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Have you ever skipped a meal?
Maybe unintentionally.
Or perhaps you've heard people talking about intermittent fasting.
Yeah, or maybe life just gets crazy busy, right?
We all go through periods without food.
Exactly.
But what's really happening inside your body then?
It's way more complex than just a rumbling stomach.
Oh, absolutely.
Your body isn't just passively waiting.
It's this incredibly sophisticated system, really a masterclass in managing fuel honed over ages.
So today, we're doing a deep dive into that world fasting metabolism.
We're pulling insights straight from a key chapter in Marx's basic medical biochemistry.
That's right.
It lays out these processes with remarkable clarity.
And our mission here is to walk you through it all, the biochemistry, the pathways, even some clinical examples and make it all feel clear, accessible.
Think of it as a guided tour inside your own energy systems.
Hopefully you'll have some real aha moments about how resilient your body actually is.
Okay, let's start with the basics then.
We think fasting means feeling hungry, but metabolically.
When does the body actually flip that switch?
Yeah, that's a common thought.
But the metabolic shift, it actually starts pretty quietly, maybe two to four hours after you last ate.
Two to four hours.
So just when your blood glucose levels get back to normal baseline levels?
Precisely.
It's not about hunger yet.
It's an internal signal.
The immediate fuel delivery is done and the body's thinking, okay, time to use the reserves.
Right.
So blood glucose dips back to baseline.
What's the first domino to fall, hormonally speaking?
Okay, so this dip triggers a really crucial hormonal seesaw.
Insulin, which was high after the meal to help store glucose, starts to drop.
And the opposite happens with glucagon.
Exactly.
Glucagon levels start to rise.
Think of insulin as the store energy signal and glucagon as the release stored energy command.
It's this constant balancing act.
Got it.
So glucagon's rising.
No.
Where does the body go first for that quick energy boost?
What's the first reserve it taps?
The very first stop is the liver.
It's got the stored form of glucose called glycogen.
Right, like a little emergency stash.
Sort of, yeah.
And the liver starts breaking it down rapidly.
That process is called glycogenolysis.
It releases glucose straight into the bloodstream.
And that glucose is absolutely essential for certain tissues, isn't it?
Critically important, especially for the brain and red blood cells.
Why them specifically?
Well, your brain, it largely lacks the machinery to burn fatty acids effectively.
So it relies heavily on glucose.
And red blood cells.
They're even more dependent.
They don't have mitochondria, the cells powerhouses.
So for them, glucose isn't just the main fuel, it's the only fuel.
Keeping their supply going is top priority.
Okay, so the liver's busy with glycogen.
But what about our fat stores?
Surely they get involved pretty quickly too.
Oh, definitely.
At the same time, your adipose tissue, your fat tissue starts lipolysis.
Lipolysis, breaking down fat.
Yep.
It breaks down its stored
triacylglycerols, which is just the technical term for body fat, really.
And that releases.
Fatty acids and a molecule called glycerol into the blood.
And those fatty acids become a major fuel source.
So let me see if I've got this.
Brain and red blood cells are running on that liver glucose.
Correct.
But other tissues, like muscles, they switch over and start burning these newly released fatty acids.
That's right.
They oxidize them completely down to CO2 and water for energy.
It's very efficient for them.
But the liver does something different, doesn't it?
With the fatty acids it takes up.
It does.
And it's fascinating.
The liver takes up fatty acids too, but it doesn't burn them all completely.
It partially oxidizes some of them into smaller molecules.
Ketone bodies.
Exactly.
Ketone bodies like acetoacetate and hydroxybutyrate.
The liver makes these and then releases them back into the blood.
And these aren't waste products, they're another fuel source.
Absolutely not waste.
They're a very clever, water soluble alternative fuel.
Tissues like muscle and your kidneys can grab these ketone bodies from the blood.
And burn them for energy in their own mitochondria.
Precisely.
In their TCA cycle, the main energy pathway, it's like the liver is pre -processing fat into a more easily transportable fuel for other organs.
Okay, but that liver glycogen, that emergency stash, it doesn't last forever, does it?
You said maybe 24 hours, 30 hours tops.
Yeah, it's a relatively short -term solution.
Which really highlights how quickly the body needs to shift gears again if fasting continues.
So what is plan B when the liver glycogen starts running low?
That's when a process called gluconeogenesis really ramps up.
A gluconeogenesis.
Making new glucose.
