Prediabetes often precedes type 2 diabetes, the form of the disease most closely tied to obesity.
A new study suggests that the timing of this transition may set the stage for dementia in later years.
Prediabetes refers to blood sugar or glucose levels that are higher than normal but not high enough to be defined as diabetes.
Researchers found that folks who were diagnosed with type 2 diabetes before their 60th birthday were three times more likely to develop dementia than people without diabetes. This increased risk for dementia dropped for people who developed diabetes at older ages. By age 80, for example, developing diabetes was not associated with higher odds for dementia.
"Slowing or preventing prediabetes progression to diabetes may be an important way to prevent dementia," said study co-author Michael Fang. He is an assistant professor of epidemiology at Johns Hopkins University Bloomberg School of Public Health in Baltimore.
Exactly how, or even if, progression from prediabetes to type 2 diabetes affects dementia risk is not fully understood, but the researchers have a theory.
Insulin resistance and high blood sugar levels are associated with diabetes and contribute to the buildup of beta-amyloid and tau in the brain. These are hallmarks of Alzheimer's disease, the most common form of dementia.
"The buildup of beta-amyloid amyloid and tau may cause the loss of brain cells, which in turn can lead to dementia," Fang said.
"It's [been] unclear whether prediabetes is an independent risk factor or if persons with prediabetes are simply at higher risk for diabetes," he explained. "Our findings support the theory that prediabetes matters for dementia mostly because these patients are at increased risk for diabetes."
For the study, the researchers analyzed data on about 11,660 people who did not have diabetes when the research began. Of these, 20% had prediabetes. Folks underwent tests measuring cognitive, or mental, function throughout the study, which spanned for nearly three decades.
The earlier people progressed from prediabetes to diabetes, the more likely they were to develop dementia, the study found. Progression to type 2 diabetes before age 60 was associated with a three times increased risk of developing dementia. This fell to 73% increased risk for those who developed diabetes aged 60-69 years, and dropped further to 23% increased risk for those who developed diabetes at ages 70-79.
So, will preventing progression from prediabetes to type 2 diabetes lower dementia risk?
That's the hope, Fang said. "Policies that effectively slow prediabetes progression may have an important impact on the overall burden of dementia," he said.
Modest weight loss and participation in programs that encourage healthier lifestyles, such as the National Diabetes Prevention Program, could make a dent in these statistics, he said.
The study is published in the May 24 issue of Diabetologia.
Type 2 diabetes is a well-established risk for dementia, said Yuko Hara. She is the director of prevention and aging at the Alzheimer's Drug Discovery Foundation in New York City.
Preventing or managing diabetes is one of the most important things you can do to reduce your dementia risk and protect your brain from cognitive decline, said Hara, who reviewed the new study.
"Healthy diet, exercise and weight control are the first steps of diabetes prevention and management," she said. Medications are also available to keep blood sugar levels at optimal levels, she said.