Question

Ketone bodies are byproducts in the metabolism of
(a) Carbohydrate
(b) Protein
(c) Fat
(d) All of the above

Answer 1

Hint: Ketone bodies when found in excess amount in the urine, is indicative of some underlying health disease because normally glucose is digested which does not have ketone as a by-product. Ketones are only released when the body is not able to digest glucose and is breaking down other compounds for energy.

Complete Answer:
Ketone bodies are water-soluble molecules that are formed as a byproduct in the metabolism of fatty acids in the liver. Fatty acids along with glycerol from fats, so during the breakdown of fats, ketones bodies are obtained. This process occurs in the body when there is a low intake of food, starvation, fasting, alcoholism, or when the carbohydrate amount is less in the diet. It is also seen in type 1 diabetes mellitus and is used as its diagnostic test.
Fats are broken down as glucose is not available and thus gluconeogenesis occurs in the liver. In gluconeogenesis, other compounds are broken down and converted into glucose to supply the energy demands of the body. When fats are utilized for gluconeogenesis, ketone bodies get formed.

Additional information: Let us look at the different substrates used by the body to gain energy.
Carbohydrates: Carbohydrates are the ideal choice for the body to use as substrate as it has high energy output as well as high respiratory quotient.
Fats: Fats have low respiratory quotients and thus are chosen to be substrate only when there are low levels of carbohydrates in the body.
Proteins: Proteins make up the structural components of cells and thus are used last in case of intense starvation when the body has no other compound left.
Organic acids: Compounds like oxalic acid, malic acid have a very high respiratory quotient such as 4, 3.9, etc but are not used by the body as their energy output is less.
So, the correct option is ‘(c) Fat’.

Note:
- Respiratory quotients is the amount of carbon dioxide evolved per oxygen consumed.
- Proteins are broken down into pyruvic acids and inserted into the respiration pathway.
- Whereas fats are broken down into acetyl CoA and dihydroxyacetone phosphate and entered into the respiratory pathway.