Hunger: Knowing When to Eat

Hunger: Knowing When to Eat

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.

21 thoughts on “Hunger: Knowing When to Eat

  1. Thanks Dave. I have found that looking into dieting and understanding the actual science about food, nutrition and exercise is difficult. My searches largely result in mostly click-bait "loose weight fast" type of "education". It is very hard to find honest information without all the marketing.

  2. Hunger is something that most people in America and other developed countries rarely experience. Some may never experience it. I started (and still do) fasting every day meaning I don't eat anything until early evening. At first I would get this almost alarming sensation for a few minutes which quickly went away. I learnt that this is "hunger".
    Anyhow, the point is that human evolved to have periods of hunger whereas in our modern society food is everywhere and constant eating is encouraged.

  3. Well, what an interesting coincidence. I’ve gained weight this semester as I greatly struggle with emotional eating! I just added books about it to my amazon and plan on working on it greatly… Thanks Prof Dave

  4. Thank you so much! Im soo excited to understand and learn this topic. Might even help me realize if nutrition is something that I will enjoy studying in the future:)

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