Theories of Hunger and Eating

Hunger, Eating, and Health Why Do Many People Eat Too Much?

12.1 Digestion, Energy Storage, and Energy Utilization

12.2 Theories of Hunger and Eating: Set Points versus Positive Incentives

12.3 Factors That Determine What, When, and How Much We Eat

12.4 Physiological Research on Hunger and Satiety

12.5 Body Weight Regulation: Set Points versus Settling Points

12.6 Human Obesity: Causes, Mechanisms, and Treatments

12.7 Anorexia and Bulimia Nervosa

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source of serious personal and health problems. Most eating-related health problems in industrialized

nations are associated with eating too much—the average American consumes 3,800 calories per day, about twice the average daily requirement (see Kopelman, 2000). For

example, it is estimated that 65% of the adult U.S. popu- lation is either overweight or clinically obese, qualifying

this problem for epidemic status (see Abelson & Kennedy, 2004; Arnold, 2009). The resulting financial and personal costs are huge. Each year in the United States, about $100 billion is spent treating obesity-related disorders (see Ol- shansky et al., 2005). Moreover, each year, an estimated 300,000 U.S. citizens die from disorders caused by their excessive eating (e.g., diabetes, hypertension, cardiovas- cular diseases, and some cancers). Although the United States is the trend-setter when it comes to overeating and obesity, many other countries are not far behind (Sofsian, 2007). Ironically, as overeating and obesity have reached epidemic proportions, there has been a related increase in disorders associated with eating too little (see Polivy & Herman, 2002). For example, almost 3% of American adolescents currently suffer from anorexia or bulimia, which can be life-threatening in extreme cases.

The massive increases in obesity and other eating- related disorders that have occurred over the last few decades in many countries stand in direct opposition to most people’s thinking about hunger and eating. Many people—and I assume that this includes you—believe that hunger and eating are normally triggered when the

body’s energy resources fall below a prescribed optimal level, or set point. They ap- preciate that many factors in-

fluence hunger and eating, but they assume that the hunger and eating system has evolved to supply the body with just the right amount of energy.

This chapter explores the incompatibility of the set- point assumption with the current epidemic of eating disorders. If we all have hunger and eating systems

whose primary function is to maintain energy resources at optimal levels, then eating disorders should be rare. The fact that they are so prevalent suggests that hunger and eating are regulated in some other way. This chapter will repeatedly challenge you to think in new ways about issues that impact your health and longevity and will provide new insights of great personal relevance—I guarantee it.

Before you move on to the body of the chapter, I would like you to pause to consider a case study. What would a severely amnesic patient do if offered a meal

shortly after finishing one? If his hunger and eating were controlled by energy set points, he would refuse the sec- ond meal. Did he?

The Case of the Man Who Forgot Not to Eat

R.H. was a 48-year-old male whose progress in graduate school was interrupted by the development of severe am- nesia for long-term explicit memory. His amnesia was similar in pattern and severity to that of H.M., whom you met in Chapter 11, and an MRI examination revealed bilateral damage to the medial temporal lobes.

The meals offered to R.H. were selected on the basis of interviews with him about the foods he liked: veal parmi- giana (about 750 calories) plus all the apple juice he wanted. On one occasion, he was offered a second meal about 15 minutes after he had eaten the first, and he ate it. When offered a third meal 15 minutes later, he ate that, too. When offered a fourth meal he rejected it, claiming that his “stomach was a little tight.”

Then, a few minutes later, R.H. announced that he was going out for a good walk and a meal. When asked what he was going to eat, his answer was “veal parmigiana.”

Clearly, R.H.’s hunger (i.e., motivation to eat) did not result from an energy deficit (Rozin et al., 1998). Other cases like that of R.H. have been reported by Higgs and colleagues (2008).

12.1 Digestion, Energy Storage, and Energy Utilization

The primary purpose of hunger is to increase the proba- bility of eating, and the primary purpose of eating is to supply the body with the molecular building blocks and energy it needs to survive and function (see Blackburn, 2001). This section provides the foundation for our con- sideration of hunger and eating by providing a brief overview of the processes by which food is digested, stored, and converted to energy.

