Addictive behaviors present difficult, since it is hard for an observer to realize why clearly harmful behaviors are continued. However, we frequently see in practice alcoholics who drink despite liver damage or social consequences, smokers who smoke despite lung or heart damage, and morbidly obese patients who continue to overeat despite the resulting health risks and compromised function. In recent years, we have learned a good deal about the reinforcing ramifications of addictive substances on the brain. Neurochemical pathways involving such potent neurotransmitters as dopamine, serotonin, gamma-aminobutyric acid, and glutamate are influenced by exposure to the majority of substances with the potential for addiction or abuse. Repeated contact with these addicting substances contributes to stimulation and modulation of reward pathways that, consequently, make such substances more attractive and harder to resist. There is some evidence from neuroimaging that, even after years of abstinence, these pathways may never return with their preaddiction state, raising the chance of relapse with reexposure. Overeating clearly shares factors with other addictive behaviors. The overeater persists even yet in the face of negative physical consequences and societal disapproval, has a high frequency of relapse, and clearly finds rewards in the behavior which are not readily apparent to the observer and that override the most obvious negative consequences. Recent studies in human and animal subjects declare that a few of the same neurotransmitter systems active in addiction might be involved in the persistence of harmful eating behaviors. Nora Volkow, one of the nation’s premier researchers on addictive behaviors, has proposed a standard model for both obesity and drug abuse and addiction, with dopamine pathways in the mind playing an integral role. In every addictive behaviors, she hypothesizes that the addicted individual assigns an enhanced salience or meaning to a particular reinforcer (drugs or food) at the cost of other reinforcers.
This is a result of conditioned learning and resetting of the brain’s reward thresholds by repeated stimulation either by substances of abuse or by large quantities of palatable food, usually nutrientdense sugars or fats. In this model, experience of the foodstuff or drug reinforcer or conditioned cues for consumption triggers the dopamine-modulated neuronal reward and motivation circuits, while inhibiting cognitive control.5 Thus, the drive to eat is stimulated, and the ability to inhibit the drive is diminished, a difficult combination to overcome despite the best of intentions. In a study considering gender differences in brain a reaction to food stimulation, Volkow and her colleagues6 found male subjects had a better ability to inhibit brain activation in a reaction to food stimulation in multiple regions, such as the amygdala, hippocampus, insula, orbitofrontal cortex, and striatum. Volkow et al hypothesized that difference in ability to dampen a reaction to food stimulation can lead to a lowered ability to suppress hunger in women and the resulting gender difference in obesity. Alternation of access and restriction may improve the addictive eating pattern, ultimately causing the familiar cycle of successful dieting followed by way of a rebound to the pre-diet weight or even higher.7 Imaging studies of obese and lean individuals who are anticipating calorie-rich tasty food versus a tasteless solution demonstrate that obese subjects respond with greater activity in the gustatory and somatosensory regions of the brain and decreased activation in the striatum. This decreased activity may reflect a genetically mediated reduction in dopamine signaling in that region, leading these individuals to overeat to pay for the hypofunctional dorsal striatum.8,9 The continued growth of obesity in our society regardless of the proliferation of “lite” and “low-fat” foods emphasizes that there is no shortcut to healthy eating, and a recent commentary in the Journal of the American Medical Association highlights that even efforts to curb calorie intake through the utilization of artificial sweeteners may not trump neurobiology. Mcdougal cites an observational study that found a dose–response relationship between consumption of diet drinks and measurement of adiposity over time, along with studies finding daily use of such drinks connected with metabolic syndrome and danger of diabetes, even though the direction of the causality was not clear. Even more concerning, studies with experimental animals found the drive for sweetness led them to choose saccharine over intravenous cocaine, previously thought to be one of the most addictive substances. The danger for the human would-be dieter is that the usage of these hyper-sweet sugar substitutes may overstimulate sweetness receptors, causing them to revert to an infantile state and creating a healthy diet of energy-dilute foods less palatable.10 David Kessler,11 the former chief of the Food and Drug Administration who was simply widely praised for addressing tobacco addiction during his term in office, has joined the struggle against obesity. In a recently available book, he calls for retraining our brains, suggesting that people must make the same change in societal attitudes toward the acceptability of eating calorie-rich foods that we did toward the acceptability of smoking. He challenges our society to trade the transient neurologic reward of high-sugar/high-fat foods for healthier eating, while acknowledging their own lifelong battle to resist those very foods. However intellectually appealing his arguments might be, experience, including his own, suggests that these behavioral changes are difficult to keep up long-term, and the information from neuroimaging studies confirm a formerly obese individual’s neurobiology might be creating a silent but very powerful opposition to such change. For most would-be dieters, an ice cream soda will continue to look more desirable than brussels sprouts, and their own neurotransmitters will continue to urge them in the direction of the fats and sugars that will bring satiation and neurologic reward. A recently available article in the New Yorker reviewed a few of the societal factors that also militate against dietary behavior change, including a decline in the cost of fats, oils, and sugars in accordance with other foods, and the successful commercial promotion of larger servings extending even into your home, as newer editions of old standby cookbooks now calculate fewer servings from standard recipes.