The Evolution of Obesity

9 Getting Ready to Eat

206 CHAPTER NINE Getting Ready to Eat Eating requires a complex set of coordinated physical, physiological , and behavioral actions. Of course when we eat we don’t really have to think about it. We can do it easily and naturally. But when we consider it as scientists, we can see that eating is a very complex process. It is also episodic. We don’t eat all the time. We don’t even think about eating all the time. However, if you hadn’t eaten for a while and you smelled an enticing aroma wafting from the kitchen, your body would react. Your physiology would begin to gear itself up to receive food. Your mind would turn to eating. And perhaps you would get out of your chair and go into the kitchen to find out when dinner was ready. Already your body would have changed its metabolic state. It would be ready for dinner, regardless of what the cook says. This anticipatory response to the smell, sight, and taste of food was first discovered by Pavlov in the late 19th century. Pavlov Revisited In 1904 Ivan Petrovich Pavlov received the Nobel Prize in Physiology for his work on the digestive system. A key facet of his work was the demonstration that the gut and the central nervous system (CNS) work together in the process of digestion. Digestive secretions, saliva, gastric acid, and so forth could be stimulated by the brain. Thus even before the peptide revolution and the identification of the many gut-brain peptides, the concept of a link between central and peripheral feeding physiology was proposed. GET TING READY TO EAT 207 The concept of factory is important in understanding Pavlov’s approach toward and contribution to science: many investigators sharing resources and technologies in the pursuit of knowledge about a common theme, performing related but not identical investigations. Pavlov established a factory for the study of physiology (Figure 9.1). He also conceived of the digestive system as a complex chemical factory, with many parts working in concert to provide the nutrients necessary for life (Todes, 2002). The orderliness of his mind was reflected in both his methods of investigation and his conception of physiology. (For more details on Pavlov ’s life and accomplishments see Pavlov’s Physiology Factory by Daniel P. Todes [2002]). At the time of Pavlov’s early investigations there was controversy over whether the nervous system contributed to digestive secretions. The consensus among physiologists at that time was that the central nervous system was not involved (Todes, 2002). Pavlov disagreed. In a series of experiments at his factory, each building upon and expanding the results of the ones before, Pavlov was able to demonstrate the connection between FIGURE 9.1. The vivarium at Pavlov’s laboratory in Saint Petersburg, Russia. 208 THE EVOLUTION OF OBESITY the brain and the gut by investigating the phenomenon now known as the cephalic-phase response in digestive physiology (Smith, 1995). In this chapter we examine the role of cephalic-phase responses in the regulation of food intake. We examine cephalic-phase responses from an adaptive perspective. We focus on the functions of various cephalic-phase responses, their adaptive value, and possible selective pressures that have influenced their evolution. Cephalic-Phase Responses We eat food; we need nutrients. Nutrients are essential, but many are also toxic in high concentrations, and food contains many things, good and bad, besides nutrients. The function of our digestive physiology is to render food into nutrients safely and efficiently. The term cephalic-phase response refers to anticipatory physiological regulation related to food and feeding. It refers to digestive and metabolic responses to food cues generated by the central nervous system that act to 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 turns food into nutrients. One result is an increase in the amount of food that can be processed at a given time—an advantage in our evolutionary history , but possibly one source of our species’ susceptibility to obesity in the modern milieu. For certain foods (e.g., those high in simple sugars) maybe we are too efficient. 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...