Why Do We Eat? A Neurobiological Perspective. Part VI

In previous posts in this series, I explained that the brain (primarily the mesolimbic system) integrates various factors to decide whether or not to drive food seeking and consumption behaviors.  These include homeostatic factors such as hunger, and non-homeostatic factors such as palatability and the social environment.

In this post, I'll examine the reward system more closely.  This is the system that governs the motivation for food, and behavioral reinforcement (a form of learning).  It does this by receiving information from other parts of the brain that it uses to determine if it's appropriate to drive (motivate) food seeking behavior.  I covered its role in motivation in the first post of the series, so in this post I'll address reinforcement.

Behavioral Reinforcement


In the last post, I discussed palatability, or the pleasure associated with eating food, and how that influences food intake.  But I saved a key element of the discussion for this post: behavioral reinforcement.  Behavioral reinforcement is a form of learning that happens when you engage in a behavior that the reward system decides is favorable.  The reward system initiates a program that makes you feel good, and increases the probability that you'll engage in that behavior again.  Conversely, if you engage in a behavior that the reward system considers unfavorable, you may develop an aversion to it and your likelihood of engaging in that behavior will decrease.  This is a key part of our species' ability to survive and navigate a constantly changing world.  It allows us to adapt to novel environments rather than rely exclusively on hard-wired behaviors.

The reward system evolved to reinforce behaviors that enhance survival and reproduction, and oppose behaviors that reduce survival and reproduction.  But it's important to remember that the reward system didn't evolve to handle a world of ice cream sundaes and pornography.  It reinforces behaviors that would have increased survival in the ancestral environment*.  That's why it favors calorie-dense easily digested food, and tends to oppose unnecessary physical activity.  In the ancestral environment, the risk of suffering reproductive consequences due to excessive consumption of energy-dense food and too little physical activity was probably low-- the main concern was eating enough to sustain your calorie expenditure.  Here's Dr. Bruce King** from a very recent article (1):

The gastrointestinal, sensory (taste and olfaction), and brain feeding mechanisms that developed during the past 2 million years were highly adaptive for ancestral hunter-gatherers living in an environment with limited high-density foods and periods of food deprivation. Today, however, humans in industrialized countries live in what has been called an “obesogenic environment.” The non-homeostatic brain reward circuitry that was acquired during evolution to seek out and eat as many nutritionally high-dense foods as possible is able to overrule the physiological inhibitory mechanisms that were designed to limit meal size and weight gain.

Regardless of the evolutionary justification, we know today that certain food properties reinforce behavior in animals and humans.  If a food contains these properties, in general when you eat it repeatedly the reward system will make you more likely to obtain and eat it over time, sometimes even if you aren't hungry.  The reward system makes the food more palatable and more tempting, reinforcing consumption behavior.  Here is a list of some of the core factors that drive reward.  You may recognize them from the post on palatability:

• Sweetness
• Fat
• Starch
• Calorie density
• Salt
• Free glutamate ('umami')
• Absence of bitterness

Let's illustrate this with some research, then an everyday example.  Dietary fat and calorie density are two of the most rewarding food properties, and they are linked to one another since fat is the most calorie-dense macronutrient.  In an experiment published in Physiology and Behavior in 1991, children were given yogurts that contained novel flavors (i.e. it was the first time they had encountered those flavors), in either high-fat or low-fat form (2).  Researchers found that at the end of the experiment, children ended up preferring flavors that were previously paired with the high-fat yogurt, even if they were no longer associated with high fat.  The reward system caused them to 'learn' to prefer flavors associated with fat and calorie density, which subsequently drove their eating behavior.  Similar findings have been reported using beverages fortified with different amounts of carbohydrate (3).

Conversely, if a food is paired with nausea, the reward system will learn to avoid the food and cause it to seem less appealing (4, 5).  This is called a learned taste aversion.

Chocolate

Now, a common sense example.  Initially, people tend to prefer sweet milk chocolate because sweetness is an inherently preferred food quality (from birth), and the bitterness of dark chocolate is inherently disliked.  As they're exposed to chocolate repeatedly, dark chocolate often becomes more pleasant as the brain learns that dark chocolate contains more drug than milk chocolate, and is not toxic despite its bitterness.  This is the same process that occurs for coffee and beer, both of which are bitter but contain a habit-forming drug.  The unpleasant aspects are overcome as they're associated with positive properties over time, and behaviors to seek and consume those foods/beverages are reinforced.  The same thing happens for many foods, for example, we acquire a taste for vegetables over time if they're repeatedly paired with fats and other calorie sources.  No one would ever acquire a taste for kale if we ate it plain every time-- it's bitter and contains virtually no calories.

Chocolate is an archetypal high-reward food, and most people reading this would probably feel tempted if a square were right in front of them, regardless of hunger (including me).  That's because it's a killer combination from a food reward perspective: extremely high energy density, high fat, sweetness, a nice texture, and a drug.  It doesn't get much more rewarding than that.  But the world isn't black-and-white: dark chocolate also seems to have meaningful health benefits if eaten in moderation.  Still, it can facilitate overeating and fat gain.  My solution is to keep a bag of plain roasted cocoa nibs in my kitchen.  They taste good, but not good enough that I'll eat them if I'm not hungry.

This brings us to another important point.  I've noticed that my writing on the subject of food reward has frequently been misunderstood and/or misrepresented, e.g. "we should eat food that tastes bad", or "we should reduce food reward and palatability as much as possible".  I think a lot of that is coming from people who don't want to understand food reward.  But for those who do, I'll clarify.  Food reward and palatability are factors that influence food intake and body fatness.  They aren't inherently bad or good-- they're tools that you can use to achieve your goals, whether that's fat loss, weight gain, or eating food that you enjoy.  There are other ways to lose or gain fat, and food reward doesn't erase those-- it only adds to them.  Food reward is one of the reasons why many of us eat beyond our true calorie needs, but that doesn't mean it needs to be ruthlessly minimized in all situations.  Life is a balancing act, and enjoying food is part of the equation.

The Updated Model

We haven't uncovered the last 'module' yet, so the model looks the same as last time:

* Whatever that is exactly.  But I think we can agree it's not 2012 USA.

** Bruce King is a seasoned obesity researcher currently at Clemson.  He wrote an outstanding review paper titled "The rise, fall, and resurrection of the ventromedial hypothalamus in the regulation of feeding behavior and body weight" that I often refer to (6).  The VMH is one of the key hypothalamic nuclei that regulates food intake and body fatness.  I highly recommend this article to anyone who has a serious interest in the VMH, and the hypothalamic control of food intake and body fatness in general.