Why Do We Eat? A Neurobiological Perspective. Part II

In the last post, I explained that eating behavior is determined by a variety of factors, including hunger and a number of others that I'll gradually explore as we make our way through the series.  These factors are recognized by specialized brain 'modules' and forwarded to a central action selection system in the mesolimbic area (the reward system), which determines if they are collectively sufficient cause for action.  If so, they're forwarded to brain systems that directly drive the physical movements involved in seeking and consuming food (motor systems).

The term 'homeostasis' is important in biology.  Homeostasis is a process that attempts to keep a particular factor within a certain stable range.  The thermostat in your house is an example of a homeostatic system.  It reacts to upward or downward changes in a manner that keeps temperature in a comfortable range.  The human body also contains a thermostat that keeps internal temperature close to 98.6 F.  Many things are homeostatically regulated by the body, and one of them is energy status (how much energy the body has available for use).  Homeostasis of large-scale processes in the body is typically regulated by the brain.

We can divide the factors that determine feeding behavior into two categories, homeostatic and non-homeostatic.  Homeostatic eating is when food intake is driven by a true energy need, as perceived by the brain.  For the most part, this is eating in response to hunger.  Non-homeostatic eating is when food intake is driven by factors other than energy need, such as palatability, habitual meal time, and food cues (e.g. you just walked by a vending machine full of Flamin' Hot Cheetos).

We can divide energy homeostasis into two sub-categories: 1) the system that regulates short-term, meal-to-meal calorie intake, and 2) the system that regulates fat mass, the long-term energy reserve of the human body.  In this post, I'll give an overview of the process that regulates energy homeostasis on a short-term, meal-to-meal basis.

The Satiety System (Short-Term Energy Homeostasis)


The stomach of an adult human has a capacity of 2-4 liters.  In practice, people rarely eat that volume of food.  In fact, most of us feel completely stuffed long before we've reached full stomach capacity.  Why?



The digestive tract uses a complex suite of signals to communicate with the brain, indicating the type and amount of food a person has eaten.  These signals are part of a system that attempts to keep meal size in the optimal range, not too large and not too small, either of which could be harmful to the organism.  We feel full before the stomach has reached capacity because or brain decides, based on input from the digestive tract, that we've had enough to eat.

One of the most important signals is stomach distension.  The stomach contains stretch receptors that inform the brain of the volume of food consumed.  These are indirectly connected to the hindbrain via spinal sensory nerves, as well as the vagus nerve (1, 2).  The vagus nerve is intimately involved in two-way communication between the brain and the digestive system.

In addition, there are a variety of 'gastrointestinal peptides' that are secreted as food passes through the stomach and small intestine.  These respond both to the amount and the type of food consumed (i.e. protein, carbohydrate and fat).  These peptides play many roles in the body, but one important role is to communicate information to the brain about the food you just ate.  These peptide signals are transmitted to the brain both via the bloodstream, and via local actions on the vagus nerve in the intestine.  It's beyond the scope of this series to discuss these peptides individually, but if you want more information there are several good review papers available (3, 4, 5).

The hindbrain, and particularly the nucleus tractus solitarius (NTS) and area postrema (AP), is the primary brain site responsible for receiving these signals and promoting satiety and meal termination.  In a series of gruesome but fascinating experiments, researchers disconnected the forebrain and the hindbrain of rats, and demonstrated that the hindbrain contains all of the necessary hardware to terminate meals appropriately (6).  However, the hindbrain is not able to alter meal size in response to changes in long-term energy status, so that function (discussed in the next post) resides elsewhere in the brain (7).

What Food Properties Determine Satiety?

The satiety system is designed to make sure the body gets enough energy from an individual meal, without overdoing it.  The primary variable it responds to is calories.  The more calories you eat, the more full you feel.  Eating a huge bowl of lettuce with no dressing may have a lot of volume, but it won't really make you feel full, in the sense that you no longer have a desire to seek food.  Conversely, drinking a pint of heavy cream may not fill up your stomach as much as the aforementioned lettuce, but it will make you feel full!  The cream contains 900 calories, while the salad contains virtually none.

