Question

After several days of not eating, one's nitrogen excretion is higher than normal. What causes this high level of nitrogen excretion initially?

After a week and a half of no food, one's nitrogen excretion falls. Why causes it to fall?

After nitrogen excretion falls and all fat sources have been drained, the nitrogen excretion increases, once more. Why is that?

Answer 1

During starvation our body gives priority to supply glucose to brain continuously. After starting  fasting all the gllucose in blood is used . Then depletion of insulin an d production of glucagon favours glygogenolysis ( glycogen breakdown) and gluconeogenesis (synthesis of glucose from non carbohydrates ie from proteins majorly). So due to the use of amino acids for the production of glucose after a week of fasting, the nitrogen excretion is more.

The second priority is to protect proteins. If fasting is prolonged then the body changes the metaboilism of using fat stored as triacylglycerols (TGs). So now to run the TCA cycle oxlo acetate is needed and gluconeogenesis stoped. During this time brain uses ketone bodies that are produced from Acetyl coA which is the end product of beta oxidation pathway of fatty acids. so after a week and half no food nitrogen excretion falls bec of the utilisation of stored fat by body.

Ater the depletion of all TGs again the body depend on the available proteins for energy supply so again protein breakdown and gluconeogenesis favoured which leads to nitrogen excretion