Prediabetes and diabetes often go undiagnosed in people with early Alzheimer’s, new research shows.

A Georgetown University neurologist made the discovery when enrolling people with mild to moderate Alzheimer’s in a study. R. Scott Turner, M.D., wanted to determine whether resveratrol, a compound found in red grapes and red wine, could change blood sugar levels in patients with mild to moderate Alzheimer’s.

Turner said he “was shocked” to find 48% of his study enrollees showed signs of undiagnosed prediabetes and diabetes. Potential subject with diagnosed diabetes had already been excluded.

“I was surprised by how many people didn’t know they were prediabetic, and these are individuals who already get the best medical care,” Turner said in a press release.

Study participants took a fasting glucose tolerance test and retested two hours after eating. A high sugar level after two hours demonstrates glucose intolerance (prediabetes) or diabetes if the level is very high.

Thirty percent of the 125 study participants had glucose intolerance and 13% had results consistent with diabetes, Turner found.

The findings suggest that people with Alzheimer’s should be tested for glucose intolerance, Turner said.

“It’s a simple, inexpensive study that reveals critical health information,” he said.