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Hunger and the Brain
303 Eating FIGURE 8.2 – Anabolic The Hypothalamus and Hunger Regions of the hypothalamus generate signals that either increase hunger and reduce energy expenditure, called anabolic effects, or reduce hunger and increase energy expenditure, called catabolic effects. Brain Catabolic + – Food intake – + + Energy expenditure Fatness signals • Metabolic rate • Physical activity Insulin/leptin + – + Energy balance Fat stores + Source: Adapted from Schwartz et al. (2000). far less effective in people who are obese because they eat a high-fat diet (Gura, 1999; Heymsfield et al., 1999). In these far more common cases of obesity, the brain appears to have become less sensitive to leptin’s satiety signals (Ahima & Flier, 2000; Lin et al., 2000; Lustig et al., 2004). LINKAGES How does the brain know when we are hungry? (a link to Biology and Behavior) Hunger and the Brain Many parts of the brain contribute to the control of eating. However, research has focused on regions of the hypothalamus that may play primary roles in detecting and reacting to the blood’s signals about the need to eat. As shown in Figure 8.2, the hypothalamus influences both how much food is taken in and how quickly its energy is used, or metabolized. Some regions of the hypothalamus detect leptin and insulin; these regions generate signals that either increase hunger and reduce energy expenditure or reduce hunger and increase energy expenditure. There are at least twenty neurotransmitters that convey these signals to networks in various parts of the hypothalamus and in the rest of the brain (Cota et al., 2006; Woods et al., 1998, 2000). Activity in a part of the network that passes through the ventromedial nucleus in the hypothalamus tells an animal that there is no need to eat. So if a rat’s ventromedial nucleus is stimulated, the animal will stop eating (Kent et al., 1994). However, if the ventromedial nucleus is destroyed, the animal will eat much more than usual and maintain a much higher body weight. In contrast, the lateral hypothalamus contains networks that tell an animal to start eating. So when the lateral hypothalamus is stimulated, rats begin to eat huge quantities, even if they have just had a large meal (Stanley et al., 1993). When the lateral hypothalamus is destroyed, however, rats stop eating almost entirely. Decades ago, these findings led to the suggestion that these two hypothalamic regions interact to maintain some homeostatic level, or set point, based on food intake, body weight, or other eating-related signals (Powley & Keesey, 1970). According to this