Digestion The gastrointestinal tract and the process of digestion are illustrated in Figure 12.1 on page 300. Digestion is the gastrointestinal process of breaking down food and ab- sorbing its constituents into the body. In order to appre- ciate the basics of digestion, it is useful to consider the body without its protuberances, as a simple living tube

29912.1 ■ Digestion, Energy Storage, and Energy Utilization

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Eating is a behavior that is of interest to virtuallyeveryone. We all do it, and most of us derive greatpleasure from it. But for many of us, it becomes a

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(a simple sugar that is the breakdown product of complex carbohydrates, that is, starches and sugars).

The body uses energy continuously, but its consump- tion is intermittent; therefore, it must store energy for use in the intervals between meals. Energy is stored in three forms: fats, glycogen, and proteins. Most of the body’s energy reserves are stored as fats, relatively little as glycogen and proteins (see Figure 12.2). Thus, changes in the body weights of adult humans are largely a conse- quence of changes in the amount of their stored body fat.

Why is fat the body’s preferred way of storing energy? Glycogen, which is largely stored in the liver and muscles, might be expected to be the body’s preferred mode of energy storage because it is so readily converted to glucose—the body’s main directly utilizable source of energy. But there

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Chewing breaks up food and mixes it with saliva.1 Saliva lubricates food and begins its digestion.2 Swallowing moves food and drink down the esophagus to the stomach.3 The primary function of the stomach is to serve as a storage reservoir. The

hydrochloric acid in the stomach breaks food down into small particles, and pepsin begins the process of breaking down protein molecules to amino acids.

The stomach gradually empties its contents through the pyloric sphincter into the

duodenum, the upper portion of the intestine, where most of the absorption takes place.

Digestive enzymes in the duodenum, many of them from the gall bladder and pancreas,

break down protein molecules to amino acids, and starch and complex sugar molecules to simple sugars. Simple sugars and amino acids readily pass through the duodenum wall into the bloodstream and are carried to the liver.

Fats are emulsified (broken into droplets) by bile, which is manufactured in the liver and

stored in the gall bladder until it is released into the duodenum. Emulsified fat cannot pass through the duodenum wall and is carried by small ducts in the duodenum wall into the lymphatic system.

Most of the remaining water and electrolytes are absorbed from the waste in

the large intestine, and the remainder is ejected from the anus.

Steps in Digestion

Parotid gland

Salivary glands

Esophagus

Liver

Stomach

Gall bladder

Pyloric sphincter

Pancreas

Duodenum

Large intestine or colon

Small intestine

Anus

with a hole at each end. To supply itself with energy and other nutrients, the tube puts food into one of its two holes—the one with teeth—and passes the food along its internal canal so that the food can be broken down and partially absorbed from the canal into the body. The leftovers are jettisoned from the other end. Although this is not a particularly appetizing description of eating, it does serve to illustrate that, strictly speaking, food has not been consumed until it has been digested.

Energy Storage in the Body As a consequence of digestion, energy is delivered to the body in three forms: (1) lipids (fats), (2) amino acids (the breakdown products of proteins), and (3) glucose

FIGURE 12.1 The gastrointestinal tract and the process of digestion.

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are two reasons why fat, rather than glycogen, is the pri- mary mode of energy storage: One is that a gram of fat can store almost twice as much energy as a gram of glyco- gen; the other is that glycogen, unlike fat, attracts and holds substantial quantities of water. Consequently, if all your fat calories were stored as glycogen, you would likely weigh well over 275 kilograms (600 pounds).

Three Phases of Energy Metabolism There are three phases of energy metabolism (the chem- ical changes by which energy is made available for an

organism’s use): the cephalic phase, the absorptive phase, and the fasting phase. The cephalic phase is the preparatory phase; it often begins with the sight, smell, or even just the thought of food, and it ends when the food starts to be absorbed into the bloodstream. The absorptive phase is the period during which the energy absorbed into the bloodstream from the meal is meet- ing the body’s immediate energy needs. The fasting phase is the period during which all of the unstored en- ergy from the previous meal has been used and the body is withdrawing energy from its reserves to meet its immediate energy requirements; it ends with the begin- ning of the next cephalic phase. During periods of rapid weight gain, people often go directly from one absorp- tive phase into the next cephalic phase, without experi- encing an intervening fasting phase.