Exactly, genesis of new glucose.
The liver starts synthesizing glucose from scratch using non -carbohydrate sources.
It's like switching from using stored supplies to manufacturing the essential product.
Manufacturing it from what, though?
Where do the building blocks come from?
Three main places provide the carbon skeletons.
Lactate, glycerol, and amino acids.
Lactate, like from muscles during exercise.
Yep, lactate from muscle and also from red blood cells.
The body's recycling.
Glycerol comes from that fat breakdown.
Lepolysis, we mentioned, it's the small backbone part of the fat molecule.
And amino acids.
That means breaking down muscles.
Primarily, yes.
Amino acids from muscle protein breakdown become a major source for gluconeogenesis.
And this is a key point.
The body starts sacrificing its own functional proteins.
That's not ideal long -term.
It's quite a trade -off.
And when you use amino acids for glucose, you have to deal with the nitrogen part, don't you?
That leads to urea.
Correct.
The carbon part goes to glucose, but the nitrogen is potentially toxic.
So the liver converts that nitrogen into urea.
Which is safe and easily excreted by the kidneys.
Exactly.
It's a crucial detoxification step coupled with glucose production.
Let's make this real.
The text mentions a patient per CV with mild malnutrition.
After an overnight fast, his blood glucose was a bit low, 72mgdL.
Normal is like 80 -100, too.
Right.
That 72 tells us his liver was already working pretty hard.
Glycogen was likely low or gone.
So gluconeogenesis was plugging away, trying to keep that glucose level up.
And his ketone bodies were slightly elevated, too, at a 110 mying compared to a normal of maybe 70.
Yes.
That shows his fat breakdown lipolysis was active and the liver was already starting to make ketones.
Even in just mild malnutrition after an overnight fast, his body was already shifting metabolic gear significantly.
So that covers the early stages.
Quick switches, using glycogen, starting fat breakdown and gluconeogenesis.
But what happens if the fast continues, not just overnight, but for days?
We enter starvation, right?
Right.
When you go beyond about three days without food, you're in the starved state.
And the body's priorities shift fundamentally.
Oh, so.
The absolute top priority now becomes conserving protein.
If the body kept breaking down muscle for glucose at the initial rate, you'd run into serious trouble very quickly.
Ah, the protein -sparing effect.
This is where the body gets really clever, isn't it?
How does it manage that?
It's an amazing adaptation.
So muscles keep burning fatty acids as their main fuel, but they actually decrease their use of ketone bodies.
They use less ketones.
Why?
Because it leaves more ketones available in the blood.
The liver keeps making them from fatty acids.
So ketone levels in the blood rise significantly, maybe into the millimolar range, much higher than Percy's 110 micromolar.
OK, so ketone levels are way up.
And this is where the brain comes in differently.
This is the absolute key.
With ketone levels that high, the brain starts actively taking them up and using them for energy.
It adapts its own metabolism.
So the brain starts burning ketones.
A significant amount, yes.
It doesn't completely stop using glucose, but its reliance on glucose drops substantially, maybe by 40 % or even more.
It learns to use this alternative fuel floating around.
And if the brain needs less glucose.
Then the liver doesn't have to make as much glucose via gluconeogenesis.
Which means less breakdown of muscle protein to supply those amino acids.
Exactly.
That's the protein sparing effect in action.
Less protein breakdown means less muscle wasting.
And it also means less urea production because you're processing fewer amino acids.
It dramatically extends survival time.
Wow.
So throughout all this, from the first hours to deep starvation adipose tissue,
our fat is the main hero fuel supplier.
Absolutely.
It just keeps steadily releasing fatty acids and glycerol.
Our fat stores are fundamentally our long -term energy reserves designed for exactly these situations.
But even this adaptation has limits.
People can't survive indefinitely without food.
What's the breaking point?
Right.
Metabolism can only do so much.
Survival ends when critical body mass is lost.
Usually around 40 % of total body weight.
And that relates to losing too much protein or fat.
Both.
It corresponds roughly to losing maybe 30, 50 % of your body protein, which means organs start failing or depleting almost all your fat reserves, maybe 70, 95 % gone.
That translates to a very low BMI, doesn't it?
Extremely low.
Around BMI 13 for men, maybe 11 for women.
At that point, it's the loss of essential tissue protein combined with severe electrolyte imbalances and micronutrient deficiencies that leads to organ failure and death.
Let's look at the other clinical example, Anar with severe anorexia nervosa.
