Hockenbury Discovering Psychology 5th txtbk

Biological Motivation331FOCUS ON NEUROSCIENCEDopamine Receptors and ObesityEating to Stimulate Brain Reward? In Chapter 2, we notedthat dopamine brain pathways are involved in the reinforcingfeelings of pleasure and satisfaction. In Chapter 4, we alsonoted that many addictive drugs produce their pleasurable effectsby increasing brain dopamine levels. These pleasurable effectsare most reinforcing in people who have a low level ofdopamine brain receptors (Volkow & others, 1999b, 2007).Given that eating can be highly reinforcing and produces pleasurablesensations, could the same mechanisms also play a rolein obesity?In a landmark study, researchers injected obese andnormal-weight individuals with a slightly radioactive chemical“tag” that binds to brain dopamine receptors. Positron emissiontomography (PET) scans detected where the chemical tagbound with dopamine receptors, shown in red. The two PETscans reveal significantly fewer dopamine receptors for obeseindividuals (left) as compared to the normal-weight control subjects(right). And, among the obese people in the study, thenumber of dopamine receptors decreased as BMI increased(Wang & others, 2001, 2004).Researchers don’t know yet whether the reduced number ofdopamine receptors is a cause or a consequence of obesity. Oneclue may come from a new study that compared geneticallyobese, leptin-deficient rats with normal, lean rats (Thanos &others, 2008). The genetically obese rats had significantly lowerlevels of dopamine receptors than normal, lean rats. Takentogether, such findings suggest that compulsive or binge eatingmight compensate for reduced dopamine function by stimulatingthe brain’s reward system (Volkow & Wise, 2005).ObeseNormalThe key phrase here is susceptibility to obesity. Even thoughsomeone may be genetically predisposed to obesity, environmentalfactors still play a role. If an individual is genetically susceptible andlives in a high-risk environment, obesity is more likely to occur.And what constitutes a high-risk environment for obesity? An environmentcharacterized by ample and easily obtainable high-fat,high-calorie, palatable foods. A real-world example of this interactionis shown in Figure 8.7. As countries develop stable economiesand food supplies, the prevalence of obesity rapidly escalates.Leptin Resistance Having greater fat stores, most obese peoplehave high blood levels of leptin. So why don’t these high blood levels ofthe leptin hormone reduce eating behavior and induce weight loss?Many obese people experience leptin resistance, in which the normal mechanismsthrough which leptin regulates body weight and energy balance are disrupted (Enriori& others, 2006; Morrison, 2008). Although leptin levels are high in the blood,they are often low in the obese person’s cerebrospinal fluid. This suggests that leptinis not sufficiently transported from the blood to the brain. One possibility is that theobese person’s high blood levels of leptin are overwhelming the transport system tothe brain (Schwartz & others, 2000).Dieting: BMR Resistance to Maintaining Weight Loss Any diet that reducescaloric intake will result in weight loss. The difficult challenge is to maintain theweight loss. Many overweight or obese dieters experience weight cycling, or yo-yodieting—the weight lost through dieting is regained in weeks or months and maintaineduntil the next attempt at dieting.One reason this occurs is because the human body is much more effective at vigorouslydefending against weight loss than it is at protecting against weight gain(Keel & others, 2007). As caloric intake is reduced and fat cells begin to shrink,the body actively defends against weight loss by decreasing metabolism rate andenergy level. With energy expenditure reduced, far fewer calories are needed tomaintain the excess weight. In effect, the body is using energy much more efficiently.If dieters continue to restrict caloric intake, weight loss will plateau in amatter of weeks. When they go off the diet, their now more energy-efficient bodiesquickly utilize the additional calories, and they regain the weight they lost.Percent of population25201510508.91.8Least developedcountries (45)BMI < 17 (underfed)BMI ≥ 30 (obesity)6.94.8Developingcountries (75)2.417.1Economies intransition (27)Level of economic development1.620.4Developed marketeconomies (25)Figure 8.7 Genetic Susceptibility andEnvironmental Conditions If differentpopulations have similar percentages ofpeople who are genetically susceptible toobesity, then the critical factor becomesenvironmental conditions. In a low-riskenvironment, the availability of food,especially high-calorie foods, is limited. Incountries with poorly developed econo -mies and food supplies, the prevalence ofunderfed people (BMI 17) is higher thanthe prevalence of obesity (BMI 30). Butin countries with established or welldevelopedeconomies, such as most Westerncountries, food is plentiful and easilyattainable, creating high-risk conditionsfor obesity. In highly developed marketeconomies, obesity rates have soared.Sources: Barsh & others (2000); World Health Organization(2001).