The flow of energy during the three phases of energy metabolism is controlled by two pancreatic hormones: insulin and glucagon. During the cephalic and absorptive phases, the pancreas releases a great deal of insulin into the bloodstream and very little glucagon. Insulin does three things: (1) It promotes the use of glucose as the pri- mary source of energy by the body. (2) It promotes the conversion of bloodborne fuels to forms that can be stored: glucose to glycogen and fat, and amino acids to proteins. (3) It promotes the storage of glycogen in liver and muscle, fat in adipose tissue, and proteins in muscle. In short, the function of insulin during the cephalic phase is to lower the levels of bloodborne fuels, primarily glucose, in anticipation of the impending influx; and its function during the absorptive phase is to minimize the increasing levels of bloodborne fuels by utilizing and storing them.

In contrast to the cephalic and absorptive phases, the fasting phase is characterized by high blood levels of glucagon and low levels of insulin. Without high levels of insulin, glucose has difficulty entering most body cells; thus, glucose stops being the body’s primary fuel. In effect, this saves the body’s glucose for the brain, because insulin is not required for glucose to enter most brain cells. The low levels of insulin also promote the conversion of glycogen and protein to glucose. (The conversion of protein to glucose is called gluconeogenesis.)

On the other hand, the high levels of fasting-phase glucagon promote the release of free fatty acids from adi- pose tissue and their use as the body’s primary fuel. The high glucagon levels also stimulate the conversion of free fatty acids to ketones, which are used by muscles as a source of energy during the fasting phase. After a pro- longed period without food, however, the brain also starts to use ketones, thus further conserving the body’s re- sources of glucose.

Figure 12.3 summarizes the major metabolic events as- sociated with the three phases of energy metabolism.

30112.1 ■ Digestion, Energy Storage, and Energy Utilization

Fat in adipose tissue (85%)

Protein in muscle (14.5%)

Glycogen in muscle and liver (0.5%)

FIGURE 12.2 Distribution of stored energy in an average person.

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12.2 Theories of Hunger and Eating: Set Points versus Positive Incentives

One of the main difficulties I have in teaching the funda- mentals of hunger, eating, and body weight regulation is the set-point assumption. Although it dominates most people’s thinking about hunger and eating (Assanand, Pinel, & Lehman, 1998a, 1998b), whether they realize it or not, it is inconsistent with the bulk of the evidence. What exactly is the set-point assumption?

Set-Point Assumption Most people attribute hunger (the motivation to eat) to the presence of an energy deficit, and they view eating as the means by which the energy resources of the body are returned to their optimal level—that is, to the energy set point. Figure 12.4 summarizes this set-point assumption. After a meal (a bout of eating), a person’s energy resources are assumed to be near their set point and to decline there- after as the body uses energy to fuel its physiological processes. When the level of the body’s energy resources falls far enough below the set point, a person becomes motivated by hunger to initiate another meal. The meal continues, ac- cording to the set-point assumption, until the energy level

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Cephalic Phase Preparatory phase, which is initiated by the sight, smell, or expectation of food

Absorptive Phase Nutrients from a meal meeting the body’s immediate energy requirements, with the excess being stored

Fasting Phase Energy being withdrawn from stores to meet the body’s immediate needs

Promotes • Utilization of blood glucose as a source

of energy • Conversion of excess glucose to

glycogen and fat • Conversion of amino acids to proteins • Storage of glycogen in liver and muscle,

fat in adipose tissue, and protein in muscle

Inhibits • Conversion of glycogen, fat, and protein

into directly utilizable fuels (glucose, free fatty acids, and ketones)

Promotes • Conversion of fats to free fatty acids

and the utilization of free fatty acids as a source of energy

• Conversion of glycogen to glucose, free fatty acids to ketones, and protein to glucose

Inhibits • Utilization of glucose by the body but

not by the brain • Conversion of glucose to glycogen and

fat, and amino acids to protein • Storage of fat in adipose tissue

Glucagon levels low

Insulin levels high

Glucagon levels high

Insulin levels low

FIGURE 12.3 The major events associated with the three phases of energy metabolism: the cephalic, absorptive, and fasting phases.

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returns to its set point and the person feels satiated (no longer hungry).

