During starvation the body does everything possible to preserve the brain. The brain has highest priority when it comes to gaining access to essential nutrients and fuel. The brain is, therefore, the last area to suffer from a shortage of food. There is even some evidence that short periods of starvation may be healthy for the brain. However, even the brain must eventually metabolize its own neurons to keep the rest of the brain alive.
During the initial phases of starvation or water fasting, the body converts glycogen stored in the liver and muscles into glucose. After about 24 hours of using stored glycogen as an energy source, the body switches to fat burning. Some muscle and connective tissue also is used to provide the building blocks for important biological catalysts. As fats cannot cross the blood-brain barrier, the brain cannot use fats as a source of energy. However, fat metabolism in the liver produces ketone bodies as a byproduct. The brain can use these substances as an energy source. A state in which ketone bodies have become the main source of energy for the brain is called "ketosis."
Ketosis is not unhealthy for the brain, according to Lyle McDonald, author of "The Ketogenic Diet." In fact, there is some evidence that ketosis can stabilize neurons in the brain. Ancient Greek physicians successfully cured some cases of epilepsy with starvation. The seizures that occur in epilepsy happen when localized neurons get hyperexcited and the intense firing spreads to larger areas of the brain. Though it is not entirely clear why, ketone bodies can stabilize the neurons and protect against seizures. In more recent years, the so-called ketogenic diet has been used as a treatment of epilepsy. This diet consists primarily of fat.
Breakdown of Muscle and Neurons
When starvation is allowed to continue for an extended time period, most of the body's stored fat has been used up and the shortage of vitamins and minerals becomes significant. At this point, the body's only chance of survival is to metabolize its own muscles and connective tissue. The brain too lacks essential nutrients and begins to break down its own neurons to keep the rest alive. According to a study published in the January 2009 issue of "Journal of Biological Chemistry," male and female brains react differently during starvation. The male brain more quickly begins to use its own tissue to supply nutrients.
Reduced Brain Volume
When the brain starts to break down its own neurons, the brain literally shrinks. This shrinkage, however, is reversible if starvation is ended. A study published in the May 2010 issue of "International Journal of Eating Disorders" showed reduced brain volume in people with anorexia nervosa. When anorexics starve themselves to lose weight, their brain starts metabolizing its own gray matter. However, those subjects who regained weight also regained brain volume.