Type 2 diabetes is a chronic, progressive illness that can have devastating complications, including hearing loss, blindness, heart disease, stroke, kidney failure and vascular damage so severe as to require limb amputation. Now a new study underscores the toll that diabetes can take on the brain. It found that Type 2 diabetes is linked to an increased risk for Alzheimer’s disease and other forms of dementia later in life, and the younger the age at which diabetes is diagnosed, the greater the risk.

The findings are especially concerning given the prevalence of diabetes among American adults and rising rates of diabetes in younger people, which is largely tied to rising rates of obesity. The Centers for Disease Control and Prevention estimates that more than 34 million American adults have Type 2 diabetes.

“This is an important study from a public health perspective,” said Yale Diabetes Center Director Dr. Silvio Inzucchi, who was not involved in the research. “The complications of diabetes are numerous, but the brain effects are not well studied. Type 2 diabetes is now being diagnosed in children, and at the same time there’s an aging population.”

For the new study, published in JAMA, British researchers tracked diabetes diagnoses among 10,095 participants who were ages 35 to 55 at the start of the project in 1985 to 1988, and free of the disease at the time.

They followed them with clinical examinations every four or five years through 2019. At each examination, researchers took blood samples to evaluate fasting glucose levels used to detect diabetes, and recorded cases of the Type 2 disease.

Researchers also determined dementia cases using British government databases. Over an average follow-up of 32 years, they recorded 1,710 cases of Type 2 diabetes and 639 of dementia.

Researchers calculated that each five-year-earlier increment of the onset of diabetes was associated with a 24% increased risk of dementia. Compared with a person without diabetes, for instance, a 70-year-old diagnosed with Type 2 diabetes less than five years earlier had an 11% increased risk for dementia. But a diagnosis at age 65 was associated with a 53% increased risk of later dementia; and a diagnosis at 60, with a 77% increased risk. A person diagnosed with Type 2 diabetes at ages 55 to 59 had more than twice the risk of dementia in old age compared with a person in the same age group without diabetes.

The study was observational, so it could not prove that diabetes causes dementia. But it was long-running, with a large study population. Researchers controlled for many factors that affect the risk for dementia, including race, education, heart conditions, stroke, smoking and physical activity — and the diabetes-dementia link persisted.

“These are exceptional data,” said Daniel Belsky, an assistant professor of epidemiology at Columbia Mailman School of Public Health who was not involved in the research. “These associations between the timing of onset of diabetes and development of dementia show the importance of a life-course approach to preventing degenerative disease.”