Why is Eating Behavior Regulated?
Let's start at the most fundamental level. To be competitive in a natural environment, organisms must find rational ways of interacting with their surroundings to promote survival and reproduction. One of the most important elements of survival is the acquisition of energy and chemical building blocks, either by photosynthesis, or (in the case of animals) eating other organisms. This imperative drove the evolution of rational food seeking behaviors long before the emergence of humans, mammals, reptiles, amphibians, fish, worms, and even eukaryotes (organisms with nuclei).
For example, bacteria that are able to move typically display a behavior called 'chemotaxis': movement toward or away from particular chemicals. These bacteria move away from noxious stimuli such as concentrated salt solutions, and toward chemicals that signal food. Chemotaxis toward a food source is a simple example of a rational food-seeking behavior that presumably existed long before the evolution of eukaryotes.
Since the natural world is complex, there are ways to improve on the simple food-seeking behaviors of bacteria in order to optimize survival*. The brain is an organ that is dedicated in large part to doing just this. As I've explained in previous posts, it is a centralized behavior control organ that senses the internal and external environment, and attempts to optimize when and what an organism eats in the context of those particular conditions. As one might expect, the brain is 'tuned' to optimize fitness within a particular ecological context-- a point we'll return to later in the post. Now, let's consider the rational process that the brain uses to make its decisions.
Deciding When to Eat
When you have an important decision to make, and you want to make the best possible choice, what do you do? Many of us create a list of pros and cons, weigh each factor according to its importance, and use that list to guide our choice. Although we have a lot to learn about how this happens, the brain (likely the mesolimbic 'reward' system) probably uses a process much like this to decide when to eat. Here is a list of some of the pros and cons that the brain considers:
Looking at the list above, it should start to become obvious why in the modern world we often eat more than we need to satisfy hunger, and more than we need to meet our body's energy requirement. But this poses a problem. If our brains evolved to make rational decisions about food, then why do they lead us to eat too much, eat the wrong foods, and ultimately make us sick?
Our Brains Evolved to Make Rational Decisions About Food-- In a Particular Context
Why are we drawn to pizza, ice cream, potato chips, fried chicken, and pastries, when we know they're fattening and unhealthy? These don't appear to be rational motivations that encourage survival and reproduction in the 21st century, but in fact, they make great sense-- in the context of our ancestors' world.
In our ancestors' world, food tended to be harder to obtain and prepare than it is in our world. Starvation was a much larger threat than it is to most people in affluent societies today, while eating too much was a rare luxury. Therefore, our brains are tuned by evolution to seek calorie-dense, easily digested food that requires minimal effort and risk to obtain and consume. This is the foundation of food reward. The fattier, starchier, sweeter, milder, and more calorie-dense the food, the more 'valuable' the reward system considers it**-- and motivation to eat is scaled accordingly. Why accept the effort and risk of chasing a deer or growing rice when you can simply push a pedal in your car, drive up to a fast food joint and order a hamburger, fries and milkshake? This is what our reward system compels us to do, and since our modern environment makes it so easy for us to indulge these deep-seated desires, we have to consciously guide ourselves onto a healthier path.
Since our ancestors' food required work to acquire and prepare, effort cost was higher. This means there had to be a higher level of motivation to drive food seeking behavior. In addition, the food was typically more satiating per calorie, because it was richer in protein, water and fiber. Yet there were fewer motivating factors to be found: our ancestors generally ate simple food by today's standards, and they didn't have as much access to habit-forming drugs (alcohol, coffee) as we do today. So the factors opposing food intake were stronger, and the factors promoting it were weaker. Hunger (homeostatic eating) was presumably a more prominent motivator than today, relative to other (non-homeostatic) motivators, since non-homeostatic motivators tended to be weaker. This suggests that food intake would have corresponded more closely to the body's true energy needs as signaled by hunger-- and there was less of a drive to eat beyond hunger.
In the following image, I've summarized how these factors may have interacted in the ancestral environment, and how they interact today.
As you can see, in the ancestral world, there was somewhat of a balance between the factors that promote eating, and those that oppose it. In contrast, in the modern world, the factors that promote eating are stronger, while those that oppose it are weak. The only major opposing factor we have today is cognitive restraint, or willpower, which frankly isn't a great resource for most people. The fallibility of willpower is evident in the fact that many people continue to eat foods that are unhealthy and fattening, despite the fact that they know they're unhealthy and fattening. For a fictional account that illustrates how some of these factors may operate in everyday life, see below***.
