Control-theory models of body-weight regulation and body-weight-regulatory appetite

Human body weight (BW), or some variable related to it, is physiologically regulated. That is, negative feedback from changes in BW elicits compensatory influences on appetite, which may be called BW-regulatory appetite, and a component of energy expenditure (EE) called adaptive thermogenesis (AdEE). BW-regulatory appetite is of general significance because it appears to be related to a variety of aspects of human appetite beyond just energy intake. BW regulation, BW-regulatory appetite and AdEE are frequently discussed using concepts derived from control theory, which is the mathematical description of dynamic systems involving negative feedback. The aim of this review is to critically assess these discussions. Two general types of negative-feedback control have been invoked to describe BW regulation, set-point control and simple negative-feedback control, often called settling-point control in the BW literature. The distinguishing feature of set-point systems is the existence of an externally controlled target level of regulation, the set point. The performance of almost any negative-feedback regulatory system, however, can be modeled on the basis of feedback gain without including a set point. In both set-point and simple negative-feedback models of BW regulation, the precision of regulation is usually determined mainly by feedback gain, which refers to the transformations of feedback into compensatory changes in BW-regulatory appetite and AdEE. Stable BW most probably represents equilibria shaped by feedback gain and tonic open-loop challenges, especially obesogenic environments. Data indicate that simple negative-feedback control accurately models human BW regulation and that the set-point concept is superfluous unless its neuroendocrine representation is found in the brain. Additional research aimed at testing control-theory models in humans and non-human animals is warranted.