Obesity increases your risk of cardiovascular disease and some cancers, past research indicates. But a new Iowa State University study suggests obesity, or more specifically the insulin resistance that is common in those who are overweight, is strongly associated with memory loss, too. This in turn ups the ante on a person’s risk of Alzheimer's disease, the researchers said.

“Even people with mild or moderate insulin resistance who don't have type 2 diabetes might have an increased risk for Alzheimer's disease because they're showing many of the same sorts of brain and memory relationships,” said Dr. Auriel Willette, a research scientist in the Department of Food Science and Human Nutrition at Iowa State, in a press release. Insulin resistance, he says, is common in people who are obese or diagnosed with either pre-diabetes or type 2 diabetes.

The study involved 150 adults whose average age was 60, and who were at risk for Alzheimer’s disease even though none had shown signs of memory loss just yet. The researchers recruited participants through the University of Wisconsin-Madison and Wisconsin Registry for Alzheimer's Prevention (WRAP) study, an ongoing examination of genetic, biological and lifestyle factors that contribute to dementia.

Willette and his colleagues sought to answer a single question: Is a pattern of decreased use of blood sugar in the brain linked to memory decline?

To determine this, they measured insulin resistance using a standard and reliable method known as the homeostasis model assessment, and then gathered additional data from the participants in the form of cognitive testing, BMI, blood tests, and a brain scan. Then, they searched the scans to determine whether people with higher levels of insulin resistance used less blood sugar in their brains. With less blood sugar, the brain has less energy available for relaying information. The researchers focused on the hippocampus, the first area of the brain to show atrophy or shrinkage during Alzheimer's disease. This brain region is crucial to learning and transferring information from short- to long-term memory.

Willette and his colleagues discovered that higher insulin resistance was associated with lower glucose metabolism across large portions of the frontal, lateral parietal, lateral temporal, and medial temporal lobes. The link was especially strong in the medial temporal lobe, where the hippocampus is located, and was significantly related to worse performance on memory tests.

"Over the course of [Alzheimer’s disease], there is a progressive decrease in the amount of blood sugar used in certain brain regions. Those regions end up using less and less,” Willette said. “If you don't have as much fuel, you're not going to be as adept at remembering something or doing something.”

Source: Willette AA, Bendlin BB, Starks EJ, et al. Association of Insulin Resistance With Cerebral Glucose Uptake in Late Middle–Aged Adults at Risk for Alzheimer Disease. JAMA Neurology. 2015.