Brain functions such as thinking, memory, and learning are closely linked to glucose levels and how efficiently the brain uses this fuel source. If there isn’t enough glucose in the brain, for example, neurotransmitters, the brain’s chemical messengers, are not produced and communication between neurons breaks down. In addition, hypoglycemia, a common complication of diabetes caused by low glucose levels in the blood, can lead to loss of energy for brain function and is linked to poor attention and cognitive function.

“The brain is dependent on sugar as its main fuel,” says Vera Novak, MD, PhD, an HMS associate professor of medicine at Beth Israel Deaconess Medical Center. “It cannot be without it.”

Although the brain needs glucose, too much of this energy source can be a bad thing. A 2012 study in animals by researchers at the University of California at Los Angeles indicated a positive relationship between the consumption of fructose, another form of sugar, and the aging of cells, while a 2009 study, also using an animal model, conducted by a team of scientists at the University of Montreal and Boston College, linked excess glucose consumption to memory and cognitive deficiencies.

The effects of glucose and other forms of sugar on the brain may be the most profound in diabetes, a group of diseases in which high blood glucose levels persist over a prolonged period of time. Type 1 diabetes is a disease in which the immune system destroys the cells in the pancreas that produce insulin, a hormone used by the body to keep blood glucose levels in check. Type 2 diabetes, caused by dietary and other environmental factors, is a condition in which cells become overwhelmed by insulin and fail to properly respond; they become resistant to insulin.