The Tau of Continuous Feeding on Simple Foods

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Abstract
Chemical,reactor theory,under,the premise,of maximization of net rate of nutrient absorption,has been,used to predict throughput time, t, of digesta in animals. Animals that feed on hexoses, such as many vertebrate fruit and nectar eaters, are of central interest in testing reactor theory because they use no hydrolysis before absorption and, hence, should provide valuable, simplified test cases. Graphical methods based on batch reactors and used to make,such predictions in the past can,give ) identical with predictions from,continuous,plug-flow reactor models,derived here: in animals with passive, linear uptake alone, topt should decline as hexose concentration,of food increases. If saturating active uptake is involved, however, a minimum in topt (maxi- mum,in ingestion,rate) is predicted,at intermediate,hexose concentration, the exact location of this minimum depending on costs of ingestion as well as on uptake kinetics. That is, topt first falls to a minimum,with increasing hexose concentration and,then increases. Optimal throughput,time rises as uptake sites 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 solutions provided,here further make,the novel and very general prediction of high sensitivity to decreasing t below,topt.