Why Do We Eat? A Neurobiological Perspective. Part VII

Welcome back to the series, after a bit of a hiatus!  In previous posts, we covered the fact that humans eat because we're motivated to eat, and many things can motivate us to eat.  These include factors related to energy need (homeostatic factors), such as hunger, and factors that have little to do with energy need or hunger (non-homeostatic factors).  These many factors are all processed in specialized brain 'modules' that ultimately converge on a central action selection system (part of the reward system); this is the part of you that decides whether or not to initiate eating behaviors.

This will be somewhat of a catch-all post in which I discuss cognitive, emotional, and habit influences on food intake.  Since these factors are not my specialty, I'll keep it brief, but I don't mean to suggest they aren't important.

Food 'Cost'


One of the primary factors in deciding when and what to eat is determining how much you have to give up to obtain the food in question.  We can call this the 'cost' of food, but in this context a food's 'cost' refers to effort cost in addition to monetary cost.  If you like hamburgers, and there's a hamburger on your desk, you may eat it even when you aren't hungry and your general motivation to eat is relatively low.  If the same hamburger is across the street, you'll need to be at least a little bit hungry (or motivated by something else, for example boredom or a great love of hamburgers) to have enough motivation to get it.  If you had to walk three miles and climb a tree to get your hamburger, you'd have to be quite hungry!  But this was precisely the situation for our non-industrial ancestors-- food was not at arm's reach at all times of day.  The 'cost' of food was higher because it took effort to obtain and prepare, and therefore it required a higher level of motivation (hunger) to drive food seeking behavior.

The effort cost of food is a major determinant of food intake.  Even small increases in effort cost can have a significant impact on food and beverage intake (1, 2).  For example, placing a snack bowl 140 cm (4'7") away from a person leads to less snack intake than placing the same bowl 20 cm (8") away (3).  Getting up simply requires a bit more motivation than reaching out your arm.  Having food in your immediate vicinity, and particularly within arm's reach, is generally a bad idea if you're concerned about your weight.  The monetary cost of food also influences intake and selection.

This concept is the domain of a research discipline called 'behavioral economics'.  It's not my field, but I do find it interesting.  I'll explore this a bit further in the next post in the series.

Cognitive Restraint and Goals

Our behavior is strongly influenced by unconscious factors, but clearly we have some degree of conscious control over when and what we eat.  Our 'higher' cognitive functions, many of which are seated primarily in the cerebral cortex, serve to guide our behavior in a manner that supports our long-term goals.  Your reward system may want you to eat a donut, but your cortex can inhibit that behavior because you know eating the donut is inconsistent with your long-term goal of health and leanness.  The technical term for this is 'cognitive restraint', the common-sense term for it is willpower, and it is perceived as a conflict between unconsciously driven desires and conscious goals.

Negative Emotions

Emotions such as stress and sadness can drive food intake in susceptible people, and they may do so at least in part by increasing reward-seeking behavior in an effort to combat negative emotions.  The American Psychological Association reported in 2007 that 43 percent of Americans overeat when stressed, while 36 percent skip a meal (4).  Clearly the response to stress is highly individual.  However, among those who overeat, the foods consumed tend to be calorie-dense and highly palatable, such as candy, chocolate, ice cream, potato chips, cookies, cake, fast food, pizza, etc.

Social Environment

Who you eat with has a large impact on food intake.  For example, a person eating with 6 or more other people will eat on average nearly twice as many calories as a person eating by herself (5).  Eating is part of socializing, and the alcohol consumed in such social situations also disinhibits eating behavior.  People often wrongly believe they are unaffected by these factors.

Habit

To begin to illustrate the importance of habit in food intake behaviors, all we need is a common-sense example: how many times have you eaten lunch not because you're hungry, but simply because it's lunchtime?      The power of time cues is evident in a very interesting experiment that was conducted in amnesic patients in 1998 (6).  Researchers brought the patients a meal, told them it was lunchtime, and they ate a full meal completely.  15 minutes later, after the patients had forgotten they had eaten, researchers brought in a second full meal and told them it was lunchtime again.  They ate the second meal completely.  15 minutes later, researchers brought in a third meal, told them it was lunchtime again, and some of the patients ate part of the meal.  Satiety eventually kicked in and prevented further food intake, but not before these people had consumed more than twice their usual calorie intake simply due to their habit-driven response to a mealtime cue.

Habits serve a very important role in our everyday lives: they allow us to outsource information processing tasks from our conscious mind to our unconscious mind, freeing our conscious mind for other tasks.  If you had to consciously focus on brushing your teeth, how to move each finger on a keyboard to type a sentence, how to  hold a fork, and how to react when your phone rings, you would be a much less effective person.  When these behaviors are repeated enough times, they're 'outsourced' from conscious areas including the cortex, to unconscious structures such as the basal ganglia (7).

The influence of habits on our lives runs deep.  The large majority of our behaviors, including food selection, are driven by habit.  Therefore, habits to a large extent determine our weight and health over time.

The Complete Model

We've finally arrived at the complete version of my simplified model of food intake behavior.  Each colored shape represents a functional and (sort of) anatomical brain module, while the words that surround them represent the external and internal factors they respond to.  One thing I want to note here is that the right-most module, "Cognition and Emotions", is a catch-all category that represents a variety of processes that are regulated by a number of different brain areas.

As you can see, even my highly simplified model is quite complex-- this reflects the complexity of human biology and culture.