The primary role of altered metabolism during fasting is to maintain blood glucose through the synthesis (gluconeogenesis) and utilization of alternate substrates. Fasting-starvation is divided in three broad categories:?Initial stage :- Lasts upto 2-3 daysIntermediate stage :- Lasts upto 24 daysAdvanced stage:- begins after 24 days.a) Initial stage (first stage):- Liver glycogen is the first main provider of energy. But due to limited storage it can not last longe (16-18 hrs.). Fuher glucose is provided by gluconeogenesis by non-carbohydrate sources (muscle protein breakdown is increased which provides amino acids alanine and glutamine for gluconeogenesis). Utilization of alternate substrates (other than glucose) stas increasing in terminal period of this stage. Free fatty acids and ketone bodies sta rising in this stage. Glycogen is completely depleted by the end of this stage. Blood glucose is slightly decreased.The first priority of metabolism in starvation is to provide sufficient glucose to brain and other tissues that are absolutely dependent on glucose.Liver glycogen provides glucose only for a sho period. Whereas, gluconeogenesis plays an essential role in maintaining blood glucose during both sho-term and prolonged starvation.The substrate for gluconeogenesis are pyruvate, lactate (from RBCs and exercising muscles), glucogenic amino acids released from muscle protein breakdown, and glycerol released from lipolysis.b) Intermediate stage (Second stage):- Free fatty acids are the major substrates for energy production. They are derived from hydrolysis of fat (triglycerides) in adipose tissue. Ketone bodies serve as additional source of energy. As glycogen source is already depleted, the only source of glucose is increased gluconeogenesis. After sometimes muscle breakdown decreases due to decreased need of glucose as a fuel for brain which has began using ketone bodies as a source of energy.c) Advanced stage (Third stage):- When fat stores are almost exhaused, energy requirment is obtained from breakdown of tissue proteins and protein stores once again enter in stage of rapid depletion.Thus the second priority of metabolism in starvation is to preserve protein. This is accomplished by using fatty acids and ketone bodies in place of glucose as a fuel.