Why Do We Eat When We're Not Hungry?
The neuroscience of overeating and how to prevent It
Posted Jan 25, 2016
Everyone knows that overeating is unhealthy. Weight gain, heart disease, abdominal pain--it's all common knowledge at this point. Yet, all too often we find ourselves eating when we aren’t hungry, a behavior that most would agree is just a "bad habit," but nobody means that literally or scientifically. However, research shows that eating when we’re full may be controlled by the same neurological system that controls all our habits, and this finding may be the key to understanding the cause, and the cure, for overeating.
In a study, thirty-two healthy volunteers were asked to sit in front of a computer screen and press a button whenever an image appeared on the screen signaling them to do so. When they pressed the button, a machine next to them released either a Fritos corn chip or an M&M. They ate whichever snack the machine ejected. Half of the subjects did this task for only two sessions of eight minutes each, while the other half did twelve eight-minute sessions. The second group had six times as much practice with the task as the first group and was more likely to eventually start pressing the button out of habit. With this in mind, we’ll call this group the habit group and the first group the non-habit group.
To determine how the development of habits affects our eating behavior, the researchers were interested in the activity of a certain brain region known as the ventromedial prefrontal cortex, located in the mid-lower section of the frontal lobe. A major function of this region is to anticipate the value of an expected event. This is important in the brain’s reward pathway, which manages positive and negative reinforcement of behavior. For example, when we are sitting hungrily at a restaurant and the waiter approaches the table with plates of food, neuronal fireworks light up the brain in anticipation of the meal. The ventromedial prefrontal cortex is firing away because it detects high reward. Once we are full, however, the response is vastly diminished. If the waiter were to bring another plate of food, the ventromedial prefrontal cortex would barely respond at all. The low response devalues the experience of eating, discouraging us from continuing to chow down. In short, the ventromedial prefrontal cortex participates in a feedback loop: It positively reinforces eating when we are hungry, but that very act of eating eventually causes the ventromedial prefrontal cortex to discourage us from eating and to recognize that we are full.
Using fMRI (a technique that monitors brain activity in real time), the researchers compared the responses of the ventromedial prefrontal cortex in the habit group and the non-habit group. In the non-habit group, the ventromedial prefrontal cortex was activated before each press of the button in anticipation of the snack, encouraging the subjects to eat. But that was while they were hungry. Next, the subjects ate a big meal. Now full, they pressed the button on the machine and the fMRI showed that the activation of the ventromedial prefrontal cortex was diminished. The participants were not hungry, so the projected reward of eating an M&M or a corn chip was minimal. The ventromedial prefrontal cortex downgraded the reward value of the snack to discourage further consumption.
The habit group was tested next, and things went a little differently. While the participants were hungry, their ventromedial prefrontal cortices again showed a big signal, indicating that they assigned a high reward to the food. But what would happen once they were full? This time, the fMRI results revealed that the ventromedial prefrontal cortex activity was just as strong as it was when the subjects still had their appetites. The anticipated reward value of the snack was not downgraded, even though they were full. The feedback loop was broken. Apparently, because the subjects were pressing the button and eating the snacks out of habit, their brains failed to dissuade them from eating. In fact, by maintaining the reward signal, the ventromedial prefrontal cortex was doing the opposite: positively reinforcing the behavior of eating without being hungry. The development of habit changed the act of eating from something dependent on the need for nourishment and transformed it into something automated.
This may explain why we often eat despite not being hungry. We let our habit system take over, and our eating becomes automatic. But how do we permit the habit system to seize command? Can we control it? Think of it this way: There are two systems for directing our behavior, the procedural habit system and the thoughtful conscious system. Consider how we drive a car. When driving a new route, we are completely consciously aware of our decisions on the road. But after driving a common route, such as to work, we might not even remember the trip. We can drive on autopilot, especially if our minds are busy thinking about things other than driving.
The conscious system can drive, and it can reflect on the events of the day, but it can’t do both at the same time. If the conscious system is preoccupied, the habit system is assigned the driving duties. By passively allowing thoughts to flood our minds (what we might call "spacing out"), we take our conscious system out of commission and the habit system takes over.
The habit system can similarly take over the process of eating. This often occurs when we are distracted by something, such as television. The reason doctors discourage people from eating in front of the TV is because it leads to overeating. When we watch TV, we allow the television to monopolize our conscious attention. Therefore, if we are doing something routine while watching, such as eating potato chips, the habit system will take control of that behavior. Just as a preoccupied driver may navigate on autopilot, the preoccupied diner may thoughtlessly consume five bags of chips while the mind is distracted by watching sports or an episode of The Bachelor.
When we allow our minds to be preoccupied, our ability to consciously control our behavior is suspended, and our behavior seems to follow a preprogrammed course. But we can choose to take control at any time, seize our brain circuitry from the habit system, and make healthier decisions.
For more about the hidden brain patterns that explain our behavior, check out my book: NeuroLogic: The Brain's Hidden Rationale Behind Our Irrational Behavior. Available now!