A group of Canadian scientists recently studied the brain activity of fruit flies and found that acute fasting directly influences the stability of neuronal circuits, a type of wiring that dictates the flow of information in the brain and nervous system. According to their paper, the cellular stress and lack of nutrition catalyzed by fasting blocks the synaptic activity of neurons that normally occurs in the brain, which essentially means that the brain slows down.
And although a brain "slowing down" sounds undesirable generally speaking, it may actually be beneficial for brain health. In fact, overactive synaptic activity has been associated with diseases like Parkinson's, Alzheimer's, and other degenerative diseases. So in a way, when we slow down our brain activity, it's possible that we are protecting the organ by allowing it to recharge.
Results also showed an increase in ketone bodies due to fasting, which are compounds known to be neuroprotective, essentially guarding brain cells against degenerating to the point of disease. The ketogenic (low-carb and high-fat) diet has been used since the 1920s as an extremely effective treatment for epilepsy, even though the mechanism by which is works was mostly unknown.
This study is really worth our attention because it doesn't just draw a connection between fasting and the brain—or describe an association between calorie restriction and a lower risk of disease—it describes the actual mechanism of slowing down the communication between neurons to conserve energy during nutrient deprivation.