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324 CHAPTER 8 Motivation and EmotionGain(+)FatCarb.ProteinEnergyintakeWeight=Loss(–)Physical activityand exerciseBMREnergyexpenditureFigure 8.1 Energy Balance The tendencyof our bodies to maintain a stablebody weight is well documented (Schwartz& others, 2000). Maintaining a stable bodyweight occurs when you experience energybalance—that is, when the caloriesyou take in almost exactly match the caloriesyou expend for physical activity andmetabolism.Source: Adapted from Ravussin & Danforth (1999).DigestionPositive Versus Negative Energy BalanceFor most of us, there is considerable daily variation in what, when, how often, andhow much we eat (de Castro & others, 2000; Marcelino & others, 2001). Yetdespite this day-to-day variability in eating behavior, our body weight, including ourstores of body fat, tends to stay relatively constant over the course of weeks, months,and even years (Keesey & Hirvonen, 1997). Your typical or average body weight iscalled your baseline body weight.A regulatory process called energy homeostasis helps you maintain your baselinebody weight. Over time, most people experience energy balance (see Figure 8.1).This means that the number of calories you consume almost exactly matches thenumber of calories you expend for energy. The result is that your body weight,including body fat stores, tends to remain stable.However, energy balance can become disrupted if you eat more or less foodthan you need. If your caloric intake exceeds the amount of calories expendedfor energy, you experience positive energy balance. When there is more glucosethan your body needs for its energy requirements, the excess glucose is convertedinto reserve energy—fat. If positive energy balance persists over time, thesize and number of the body fat cells that make up the adipose tissue increase.Conversely, if you diet or fast, negative energy balance occurs: Caloric intake fallsshort of the calories expended for energy. If this imbalance continues, body fatstores shrink as the reserve energy in fat cells is used for physical activity andmetabolic functions.These findings imply that energy homeostasis and a stable body weight areactively regulated by internal signals and mechanisms that influence eating behavior.But what exactly are these signals or mechanisms?Figure 8.2 Blood Glucose and the Motivationto Eat In this graph, the red dotsdepict the effects of insulin triggering asmall decline in blood glucose over thecourse of about 30 minutes. As blood glucosedecreases, the person’s subjective desireto eat increases sharply, depicted bythe blue line. Notice that although thisperson did not eat, his blood glucose levelreturned to normal within the hour andhis desire to eat diminished. In daily life, asmall decline in blood glucose level is oneof the factors that reliably predict our motivationto eat (Campfield & others, 1996).Blood glucose (mg/dl)85807570656055Desireto eatBlood glucose15 30 45 60Minutes45403530252015105Short-Term Signals That Regulate EatingConsuming food is so routine in our lives that most of us don’t really think aboutwhat motivates us to stop what we’re doing and begin eating. Psychologists andother researchers, however, are very motivated to answer that question. In the pastfew years, they’ve made important new discoveries about the physiological and psychologicalfactors involved in the motivation to eat (Strubbe & Woods, 2004).Physiological Changes That Predict EatingThe idea that some internal, biochemical factor triggers our desire to eat makesintuitive sense. But what? Many people believe that eating is triggered by a drasticdrop in blood glucose levels, which are rapidly restored by food consumption.This popular belief is not accurate. Actually, your blood levels of glucose and fatsfluctuate very little over the course of a typical day. However, about 30 minutesbefore you eat, you experience a slight increase in blood levels of insulin and aslight decrease in blood levels of glucose. In experimental studies with bothhumans and rats, these small changes reliably predict the initiation of eating(Melanson & others, 1999).Once the meal is begun, blood glucose levels return to their baselinelevel. Interestingly, glucose returns to its baseline level well before the foodis actually digested and absorbed. And, in fact, glucose will return to itsbaseline level even if you do not eat (see Figure 8.2).A more important internal signal is a new hormone discovered by Japaneseresearcher Masayasu Kojima and his colleagues in 1999. Ghrelin (pronouncedGRELL-in) is primarily manufactured by cells lining the stomach.It stimulates the secretion of growth hormone by the pituitary gland in thebrain (Kojima & others, 1999; Olszweski & others, 2008).Ghrelin was quickly dubbed “the hunger hormone” when researchshowed that it strongly stimulates appetite. When rats were deprived offood, ghrelin levels increased sharply (see Inui, 2001). More directly, rats"Desire to eat" rating