A method for estimating the available energy of incompletely digested carbohydrates in rats

We developed a method to estimate the available energy from carbohydrates that are unavailable or partially unavailable through direct digestion and absorption. Radiolabeled bacterial and plant cellulose and polydextrose were administered orally to germfree and conventional rats. Label in breath, feces, intestinal contents and carcass were determined with excellent total recovery of the administered label. Comparison of these measurements in germfree and conventional rats was used to calculate the energy directly available by digestion and that available only after fermentation. Although the method overestimates available energy because of the more efficient digestive metabolism of the germfree rat, it provides a reliable, maximum energy value based upon fewer assumptions than previous methods. In conventional rats, 65% of the administered cellulose and 54% of the administered polydextrose were recovered in feces. In conventional rats, appreciable amounts of cellulose were fermented and an available energy value of 3.5 kJ/g was calculated. Calculation of the total availability of polydextrose, taking into account the direct absorption of small amounts of monomers present and the efficiency of fermentation, indicates an available energy value of 4.7 kJ/g.