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AAPI’S NUTRITION GUIDE TO OPTIMAL HEALTH: USING PRINCIPLES OF FUNCTIONAL MEDICINE AND NUTRITIONAL GENOMICS 7 Highly Palatable Foods : The Brain Reward Pathways and Connections to Overeating Micheline Vargas, DrPH, CNS, RCEP, CSCS For decades we have been taught that the key to losing weight is a matter of energy balance; eat less and exercise more. Yet more and more people continue to gain weight. In fact, in the United States two thirds of adults are overweight and more than 72 million of them are obese. i,ii The global picture of overweight and obesity is sobering, and the number of overweight people now surpasses the number of undernourished by several hundred million. iii In 2005, over 20 million children under five years of age were overweight, and the World Health Organization has predicted that by 2015, approximately 2.3 billion adults will be overweight and over 700 million will be obese. iv Obesity rates have climbed in unison with the availability of inexpensive highly palatable foods, i.e., processed foods rich in fat, salt, and refined sugars. v Eating behaviors characterized by food 58 cravings and binging have increased concomitantly with the increased exposure to these highly palatable foods. Research in both animal models and human imaging studies shows that high calorie, highly palatable foods are directly associated with addiction and loss of control. vi,vii Emerging science has revealed that excessive food consumption involves the brain’s pleasure centers. Neuroimaging studies have shown that food, like substances of abuse, leads to increased dopamine release in the pleasure center of the brain. Positron emission tomographic (PET) imaging studies have shown that obese individuals, like drug abusers, have lower levels of dopamine D2 receptors. vii Lower dopamine D2 receptors make the obese individual less sensitive to reward stimuli. This, in turn, may make the obese individual more 2012
AAPI’S NUTRITION GUIDE TO OPTIMAL HEALTH: USING PRINCIPLES OF FUNCTIONAL MEDICINE AND NUTRITIONAL GENOMICS vulnerable to excessive food intake as a way to satisfy a reduced reward signal. This new research is important because it helps explain the reason why obese individuals either fail to succeed with weight loss efforts or regain lost weight. Few diet products or programs address the reward pathways that drive eating behaviors. A truly effective approach to weight management should encourage strategies aimed at improving dopamine functioning in the brain’s reward circuitry. This article will review emerging science that describes a direct link between the activation of the reward/pleasure pathways in the brain and eating behaviors. The Brain is Reward Driven In order to better understand the drive to overeat and food addiction, we must first understand the basics of the brain’s ‚pleasure or reward center‛. The nucleus accumbens (a collection of neurons found in the striatum) was first identified as the brain’s pleasure center in the 1950s. viii A reward circuit (figure 1), which includes the ventral tegmental area (VTA), nucleus accumbens, septum, amygdala (involved in emotion), and prefrontal cortex (thinking part of the brain), was later identified. When the brain receives a sensory stimulus that is positive (i.e., highly palatable food), signals are sent to the VTA. The VTA, in turn, releases dopamine into the nucleus accumbens, septum, amygdala, and prefrontal cortex. Dopamine leads to feelings of exuberance, desire, and positive emotions. ix The pleasure associated with dopamine’s release reinforces eating. x,xi 59 Figure 1. Reward Pathway Source: http://www.bing.com/images/search?q=brain+image s%2c+reward+centers&FORM=IGRE1#focal=efec84 e00fe6465d25e21488f7d9c7f2&furl=http%3A%2F %2Flibrary.thinkquest.org%2F04oct%2F01639%2Fe n%2Fhealth%2Fpopup%2Fnucleus.jpg Obesity & Reduced Food Reward: The dopamine reward pathways influence the fundamental drive to eat. Dopamine is required to initiate each meal, and it is associated with the duration and quantity of a meal. xi,xii Weak activation of dopamine-based reward circuitry may increase overeating. xiii Recent research has found lower dopamine D2 receptors in obese subjects. vii A genetic variation may be the cause in some of these individuals. An association between the Taq 1 A allele and lower levels of dopamine D2 receptors has been found by some researchers. xiii It may be that individuals with reduced dopamine D2 receptors use food to increase dopamine stimulation to a more satisfying level. xiv Pleasure Pathways, Habits, & Addiction: The brain is plastic, meaning it can change. If an individual repeatedly engages in an eating behavior (i.e., binging), the nervous system will actually become wired for that behavior. It will strengthen 2012
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