Two recent studies show blood-sugar levels can affect the brain—-adding new evidence that diabetes might be a significant risk factor for developing Alzheimer’s disease and other forms of dementia.

Researchers at Washington University in St. Louis found in a study of mice that raising blood sugar to abnormally high levels corresponded with increased production in the brain of amyloid beta, a protein thought to be an important factor in Alzheimer’s disease. In a separate study of middle-aged people, conducted at the University...