Why do triglycerides possesses more food calories per gram than carbohydrates? Let us continue our preview of the thermochemical interpretation of metabolism with this question and review for a moment the overall internal energy changes that nutrient molecules undergo through oxidative metabolism.

In total stoichiometric balance, oxidative metabolism involves breaking relatively weak carbon-hydrogen bonds and forming stronger carbon-oxygen and hydrogen-oxygen bonds.

Remember that bonds with greater electronegativity difference tend to be stronger because they allow a powerful nucleus to draw bonding electrons inwards. Another way to say that a bond is strong is to say that its formation leads to energy decrease (energy that ultimately translates to food calories).

The answer to the question is as simple as this: Because triglycerides have primarily carbon-hydrogen bonds, these molecules have a greater reservoir of electrons which are not already bound to oxygen which can ultimately participate in bonds with oxygen. From a thermochemical perspective, we are breaking more weak bonds with triglycerides and forming strong bonds. From an oxidation-reduction perspective, more oxygen is being reduced per gram of triglyceride.
Heats of combustion of nutrient molecules.




The heats of combustion of various types of nutrient molecules.