Here’s a couple of words you might find useful in the study of human metabolism.
They also happen to be two words that I enjoy in general.
Brunch. Buffet.
I’d throw in “all you can eat,” but that’s kind of implied, and then it would
be six words, unless you hyphenate them. But still, brunch buffet!
You’ve got eggs and bacon and french toast and biscuits and fruit and some kind of weird
sort of jello thing in there. And once you’ve paid your twelve-ninety-nine, you know there’s
a pretty good chance that it’s all gonna get eaten.
But here’s the thing.
Not everything that you fit into your face during brunch-time is going to be handled
the same way by your body.
After you’re done digesting, some nutrients will go straight to your body’s pile of
stuff to burn right away. But others will have to be converted into something else.
For example, the carbs and fats in your buttered toast can be directly oxidized into useable
energy, but the amino acids in the bacon have to converted to molecules that get broken
down like carbs, if you want to get energy out of them.
And while excess carbs and fat can be stored in larger, polymer versions of their original
forms, any extra amino acids can’t be -- they have to be converted and then stored as fat or glycogen.
So, as I’ve mentioned before, the molecules in your body are constantly changing shape,
and renewing and rearranging themselves to either build things or to use energy.
And eating food replenishes these nutrients -- especially glucose. Then, depending on
what your body needs, and when you last ate, certain hormones like insulin will help decide
what to burn, and what to store for later.
This, of course, is an important function. Which is why things can go badly if this process
doesn’t work properly because of a metabolic disorder, like diabetes.
In that case, your body can’t properly use and store nutritional energy, which makes
the very acts of eating and metabolizing more difficult, and possibly dangerous.
Exactly how glucose levels can spike or plummet, how we convert nutrients into energy, and
how all of that that relates to eating, and hunger, and weight, metabolism, and your health
in general is -- guess what? -- complicated.
But as you are a living thing that must eat in order to stay alive, I’d say it’s worth
learning how it works.
And brunch is as good a place to start as any.
Waiting in line for the all-you-can-eat brunch buffet is a good time to recall the law of
conservation of energy.
The law simply states that energy can never be created or destroyed; it can only change forms.
And the processes of metabolism involves conversions of energy -- either through catabolizing reactions
that release the energy stored in your food, or anabolizing ones that use or store that energy.
So, you can enjoy your short-stack of flapjacks and biscuits and gravy, but remember: That
energy has to either get released, or stored, somewhere inside you.
Now, the conversion of food energy is part of the great balancing act that is maintaining
homeostasis, and one of its greatest tricks is cellular respiration.
Cellular respiration is how we derive energy from the food we eat, specifically glucose,
which is where most of our food ends up.
In order for glucose to become energy we can use, we have to to give it some oxygen, and
convert each sugar molecule into six molecules of CO2, and six molecules of water, which
usually produces about 32 molecules of ATP.
Cellular respiration occurs in three simultaneous phases, where glucose is broken down by glycolysis
and other catabolic reactions; and then turned into pyruvic acid and acetyl CoA; and finally
ushered into the Kreb’s cycle.
It’s all pretty complicated, but we covered it extensively in Crash Course Biology, so
rather than getting into the blow-by-blow here, I recommend you check out that video for a refresher.
For now, just remember that cellular respiration breaks down nutrient molecules to generate
ATP. And in doing so, it captures some of the chemical energy that was in those bonds
for use in your body.
Now, when it comes to how that energy is converted and where, that depends largely on when you
ate your last meal.
We all switch back and forth between two nutritional states -- the absorptive, or fed state -- which
is during or after eating -- and the postabsorptive, or fasting state, when the GI tract is empty
and the body is running off of stored supplies.
So, say you’re just post brunch-buffet, and you’re still in the absorptive state,
unbuckling your belt as your digestive system breaks up the eggs and bacon and French toast
and syrup into a bunch of mostly-glucose molecules that pass into your bloodstream.
The first bit of glucose gets delivered throughout the body and is tapped to generate ATP on
the spot, through cellular respiration.
But since the meal was big and rich, there’s more glucose floating around than your cells
need at the moment.
And remember, ATP is too unstable to be used for storage, which means all the extra glucose
is gonna get stored as fat or glycogen.
And that storage is part of how you can end up gaining weight!.
Because: How much energy gets stored depends partly on your basal metabolic rate -- that’s
the number of daily calories your body needs to do business as usual.
And that rate can be influenced by your age, your sex, and body size or composition. A
young bodybuilder is gonna burn way more calories than that tiny grandma, but generally, if
you’re absorbing more nutrients than you’re burning, you will gain weight.
So yeah, for that you can blame the law of conservation of energy.
Now, of course all nutrients are important. I mean, your body is basically made of protein,
so that’s kinda key.
But in terms of immediate fuel, glucose is the easiest source of food for cells like
your neurons. And it is always good policy to keep your brain well-fed, so it doesn’t
get hangry, or dead.
So, your body likes to maintain a blood glucose level of 70 to 100 milligrams per deciliter.
