Disposal, Transformation, and Storage of Nutrients

When CHO availability is increased by exogenous supply, endogenous and exogenous glucose have theoretically the same oxidation chances, i.e., in the presence of a total body glucose consisting of 50% infusion and 50% production, the net amount of the oxidized glucose (indirect calorimetry) will be constituted in equal parts by the two glucose pools. Only when hepatic glucose production is totally suppressed, the energy drawn by CHO totally comes from exogenous source.

Regarding fat oxidation, the nonprotein respiratory quotient (RQnp) does not indicate the source of the lipid that is oxidized (fat stores vs fat from diet). It is known that glucose availability reduces lipid oxidation. However, it is not sure whether the increased exogenous fat availability could result in a decrease of endo- or exogenous glucose oxidation. A further very interesting physiological aspect of fat disposal is whether the exogenous fat can supply an immediate energy need and, if any, the proportion between immediate oxidation and storage of an exogenous load.

The result is that the amount of oxidized exogenous substrates of either long- chain triglycerides (MCTs are a different story) or glucose (when hepatic neo- glucogenesis is not suppressed) is less than that indicated by RQnp. The meaning is that to accomplish with REE, a part of exogenous substrates, equal or less than the need, are stored, and necessarily the oxidation of endogenous substrates continues even in feed state.