Question

Do you think differently about the process of how carbohydrate, lipids and protein are metabolized and energy is released? Or perhaps about how the pathways in metabolism work? Does it make sense now why lipids have 9 kcalories per gram and carbohydrate and protein have 4 kcalories per gram?

Answer 1

Carbohydrate , protein and fat metabolism

The Essential macronutrients are Carbohydrates, protein and fats . After the digestive process all three are absorbed to blood from the intestinal lumen. The endproducts of Carbohydrate are monosacharides such as glucose, galactose and fructose. In Carbohydrate metabolism , it is broken down through glycolyssis , then Kerb's cycle and at last the energy generation , 4 calories / per gram , through phosphorylation to generate ATPs.

The endproducts of protein digestion   is amino acids . Protein metabolism also produces the 4 calories per gram.

Fat metabolism : The end products of fat digestion are fatty acids and glycerol. Fatty acids are oxidized into Acetyl CoA in the liver , then enter into Citric acid cycle to produce ATP. 1 gram of fat produces 9 calories.  

Studies revealed that lipids or fat store more energy than carbohydrates or protien as the fatty acid contain Carbon atom which have greater number of electrons around it , when comparing with a carbon atom of glucose . During the movement of electrons from, an atom having low affinity for electrons (like Carbon), to one with high affinity for electrons ( like oxygen, there will energy release. So during fatty acid oxidation more energy ( 9 calories/ gram ) is released than when the same oxidation process happens to carbohydrates and protein.