Question

Which of the following substrates liberates the maximum energy?

• A. Carbohydrates
• B. Fats
• C. Proteins
• D. Cannot be determined

Answer 1

The correct option is B Fats

All the respiratory substrates do not provide the same amount of energy. Fats or lipids are known to provide more energy as compared to carbohydrates and proteins.

Carbohydrates are still the most preferred substrate because they can enter the respiratory pathway easily and are easily metabolised.

Proteins are rarely used as respiratory substrates. They are only used during starvation when carbohydrates and fats are no longer available.

The amount of energy liberated from complete oxidation of 1g the respiratory substrate in a bomb calorimeter (a closed metal chamber filled with oxygen) is its gross calorific value.

The actual amount of energy released by oxidation of 1g of the respiratory substrate in the body is the physiological value.

The following tabular column provides the gross calorific value and physiological values for various respiratory substrates:

$\begin{array}{ccc}\text{Respiratory}& \text{Gross Calorific}& \text{Physiological}\\ \text{Substrate}& \text{Value (kcal/g)}& \text{Value (kcal/g)}\\ \text{Carbohydrate}& 4.1& 4.0\\ \text{Lipid}& 9.45& 9.0\\ \text{Protein}& 5.65& 4.0\end{array}$