Can diabetes affect a person's memory?

Diabetes itself doesn't affect a person’s memory.  However, if your father is having many high or low blood sugar episodes, that could be the cause.  Multiple instances of hypoglycemia (low blood sugar) can cause permanent memory defects.  Typically, people with memory issues have severely low blood sugar levels -- in the 30-40 milligrams of glucose per deciliter (mg/dL) of blood range -- on numerous occasions.  Chronically high blood sugars can also cause symptoms such as fatigue and mental slowing.

If your dad's blood sugars are fairly well controlled, I would investigate other explanations for the memory loss.   A neuropsychiatric evaluation may help determine if he has true memory loss.  Common causes of memory loss include Alzheimer’s disease and other dementias.  I would consider a second opinion regarding this issue if you feel it has not been adequately evaluated by your physician.