Oxygen efficiency and the control of energy flow in animals and humans.

A new theory of feed intake regulation in ruminants has been developed as an alternative to the traditional view that feed consumption is limited by the physical and physiological constraints of the animal. Historical evidence that supports the traditional view has been examined. The new theory is based on the assumption that feed consumption has both positive and negative outcomes (benefits and costs). In a non-reproducing animal, benefits include yield of net energy for maintenance and gain. Costs are represented by the total oxygen consumption of the animal. The ratio between benefits and costs is calculated as the oxygen efficiency of feeding behavior, i.e., yield of net energy per liter of oxygen consumed. Voluntary energy intake corresponds to the feed consumption level at which oxygen efficiency is maximum. Literature examples were used to illustrate the predictive power of this theory. Differences in intake can be related to the efficiency of energy utilization, i.e., the animals' cost of processing feed. Current knowledge of the nature and causes of variation in processing costs is summarized. Secretory and absorptive processes associated with fiber consumption may explain why fiber can alter feed intake. Maximizing oxygen efficiency seems to be a unifying principle controlling other types of behavior such as locomotory behavior in humans. Possible physiological mechanisms controlling self-selected rates of locomotion are discussed in relation to the control of energy flow in ruminants.