BMI 13 .7, grade 3 malnutrition.
She's deep in that starved state.
Definitely.
Her body's physiology would fully reflect that prolonged starvation adaptation.
Her blood glucose was even lower, 65mgdL.
But she wasn't necessarily comatose from hypoglycemia.
Probably not, because her brain would have significantly adapted to using those high levels of ketone bodies.
That ketone usage protects the brain, reducing its glucose need and thus helping to spare her remaining muscle protein.
So her ketone levels would be very, very high.
Detectable in urine too.
Oh,
certainly.
Much higher than Percy's, likely in the millimolar range in her blood and easily spilling into the urine, which we call ketonuria.
The case also mentions a mannaria loss of menstrual periods.
That's linked to low body fat.
Yes, it's a common physiological response.
When body fat percentage drops below a certain critical threshold, often cited around 22%, reproductive functions shut down.
It's the body conserving all possible energy just for survival.
And her case highlights the dangers of refeeding too, right?
You have to be careful putting nutrition back in.
Absolutely critical.
Restoring nutrition has to be done carefully, often with tube feeding and expert dietitians, to avoid refeeding syndrome, which is a dangerous metabolic shift that can happen when severely malnourished people are fed too quickly.
These examples really show the power of understanding the biochemistry.
But how do doctors actually track these internal states?
We can't just peek inside cells easily.
Right, that's where those biochemical markers in blood and urine become so important.
There are windows into what's happening metabolically.
Simple tests giving big clues, like blood urea nitrogen, BUN.
Exactly.
We talked about urea being made in the liver from amino acid breakdown and excreted by the kidneys.
So low BUN might point to liver issues.
It's not making urea properly.
And high BUN.
Could mean the kidneys aren't clearing it properly, or it could just mean high protein breakdown is happening.
You have to interpret it in context.
What about creatinine and that creatinine height index, CHI?
Creatinine comes from muscle breakdown, specifically creatine phosphate, at a pretty constant rate relative to muscle mass.
So low urinary creatinine excretion over 24 hours, or a low CHI.
Which compares your output to a healthy person of the same height.
Right.
That suggests you've lost muscle mass, like Percy V, with his 85 per cchi, a mild deficit.
Now, high blood creatinine usually points towards the kidneys not filtering it out effectively.
And ketones, of course.
High levels in blood or urine generally mean fat burning and the starved state adaptation.
Generally, yes.
It signals that shift towards fat and ketone reliance, but there's that crucial exception.
Ah, diabetes.
Yes.
If you see high ketones plus high blood glucose, that screams type 1 diabetes.
The body thinks it's starving because insulin isn't there to let glucose into cells, even though blood glucose is sky high.
It's a very different, dangerous situation.
Lastly, albumin and prealbumin, proteins made by the liver used to assess nutritional status.
Correct.
They reflect the body's overall protein situation and the liver's ability to synthesize proteins.
Prealbumin changes faster half -life of just 2 -3 days, so it tells you about recent nutritional changes.
While albumin, with its longer half -life of maybe 2 -3 weeks, reflects more long -term protein status.
Exactly.
Percy V had low levels of both, indicating both recent and more chronic protein malnutrition were likely factors.
So, understanding the underlying fuel flows, glycogen, fat, glucose, ketones, is really the key to making sense of these lab tests and figuring out what the body's doing.
It absolutely is.
The tests are just numbers unless you understand the metabolic story they're telling.
It's been quite a journey, seeing how the body shifts gears from using up liver glycogen to mobilizing fat, kicking off gluconeogenesis, and then the big adaptation, using ketone bodies to spare protein.
And all orchestrated beautifully by hormones like insulin and glucagon, it's an incredibly elegant survival system.
Truly amazing.
Which leaves us with a thought to ponder.
This incredible metabolic flexibility was honed over millennia when finding food wasn't guaranteed, right?
It's an ancient survival kit.
Definitely forged by evolution.
So, as we explore things like intermittent fasting today, in our world where food is usually abundant,
how might be originally deliberately activating these ancient pathways affect us?
Our individual metabolisms, our long -term health is good, bad, complicated.
That's a fantastic question.
How do these ancestral adaptations play out in our very modern context?
Something for everyone to think about.
We hope this deep dive into the biochemistry of fasting has given you some real food for thought about your own body's amazing capabilities.
Thank you for joining us.
From the Last Minute Lecture Team, thanks for tuning in.