That being said, there are other factors besides calories that influence the satiety system.  In a very interesting paper titled "A Satiety Index of Common Foods", Susanne Holt and colleagues identified some of the factors besides calories that determine satiety (8).  It turns out, the degree of satiety per calorie consumed varies widely between foods.  The 'satiety index' refers to the amount of satiety a food produces, per calorie.  The two largest determinants of satiety were energy density (calories per gram of food) and palatability.  The lower the energy density, the higher the satiety index; conversely, the higher the palatability, the lower the satiety index.  Foods that were the highest in palatability were less than half as filling per calorie as those that were the lowest in palatability.  Palatability and energy density are related since energy density is one of the main determinants of palatability.

Many controlled studies have confirmed that palatability and energy density are major influences on meal-to-meal food intake (9, 10, 11, 12, 13, 14, 15, 16).  In a study of people living their normal lives, John de Castro's group demonstrated that people eat 44 percent more calories at meals rated as highly palatable, compared to meals that are rated as average or low palatability (17)*:

Interestingly, according to his data, there was no difference between moderate and low palatability.  It may be that most of the benefit comes from avoiding highly palatable foods.

Energy density determines the volume (technically, weight) of a food you have to eat to obtain a certain number of calories.  Foods with a high energy density include oils/fats, cookies, candy, bacon, sausage, bread, and nuts.  Foods with a low energy density include fruit, vegetables, potatoes, eggs, oatmeal porridge, seafood, and lean meats.  Energy density has a major impact on the amount of calories consumed in a meal, and over many meals in a row, because the more volume you ingest per calorie (up to a point), the more full you will feel per calorie (18).  Researcher Barbara Rolls has a diet book based on energy density called The Ultimate Volumetrics Diet, which is one of the best diet books I've seen.  Controlled trials have shown that reducing energy density is an effective way to lose body fat and maintain fat loss in free-living people, at least compared to other single-factor weight loss interventions (19, 20, 21).

Returning to the satiety index paper, there are other factors that also determine satiety per calorie.  The more protein and fiber a food contains, the higher its satiety index.  The more fat a food contains, the lower its satiety index.  Other studies have shown that if you control for energy density and palatability, fat and carbohydrate are equally satiating however (22), so fat is only less satiating because it's palatable and occupies less volume than other nutrients (this can be avoided by eating fat in the context of a lower energy density meal, for example olive oil in a salad dressing).

Now let's bring this discussion from nutrients back to foods.  Foods with a low satiety index include pastries, cookies, white bread, added fats, candy, and ice cream.  Foods with a high satiety index include fruit, seafood, lean meats, potatoes, grain porridges, eggs and beans.  Sound familiar?  These are all ancestral foods eaten by healthy, lean cultures throughout the world.  Here's a nice quote from the discussion section of the satiety index paper (23):

These results therefore suggest that 'modern' Western diets which are based on highly palatable, low-fibre convenience foods are likely to be much less satiating than the diets of the past or those of less developed countries.

In addition, food variety increases food intake.  The more types of foods a person has available at a meal, the more total food he tends to eat.  I typically lump this phenomenon together with palatability/reward for simplicity, but technically it's a related property called sensory-specific satiety (24).

The Satiety System is Influenced by Other Systems in the Brain

As one might imagine from the discussion above, the satiety system is dampened (and/or overridden) by high palatability acting in other regions of the brain to be discussed later.  Both humans and animals will continue eating past the point of fullness if the food is palatable enough.  This is the 'dessert effect'.  No one eats 250 calories of plain boiled potato after a large meal, but bring on the chocolate cake!

The satiety system is also under the control of the systems that regulate long-term energy homeostasis (body fatness).  When body energy stores are low, the energy homeostasis system tells the satiety system to increase food intake, in an attempt to recover the missing energy (25).  I'll cover this again in a post on long-term energy homeostasis.

The Model

Here is the updated model of food intake regulation, including everything we've covered so far.  The ovals represent functional and anatomical brain modules, and the terms on the outside represent the factors that influence those modules:

In the next post, I'll describe the system that regulates fat mass, which is the long-term energy reserve of the body: the energy homeostasis system.


* His findings suggest that differences in palatability only account for a small percentage (4%) of the variability in meal sizes between meals.  This is due to two reasons.  First, most people in the study consistently selected high-palatability foods.  The palatability of meals was typically high, so the differences in palatability between meals were usually small and could not account for much of the total variability in size between meals.  Despite this, palatability remains one of the most influential factors he has identified in his research (26).  The second reason is that there are other important factors that influence meal size, like hunger, body size, and social environment.  The more factors impact food intake, the less any individual factor will contribute to total variability.