Of course, we can change our diet and food environment to attempt to re-balance these factors in a more ancestral way, and many of us do so, whether it's based on ancestral logic or just common sense. We can accomplish this by using simple strategies like clearing the food environment of tempting foods, only leaving snacks out that are low in calorie density and require a bit of effort to eat (e.g., oranges), avoiding the most highly palatable foods, and eating food with a higher satiety value per calorie. There is a lot more to discuss here, but I'll leave it there for now.
I hope this series has shed some light on what regulates food intake in humans, the brain processes that are involved, and why we eat more today than our ancestors did. To consolidate the information, I've re-organized previous posts in the series so that they appear in a continuous order on the blog. I highly recommend reading these posts in order all at once, since I published them over the course of several months. They'll make much more sense that way.
* Not that the strategy bacteria use is a bad one. It seems to be working out pretty well for them. But they have a very different survival strategy than large multicellular organisms.
** Actually, this is a little bit of an oversimplification. There's a 'sweet spot' for most of these factors.
*** Let's consider a day in the life of a fictional person. Jim and his wife Samantha get out of bed in the morning. Jim takes a shower, walks into the kitchen feeling a bit hungry, and his eyes alight on an open bag of potato chips and an orange on the counter. The chips in particular are easy to access, taste good, have a high calorie density, and a seductive flavor and texture. Jim is tempted, but habit and cognitive restraint (he knows Samantha will think he's a slob if she catches him eating chips at 7:30 am) send him to the fridge instead. He puts four slices of whole wheat bread in the toaster (two for himself, two for his wife) while Samantha cooks eggs. Jim doesn't like the taste of whole wheat bread as much as white bread, but he eats it because of his conscious health goals. He makes up for the less appealing flavor by putting more jam on his toast. The jam is particularly good-- his mother made it-- so he toasts and eats a third piece of bread with jam. He also eats his egg, not because he's hungry anymore, but out of habit and because he doesn't like to waste.
Jim goes to work. First things first-- Jim pours himself a cup of coffee and adds a splash of cream. He does this because (despite its bitter flavor) coffee contains caffeine, a habit-forming drug, and cream tastes good and adds calories. Jim goes to a meeting. One of his co-workers brought donuts, but Jim has told himself he won't eat them. In any case, he's not hungry at all. Unfortunately, he's sitting within arm's reach of the donuts, and the meeting is boring. His eyes keep returning to the box of donuts. Finally, he caves in and eats one. The seductiveness, calorie density, and entertainment value of donuts overcame his conscious goal to avoid eating one. At noon, Jim takes time for lunch as he does every day. He sits down with his co-workers and eats a sandwich-- turkey cold cuts, lettuce, tomato, and mayonnaise on whole wheat bread-- and an apple. The apple has a rotten spot, so he avoids it, an example of avoiding an aversive flavor/texture. He was a little bit hungry, but he mostly ate out of habit.
As Jim walks down the hallway to leave work, he passes the secretary's desk, and she usually has a bowl of candy for passers-by. Jim takes a Hershey's kiss, because it's easily accessible, it's calorie-dense, and it tastes good. He thinks nothing of it-- it's just a Hershey's kiss. As he's walking to his car, he smells something amazing-- it's a pastry shop near his work. He suddenly feels hungry. He manages to get into his car and drive away without buying a pastry-- this time. This is an example of cognitive goals overriding a strong desire to eat that is driven by a cue (aroma) associated with a highly rewarding/palatable food.
When he arrives home, he's still feeling hungry, so his eyes again alight on the open bag of potato chips and the orange. He goes for the bag of chips, because they're easily accessed, they taste great, and they have a high calorie density. However, they will contribute very little satiety to his upcoming meal, potentially increasing total calorie intake. He pours himself a glass of wine, because it tastes good and contains a habit-forming drug. He cooks a meal with Samantha: stir-fried chicken and vegetables with rice. Jim forgets how salty soy sauce tastes, so he adds extra salt and ends up over-salting the dish. Jim and Samantha both eat less because the food tastes mediocre. For dessert, they each have an orange.
I used the example of a relatively health-conscious person to illustrate that nearly all of us live in an environmental context where it's a challenge not to overeat. The situation would be much more challenging for a person who surrounds himself with unhealthy tempting foods and cares less about his health. In most people, the modern food environment contributes to fat gain over time by default.