Hey it’s Professor Dave, what’s for lunch?
Our bodies are finely tuned machines, and they need fuel.
We’ve learned about how the body takes in oxygen as part of the respiratory system to
allow for cellular respiration.
We’ve also learned about how the body takes in food as part of the digestive system to
keep our cells functioning properly, and it will probably be worth reviewing that information
before watching this one to put things into context.
But while breathing is automatic, eating is not.
We decide when to put food in our mouths, and our brain is very good at telling us when
it thinks we need more food, by creating the sensation we call hunger.
How does the brain get this information, and how accurate is it?
Let’s get a closer look at hunger now.
It would seem like common sense that the brain receives signals that the body needs more
food, and makes us feel hungry so that we will eat.
But this is not at all the whole picture.
Certain amnesia patients will eat two, three, or even four meals in very rapid intervals
when offered, so there is more to a desire for food than simply meeting daily caloric
requirements.
First, let’s consider the phases of energy metabolism.
The cephalic phase is a prepatory phase.
This begins when someone sees, smells, or even just thinks about food.
This already initiates a physiological response.
Then comes the absorptive phase.
This happens after food has been ingested and broken down, and the small components
that have been absorbed into the bloodstream begin to meet the body’s immediate energy needs.
Finally there is the fasting phase.
The energy from the meal is gone and cells must rely on the fat and glycogen stored around
the body to get things done.
All of this activity is coordinated by hormones from the pancreas, those being insulin and
glucagon.
During the cephalic and absorptive phases, lots of insulin is released, and very little
glucagon.
This hormone does several things.
First, it promotes the use of glucose as the primary source of energy.
Second, it signals for the conversion of fuels in the bloodstream into forms that can be
readily stored, to get ready for all the new food that’s about to come in.
And third, it directs the storage of each biomolecule to the appropriate place.
That means glycogen goes to the liver and muscles, fat goes to adipose tissue, and proteins
go largely to the muscles.
By contrast, the fasting phase is characterized by high levels of glucagon and very little insulin.
This causes the body to break down glycogen to get more glucose that is ready to be metabolized,
so it’s like tapping into energy storage when we need it.
Especially high levels of glucagon during extended fasting promotes the release of fatty
acids from adipose tissue to be used as the primary fuel rather than glucose.
These fatty acids are converted to ketones, which is why this state is referred to as
ketosis, and we will discuss this in great detail in the upcoming nutrition series.
Getting back to hunger, we are now ready to deconstruct something called the set-point
assumption.
This is the seemingly common sense idea we mentioned, the notion that the brain tells
us we are hungry when we are running low on energy.
This stems from early research focused on the idea that certain levels of glucose in
the blood and a certain amount of body fat are optimal, and when levels dip below these
ideal amounts, the brain is notified and tells us to eat food.
As it turns out, this just isn’t the whole story, as is evidenced by the obesity epidemic.
We did not evolve in a world where we had the luxury to start and stop eating any type
of food we wish at any time we please.
That is simply the world we live in today, or at least many of us do.
Our ancestors had to eat as much food as possible when it was available, and store as much of
it as they could in the form of body fat, since there was no guarantee when the next
meal was coming.
In actuality, hunger is very complex, and has to do with a variety of motivations, including
social cues.
We often eat not out of hunger, but simply for pleasure, just in the way that we often
engage in sexual activity with no desire to reproduce.
We have evolved to crave these behaviors, and thus we partake in them excessively when
given the opportunities of the modern world.
Of course, there is a correlation between hunger and not having eaten for some time,
so this can also be a factor.
But let’s go through a list of factors that influence how much we eat, as well as precisely
what we eat and when we eat it.
Think of the foods that seem desirable to you.
These tend to be foods that are sweet, salty, or fatty.
From an evolutionary perspective, that’s because these are signifiers of foods with
high energy content, which means that perceiving a delectable flavor is an entirely subjective
experience.
We also develop preferences for nutrient-rich foods for similar reasons.
Most mammals choose to eat small amounts often in the day, whereas the circumstances of modern
working society have dictated that we eat larger meals at roughly predetermined times
in the day.
This conditions the mind to feel hungry at those specific times, merely in the anticipation
of an upcoming meal.
So the hunger you feel before lunch does not mean that you won’t have enough energy to
get through the rest of day if you don’t eat immediately.
This notion is falsified quite easily by extended bouts of fasting where hunger eventually dissipates
completely after several days of eating next to nothing.
Rather, the expectation that a meal is coming soon initiates the cephalic phase of metabolism,
releasing insulin and reducing blood glucose to ready the body for the reception of food.
So in these every-day situations, hunger is caused by the expectation of food, not because
of an energy deficit.
The amount that we eat in a meal is related to satiety signals.
These are responses to food intake that inhibit further consumption, but these too are not
completely dependent on the volume of food that has been eaten.
Experiments where swallowed food is diverted away from the esophagus into a tube such that
the food is not absorbed, show that rats will eat a normal-sized meal, rather than eating
indefinitely as no sustenance is achieved.
Satiety is therefore a function of previous experience rather than a direct indicator
of what nutrients have successfully made it into the bloodstream.
In addition, research has shown that ingested food interacts with receptors in the gastrointestinal
tract which causes the release of peptides into the bloodstream that act as satiety signals
in the form of hormones and neurotransmitters, so this may be the mechanism by which the
brain is informed about the food that has been consumed.
This may in turn influence our perception of the amount we eat, although this is also
influenced heavily by whether we are eating alone or with others, and where we are eating,
as well as other social cues.
Once again, an in-depth analysis of diet, health, metabolism, body weight, and so many
other things will be investigated at great length in the upcoming nutrition series.
For now, let’s move on to other topics in biopsychology.