With Thanksgiving just around the corner, we thought we would look at one of the darker traditions of the holiday: overating. Sure, the food is delicious and plentiful but we should know better. But are there other, scientific factors that can explain why we stuff ourselves at Thanksgiving?
The following is an excerpt from Neurogastronomy: How the Brain Creates Flavor and Why It Matters, by Gordon Shepherd and now available in paper. In the excerpt Shepherd begins by looking at fast food and then looks at some of the neurological reasons for why we overeat at Thanksgiving and other times of the year.
[F]ast food contains a variety of food types and flavors. This is called the supermarket, smorgasbord, or buffet effect. This idea actually originated with a blind French scientist named Jacque Le Magnen in Paris, who became a legend in research on feeding. In the 1950s he began detailed studies of laboratory rats fed different kinds of diets. He found that on daily lab chow they showed little weight gain, but if he offered them chow with different flavors they quickly began to gain weight. This effect was rediscovered in 1981 by Barbara Rolls and her colleagues at Oxford, who called it sensory-specific satiety, meaning that with one flavor the animal quickly becomes full and bored with eating more, whereas a new flavor stimulates renewed eating. This is the effect we all experience at Thanksgiving or buffets or banquets when we feel the urge to go on eating every new dish or course. It is an expression of the fact that the brain is always interested in something new or changing, a characteristic we have seen in all the sensory systems. Although the fast- food industry probably did not know of Le Magnen’s research, it designed its foods as if it did.
Another reason people overeat may lie in long-term overstimulation of the skin and membranes of the lips and mouth. These of course are activated by food when we are eating, but, surprisingly, they can also be overactive even when we are not eating. This has been shown by brain scans of activity in the somatosensory area of the cerebral cortex comparing subjects who are lean and subjects who are obese. As figure 21.1 shows (see below), from the study in 2002 by Gene-Jack Wang and his colleagues at the Brookhaven National Laboratory in New York, obese individuals even in the resting state show higher levels of brain activity in the lip, tongue, and mouth regions of the somatosensory area. The authors speculate that this could reflect hypersensitivity of the receptors there to the rewarding value of food, and could be among the factors associated with overeating.
There are several theories for why we overeat, which have been summarized by Dana Small and her colleagues in a review in 2009. One theory is based on the observation that even though a rat may have fed to satiety, it can be induced by conditioned cues to keep eating. Small and her colleagues mention an experiment that demonstrates this. In this experiment, rats learn to associate the pre sen ta tion of food with the sound of a buzzer, much like Ivan Pavlov’s dogs. If the buzzer sounds when they are sated, they will begin to eat again. Instead of buzzers, humans have many other cues that keep them eating flavorful foods. In our example, a burger cues a bag of potato chips, then the ketchup, then the soft drink, then the . . . This kind of feeding has been shown to be dependent on connections between the amygdala, a node in the emotional network, and the hypothalamus, which is involved in activating feeding. These connections become hypersensitive because of long- standing habits.
Another idea is that overeating is due to in effective inhibitory circuits in the prefrontal brain regions, combined with heightened excitability of the circuits mediating reward from the foods consumed. An analogy is the involvement of these circuits in drug cravings. This is more evidence that a tendency to overeat involves circuits at the highest cognitive, as well as emotional, levels.
Another factor is the possibility that overeating occurs because eating itself does not have adequate reward value; the brain does not register enough “pleasure” with lesser amounts of food. In his book The End of Overeating: Taking Control of the Insatiable American Appetite, David Kessler, my former dean at the Yale School of Medicine, has emphasized the combination of salt, sugar, and fat as the main flavor villains to be resisted and controlled. Neurogastronomy supports this conclusion, identifying retronasal smell and its associated multisensory brain mechanisms of flavor as underappreciated major factors.
If flavor plays this central role in what we eat, the brain must contain mechanisms for making decisions about whether a food that produces an attractive internal flavor image in the brain is also nutritious. This is a final critical part of the human brain flavor system for determining normal function in healthy people and abnormal function in people who overeat.