedly faster decline in capacity over the duration of the study. There also were strong links between cognitive performance and the amount of time a person has lived with diabetes.

Diabetes can cause damage to blood cells that travel to the brain, which might be a contributing factor to the diminished capabilities of diabetics, researchers say. They also speculate that consistently high levels of insulin might increase the body’s development of amyloid-beta plaques, which many researchers identify as a hallmark of Alzheimer’s disease.