Question

Describe the metabolic processes that occur during the post-absorptive state.

Answer 1

During the Post-absorptive Phase glucose formation occurs to keep the energy supply into the blood. This occurs mainly via three processes,

1. Glycogenolysis 2. Lipolysis    3. Gluconeogenesis

As the absorptive state comes to an end, plasma glucose levels start decreasing once it dip below about 80 mg/dl at which point alpha cells of the pancreatic islets release hormone glucagon and adrenal cortex releases epinephrine which trigger the liver cells to begin breakdown of the glycogen reserves into glucose. This process is known as glycogenolysis.

Meanwhile in adipose tissue stores , fats (triglycerides) are are broken down into glycerol and fatty acids. This process is known as lipolysis. Glycerol enters into the bloodstream reaching the liver and is used to make glucose. Fatty acids can be catabolized by many cells, especially aerobic muscle fibers. Glucagon and epinephrine also trigger lipolysis. Lipolysis begins when glycogen reserves fall to about 1/3 of maximum.Fats are not made into glucose under normal circumstances (except for the glycerol, as above), but in high fat diets or when carbohydrate fuel is unavailable the body shifts to utilizing fat to make glucose.

After about four hours of liver glycogenolysis, liver stores start to become depleted and blood glucose levels begin to fall (to about 70 mg/dl). At this point, adrenal cortex releases glucocorticoids which help liver in stabilizing the glucose levels by gluconeogenesis.

Gluconeogenesis - It is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates ( pyruvate, lactate, glycerol and amino acids)

Amino acids are made to convert into glucose under two conditions:

a) When a person is having high protein and low carbohydrate content in the body, the conversion is carried out under the influence of glucagon;

b) When other fuel reserves are low and the system is under severe stress adrenal cortex causes release of cortisol. This leads to proteins from muscles and connective tissue to be broken down into amino acids to make glucose.

Muscle glycogen is broken down into pyruvate, lactate which enters into the liver.