It is estimated that feed comprises up to 75% of livestock production costs, and energy is the greatest single cost of diets in swine (Noblet et al., 1993). Energy yielding substances make up 70 to 90% of dietary DM, and carbons from these compounds are oxidized by the animal to produce energy needed to perform biochemical work, in the form of biosynthesis of protein and lipids, active ion transport and regulation of membrane potential, and mechanical work (muscle movement). Understanding energy supply and metabolic transformations of dietary energy in the whole animal is, therefore, of critical importance to adequately supply energy to the animal for production. It is also critical to our understanding of feed efficiency in swine.