To answer your question, we will look at the physiological process of Starvation, conversion of food as energy and how the human body uses that energy.
Conversion of food/Digestion
Digestion of food starts in the mouth where our enzymes in our saliva start breaking down starch. Food is mainly digested by acid and other enzymes in the stomach to further break down food to a more absorbable form and finally absorbed in the intestines. This is with an exemption to fat which can be only absorbed with the presence of bile for fat emulsification in the duodenum (the part where bile ducts enter the 1st part of small intestine). Nutrients absorbed from food can be classified into Fat, Glucose/Sugar, Proteins, Vitamins, Minerals, and Electrolytes. In this context, we will focus on Fat, Sugar, and Proteins which makes up food calories we eat that are used as energy. -excerpt from The Physiology of Digestion
Physiology of Starvation
Our bodies maintain a particular level of glucose in the blood to maintain normal biological processes. That level varies from an individual but there is a normal range.
The blood-glucose level is kept at or above 80 mg/dl by three major factors: (1) the mobilization of glycogen and the release of glucose by the liver, (2) the release of fatty acids by adipose tissue, and (3) the shift in the fuel used from glucose to fatty acids by muscle and the liver. - Biochemistry, 5th edition: Jeremy M Berg, John L Tymoczko, and Lubert Stryer
Energy metabolism during Starvation
Each Organ Has a Unique Metabolic Profile
Brain. Glucose is virtually the sole fuel for the human brain, except during prolonged starvation.
Fatty acids do not serve as fuel for the brain, because they are bound to albumin in plasma and so do not traverse the blood-brain barrier. In starvation, ketone bodies generated by the liver partly replace glucose as fuel for the brain.
Prolonged starvation can cause increased amounts of ketones in the body to maintain normal brain functioning. There has been a study that giving ketones have improved brain functioning in patients with traumatic brain injuries. Another study claims
that ketone bodies play a neuroprotective role during starvation.
I have looked into the description of the book you've read ("Unbroken is a testament to the resilience of the human mind, body, and spirit."). The answer seems to be "will" or "spirit". Or it could just be ketones.
Common misnomer with ketosis, starvation ketoacidosis and diabetic ketoacidosis which have very different outcomes.