Anticipatory physiological regulation in feeding biology: Cephalic phase responses

Abstract

Anticipatory physiological regulation is an adaptive strategy that enables animals to respond faster to physiologic and metabolic challenges. The cephalic phase responses are anticipatory responses that prepare animals to digest, absorb, and metabolize nutrients. They enable the sensory aspects of the food to interact with the metabolic state of the animal to influence feeding behavior. The anticipatory digestive secretions and metabolic adjustments in response to food cues are key adaptations that affect digestive and metabolic efficiency and aid in controlling the resulting elevation of metabolic fuels in the blood. Cephalic phase responses enable digestion, metabolism, and appetite to be regulated in a coordinated fashion. These responses have significant effects on meal size. For example, if the cephalic phase insulin response is blocked the result is poor glucose control and smaller meals. Cephalic phase responses also are linked to motivation to feed, and may play a more direct role in regulating meal size beyond the permissive one of ameliorating negative consequences of feeding. For example, the orexigenic peptide ghrelin appears to display a cephalic phase response, rising before expected meal times. This anticipatory ghrelin response increases appetite; interestingly it also enhances fat absorption, linking appetite with digestion and metabolism.

Introduction

Appetite, food preferences, and the regulation of food intake are key aspects of energy balance and weight homeostasis. There is an ever growing list of peptides, receptors, and other gene products associated with appetite and the regulation of food intake, and investigating their interrelated roles is an important process for understanding the proximate mechanisms by which people decide when and what to eat, and when to stop. In our opinion, a comparative, evolutionary perspective will also be important to understanding why the imbalance between food intake and energy expenditure appears to be so refractory.

The acquisition of food is a necessity for animals. Strong selective pressures have acted to produce the anatomy, physiology, and behavior that serve to enhance an animal's ability to ingest, digest, absorb, and ultimately metabolize the nutrients necessary for survival and reproduction. There are anticipatory physiological responses to feeding, the cephalic phase responses, that set in motion digestive and endocrine cascades that increase the efficiency of digestion and metabolism, but also directly and indirectly regulate meal size and duration. These cephalic phase responses can be conditioned, so experience, learning, and hence social and cultural factors, can and will play a role in their expression.

In this paper, we examine the role of cephalic phase responses in the regulation of food intake. The term cephalic phase response refers to anticipatory physiological regulation related to food and feeding; i.e. digestive and metabolic responses to food cues generated by the central nervous system that prepare the organism to ingest, digest, absorb, and metabolize food (Pavlov, 1902; Powley, 1977; Smith, 1995). These anticipatory physiological responses increase the efficiency with which an organism utilizes food. One result is an increase in the amount of food that can be processed at a given time; an advantage in our past evolutionary history, but possibly a source of our species’ susceptibility to obesity in the modern milieu. Recent evidence suggests that physiological responses that serve to end feeding also can have a cephalic phase. Thus, by the first bite of food, or even before, physiological processes have been set in motion that will influence the duration of the meal and the amount of food eaten (Smith, 2000).

We will examine cephalic phase responses from an adaptive perspective. We focus on the functions of various cephalic phase responses, their effects on food intake, and the possible selective pressures that have influenced their evolution.