But let’s say that, thanks to the buffet, you’ve now got 140 milligrams per deciliter.
This means too much sugar is swimming around in your blood, and your body isn’t happy about it.
If your blood sugar levels get too high, it can damage blood vessels, especially those
in the nervous system, heart, kidneys, eyes, and extremities.
That’s why diabetes is often associated with a higher risk of heart and kidney disease,
loss of vision, and foot amputation.
But typically, rising blood sugar levels sets off a series of events that trim them back down.
Specifically, they trigger special beta cells in the pancreas to start secreting more of
the hormone that regulates everything that happens when you’re in the absorptive state.
And this hormone is the all-powerful insulin.
Insulin’s job in this instance is to move glucose out of the blood and into storage.
And to do that, it triggers a shift from catabolic reactions to anabolic ones.
For example, it puts a stop to glycogenolysis, or breaking down glycogen in your liver and
muscles to release glucose for energy, and instead ramps up the process of glycogenesis,
where extra glucose is linked together to form glycogen.
It also activates lipogenesis, where very cool chemical reactions in the liver convert
glucose to triglycerides and then ship them off to your adipose tissue for storage.
A similar thing happens with the extra fatty acids that you got from the eggs and bacon
that aren’t immediately needed for energy.
They get processed through lipogenesis, too, and are rebuilt back into triglycerides, and
then tucked away for a rainy day.
The thing is, if you’re thinking about this, all these lipids hate water, right? Fat and water don’t mix.
So you should be asking yourself, how can they be transported in the blood, when they’re so hydrophobic?
The answer is lipoproteins, four specialized proteins made by the liver that surround fats
and allow them to move in the bloodstream as an emulsion.
You’ve probably heard of some of these. One is low density lipoprotein -- also known as
LDL cholesterol. It’s what delivers some of the glucose and fat that you just ate to your body’s fat deposits.
Another is high density lipoprotein -- or HDL cholesterol, which is the “good” cholesterol.
It starts out as an empty protein packet sent out by the liver to gather up other cholesterols
from the blood, and artery walls, and other cells.
HDL then delivers the cholesterol back to the liver, or to places like the ovaries,
testes, and adrenal glands, which use it to make steroid hormones.
Alright, so now your body has put all the glucose and lipids and proteins where they
need to go, and you can just coast into the postabsorptive state.
But several hours later, even though your small intestine is still working on what’s
left of the buffet, your cells have been helping themselves to the remaining glucose in theblood,
and eventually your blood sugar level will start to drop.
Remember, your neurons are on an exclusive glucose meal plan, so they need sugar at the
ready. And if your body senses an imbalance in blood sugar levels, it sounds the alarm.
A decrease in glucose stimulates alpha cells in the pancreas to release insulin’s nemesis,
the hormone glucagon.
It starts raising the blood sugar level, by triggering the liver and adipose tissues to
metabolize their fat and glycogen stores, thereby releasing fatty acids, glycerol, and
glucose back into the blood.
But if it’s been a day or two since you’ve eaten -- for some reason -- and you’ve burned through
both your blood glucose and your glycogen stores, and you don’t have any sugars left to feed your brain?
In that case, your body will launch into gluconeogenesis, and convert fats and amino acids into glucose,
so ATP synthesis can continue in your brain cells.
This process is a sort of last-ditch effort by the body to protect the nervous system
from the damaging effects of low blood sugar.
Now, of course, this whole system has flaws. And one of them is that the whole setup is
almost entirely dependent on the proper release and reception of insulin.
People with diabetes either don’t produce enough insulin, or have abnormal receptors
for it, which is why they often have to inject a pharmaceutical version of the chemical after eating.
Otherwise, their blood sugar levels will remain too high after eating, and they’ll start
peeing out large amounts of glucose to try and balance the levels in their blood.
The problem is, because glucose is being excreted and not stored, it’s not available when
the blood sugar starts to drop, so the body has to draw on fat and protein tissues for
energy -- which is one reason why sudden weight loss can be an early sign of diabetes.
Whether you’re diabetic or not, I think we can all agree it is important to respect
your blood sugar levels, and remember to continue feeding the beast.
But, at brunch, just -- you know -- use small plates, and try to keep the trips to the buffet
to under five or so.
Today, you learned how your body uses energy from food -- including through cellular respiration,
which converts glucose into ATP; glycogenesis, which converts it to glycogen; and lipogenesis,
which converts it to triglycerides. And you also learned how insulin regulates your sugar
levels.
Thank you to our Headmaster of Learning, Linnea Boyev, and thanks to all of our Patreon patrons
whose monthly contributions help make Crash Course possible, not only for themselves,
but for everyone, everywhere. If you like Crash Course and want to help us keep making
videos like this one, you can go to patreon.com/crashcourse.
This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written
by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson.
It was directed by Nicholas Jenkins, edited by Nicole Sweeney; our sound designer is Michael
Aranda, and the Graphics team is Thought Cafe.