Set-point models assume that hunger and eating work in much the same way as a thermostat- regulated heating system in a cool climate. The heater increases the house temperature until it reaches its set point (the thermo- stat setting). The heater then shuts off, and the temperature of the house gradually de- clines until it becomes low enough to turn the heater back on. All set-point systems have three components: a set- point mechanism, a detector mechanism, and an effector mechanism. The set-point mechanism defines the set point, the detector mechanism detects deviations from the set point, and the effector mechanism acts to eliminate the deviations. For example, the set-point, detector, and ef- fector mechanisms of a heating system are the thermo- stat, the thermometer, and the heater, respectively.

All set-point systems are negative feedback systems— systems in which feedback from changes in one direction elicit compensatory effects in the opposite direction. Negative feedback systems are common in mammals be- cause they act to maintain homeostasis—a stable internal environment—which is critical for mammals’ survival (see Wenning, 1999). Set-point systems combine negative feedback with a set point to keep an internal environment fixed at the prescribed point. Set-point systems seemed necessary when the adult human brain was assumed to be immutable: Because the brain couldn’t change, energy re- sources had to be highly regulated. However, we now know that the adult human brain is plastic and capable of considerable adaptation. Thus, there is no longer a logical imperative for the set-point regulation of eating. Through- out this chapter, you will need to put aside your precon- ceptions and base your thinking about hunger and eating entirely on the empirical evidence.

Glucostatic and Lipostatic Set-Point Theories of Hunger and Eating In the 1940s and 1950s, researchers working under the as- sumption that eating is regulated by some type of set- point system speculated about the nature of the regulation. Several researchers suggested that eating is

regulated by a system that is designed to maintain a blood glucose set point—the idea being that we become hungry when our blood glucose levels drop significantly below their set point and that we become satiated when eating returns our blood glucose levels to their set point. The various versions of this theory are collectively referred to as the glucostatic theory. It seemed to make good sense that the main purpose of eating is to defend a blood glu- cose set point, because glucose is the brain’s primary fuel.

The lipostatic theory is another set-point theory that was proposed in various forms in the 1940s and 1950s. According to this theory, every person has a set point for body fat, and deviations from this set point produce com- pensatory adjustments in the level of eating that return levels of body fat to their set point. The most frequently cited support for the theory is the fact that the body weights of adults stay relatively constant.

The glucostatic and lipostatic theories were viewed as complementary, not mutually exclusive. The glucostatic theory was thought to account for meal initiation and ter- mination, whereas the lipostatic theory was thought to account for long-term regulation. Thus, the dominant view in the 1950s was that eating is regulated by the inter- action between two set-point systems: a short-term glu- costatic system and a long-term lipostatic system. The simplicity of these 1950s theories is appealing. Remark- ably, they are still being presented as the latest word in some textbooks; perhaps you have encountered them.

Problems with Set-Point Theories of Hunger and Eating Set-point theories of hunger and eating have several seri- ous weaknesses (see de Castro & Plunkett, 2002). You have already learned one fact that undermines these the- ories: There is an epidemic of obesity and overweight,

30312.2 ■ Theories of Hunger and Eating: Set Points versus Positive Incentives

Hours 1 2 3 4 5 6 7 8 9 10 11

H yp

o th

et ic

al E

n er

g y

R es

er ve

s Hunger

Meal

FIGURE 12.4 The energy set-point view that is the basis of many people’s thinking about hunger and eating.

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which should not occur if eating is regulated by a set point. Let’s look at three more major weaknesses of set-

point theories of hunger and eating.

● First, set-point theories of hunger and eating are in- consistent with basic eating-related evolutionary pressures as we understand them. The major eating- related problem faced by our ancestors was the incon-

sistency and unpredictability of the food supply. Thus, in order to survive, it was im- portant for them to eat large quantities of

good food when it was available so that calories could be banked in the form of body fat. Any ancestor— human or otherwise—that stopped feeling hungry as soon as immediate energy needs were met would not have survived the first hard winter or prolonged drought. For any warm-blooded species to survive under natural conditions, it needs a hunger and eating system that prevents energy deficits, rather than one that merely responds to them once they have devel- oped. From this perspective, it is difficult to imagine how a set-point hunger and feeding system could have evolved in mammals (see Pinel, Assanand, & Lehman, 2000).

