Question

Why is it necessary to move from glucose to ketone bodies to prevent protein degradation when glucose levels are very low?

Answer 1

Ans: When glucose levels are low, body functions will be derailed until and unless more glucose or glucose equivalent substances are made available for the cells of the body, especially brain cells. Brain needs 25% of the total glucose requirement of the body, to sustain its functions, at a given point of time.

Thus if glucose level gets low in our body, the body will depend upon stored form of energy to release more glucose. First preference will be for glycogen, which is the storage form of glucose found in muscle tissue and liver. The conversion of glycogen to glucose is facilitated mainly by glucogon hormone, which is having an opposite function to insulin hormone. ( Insulin help to store glucose, by converting excess glucose into glycogen)

When the glycogen reserves are depleted, next option is fat reservoir. Fat is a stored form of energy in our body. The excess carbohydrates we eat , will be converted into fat and will be stored in our body to be used during high energy demand or during starvation. High energy demand situations are infections, pregnancy, lactation, exercise , high metabolic rate etc.

In liver, fatty acids will be converted to acetyl- co enzyme A, and the acetyl- coA will be converted into acetone, aceto acetate, beta hydroxy butyric acid. The last 3 compounds are known as the ketone bodies. The peculiarity of ketone bodies is that, they can be utilized by brain tissue and other peripheral tissues, as a source of energy in the absence of glucose.  

The last resort is the burning of proteins. Glucose can be produced from proteins through the process of "gluconeogenesis". The protein to glucose conversion ratio will be 2:1 or 3:1, ie: burning 2 - 3 g of protein yield 1 g of glucose.But this step is not good for body. PROTEINS ARE BODY BUILDING MATERIAL, DEGRADING PROTEINS MEANS USING UP OF BODY TISSUE ITSELF. Using up of proteins will result in muscle wasting and loss of body condition. Loss of more and more protein from body will finally result in electrolyte imbalance, cardiac failure and death.

Thus our body will try to spare the protein resources as the last resort. Regaining the glycogen reserves and fat reserves are more easily achievable than rebuilding the lost muscle mass and protein components of the body, when food become again freely available.