The tau of continuous feeding on simple foods

Chemical reactor theory under the premise of maximization of net rate of nu trient absorption has been used to predict throughput time, tau, of digesta in animals. Animals that feed on hexoses, such as many vertebrate fruit an d nectar eaters, are of central interest in testing reactor theory because they use no hydrolysis before absorption and, hence, should provide valuabl e, simplified test cases. Graphical methods based on batch reactors and use d to make such predictions in the past can give optimal gut throughput time s (tau(opt)) identical with predictions from continuous plug-flow reactor m odels derived here: in animals with passive, linear uptake alone, tau(opt) should decline as hexose concentration of food increases. If saturating act ive uptake is involved, however, a minimum in tau(opt) (maximum in ingestio n rate) is predicted at intermediate hexose concentration, the exact locati on of this minimum depending on costs of ingestion as well as on uptake kin etics. That is, tau(opt) first falls to a minimum with increasing hexose co ncentration and then increases. Optimal throughput time rises as uptake sit es become saturated because there is little gross gain and no net gain from increased ingestion rate when uptake already is nearly saturated. It also rises with increasing costs of ingestion. The continuous-time analytic solu tions provided here further make the novel and very general prediction of h igh sensitivity to decreasing tau below tau(opt).