● Second, major predictions of the set-point theories of hunger and eating have not been confirmed. Early studies seemed to support the set-point theories by showing that large reductions in body fat, produced by starvation, or large reductions in blood glucose, pro- duced by insulin injections, induce increases in eating in laboratory animals. The problem is that reductions in blood glucose of the magnitude needed to reliably induce eating rarely occur naturally. Indeed, as you have already learned in this chapter, about 65% of U.S. adults have a significant excess of fat deposits when they begin a meal. Conversely, efforts to reduce meal size by having subjects consume a high-calorie drink before eating have been largely unsuccessful; indeed, beliefs about the caloric content of a premeal drink often influence the size of a subsequent meal more than does its actual caloric content (see Lowe, 1993).

● Third, set-point theories of hunger and eating are de- ficient because they fail to recognize the major influ- ences on hunger and eating of such important factors as taste, learning, and social influences. To convince yourself of the importance of these factors, pause for a minute and imagine the sight, smell, and taste of your favorite food. Perhaps it is a succulent morsel of lobster meat covered with melted garlic butter, a piece of chocolate cheesecake, or a plate of sizzling home- made french fries. Are you starting to feel a bit hun- gry? If the homemade french fries—my personal weakness—were sitting in front of you right now, wouldn’t you reach out and have one, or maybe the whole plateful? Have you not on occasion felt discomfort

after a large main course, only to polish off a substan- tial dessert? The usual positive answers to these ques- tions lead unavoidably to the conclusion that hunger and eating are not rigidly controlled by deviations from energy set points.

Positive-Incentive Perspective The inability of set-point theories to account for the basic phenomena of eating and hunger led to the development of an alternative theoretical perspective (see Berridge, 2004). The central assertion of this perspective, com- monly referred to as positive-incentive theory, is that humans and other animals are not normally driven to eat by internal energy deficits but are drawn to eat by the an- ticipated pleasure of eating—the anticipated pleasure of a behavior is called its positive-incentive value (see Bolles, 1980; Booth, 1981; Collier, 1980; Rolls, 1981; Toates, 1981). There are several different positive-incentive theo- ries, and I refer generally to all of them as the positive- incentive perspective.

The major tenet of the positive-incentive perspective on eating is that eating is controlled in much the same way as sexual behavior: We engage in sexual behavior not because we have an internal deficit, but because we have evolved to crave it. The evolutionary pressures of unexpected food shortages have shaped us and all other warm-blooded an- imals, who need a continuous supply of energy to main- tain their body temperatures, to take advantage of good food when it is present and eat it. According to the positive- incentive perspective, it is the presence of good food, or the anticipation of it, that normally makes us hungry, not an energy deficit.

According to the positive-incentive perspective, the de- gree of hunger you feel at any particular time depends on the interaction of all the factors that influence the positive- incentive value of eating (see Palmiter, 2007). These in- clude the following: the flavor of the food you are likely to consume, what you have learned about the effects of this food either from eating it previously or from other peo- ple, the amount of time since you last ate, the type and quantity of food in your gut, whether or not other people are present and eating, whether or not your blood glucose levels are within the normal range. This partial list illus- trates one strength of the positive-incentive perspective. Unlike set-point theories, positive-incentive theories do not single out one factor as the major determinant of hunger and ignore the others. Instead, they acknowledge that many factors interact to determine a person’s hunger at any time, and they suggest that this interaction occurs through the influence of these various factors on the positive-incentive value of eating (see Cabanac, 1971).

In this section, you learned that most people think about hunger and eating in terms of energy set points and

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were introduced to an alternative way of thinking—the positive-incentive perspective. Which way is correct? If you are like most people, you have an attachment to familiar ways of thinking and a resistance to new ones. Try to put this tendency aside and base your views about this impor- tant issue entirely on the evidence.

You have already learned about some of the major weaknesses of strict set-point theories of hunger and eat- ing. The next section describes some of the things that biopsychological research has taught us about hunger and eating. As you progress through the section, notice the su- periority of the positive-incentive theories over set-point theories in accounting for the basic facts.

Theories of Hunger and Eating

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