Metabolic Changes During Fasting and Starvation

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Everyone dr. Mungli here so in this video. I will be explaining you the metabolic metabolic changes that are going on in our body during fasting condition and starvation during conditions when we are going to say a person is in fasting condition strictly speaking fasting condition it begins 10 minutes after the meals thats strictly speaking otherwise loosely speaking so we just say two to three hours after meals.
It says. A person is in fasting condition. Actually that is post absorptive condition.
Anyway for our purpose here so in this video. I will be considering 2 to 3 hours after your last meal as a fasting condition. Thats what i have written here so.
Two hours after meals and then 12 hours after meals and up to 24 hours after the meals. I have this written it as a fasting condition here and then after that it is in days and then in weeks. Now anyone who is not taking food more than 24 hours then they just get into starvation that is what is referred as starvation fasting is within 24 hours.
We are not taking anything for 24 hours that is in fact that is basically fasting and if you dont take anything more than 24 hours. That is basically referred as starvation. Okay now what happens.
During fasting condition or two to three hours after the meals. So two to three hours after meals our blood glucose level falls down. So im going to write down the blood glucose concentration.
So blood glucose it continuously falling down. And then it will just make the constant phase. Okay so this is what happens under fasting conditions and the blood glucose.
This is this line is indicative of blood glucose. All the things that im writing here is in the blood. So i will write it has glucose here so glucose levels fall.
So as you can see continuous fall in the glucose level. And then it just maintains a constant phase. There now as the blood glucose level falls.
Your insulin levels will also fall down as the insulin level fall. So you all know glucagon levels will rise so look. Im going to write a glucagon level.
So. The insulin is coming from beta cells of pancreas and glucagon is coming from alpha cells of pancreas. And generally insulin has got a negative effect or inhibitory effect on alpha cells of pancreas.
So as the insulin levels fall. So. The glucagon level will rise.
So that will be rise in the glucagon levels. And the insulin level is falling down. So it will be like this so we have glucagon here.
So now. What is the metabolic effect of glucagon on our body. So you know glucagon is a catabolic hormone as opposed to insulin insulin is a anabolic hormone now a person is in fasting conditions.
A two to three hours after meals so slowly insulin is dropping down and glucagon is rising. So this glucagon is going to break down or energy sources. What are the energy sources that we have so we have glycogen in the liver.
We have glycogen in the skeletal muscle and other tissues. So glucagon. It is going to go and binds to glucagon receptors in the hepatocytes and its going to break down glycogen by stimulating.
An enzyme called glycogen phosphorylase enzyme. Now this degradation is going on in the liver. So im going to write that glycogen degradation.
Now so it starts two to three hours after meals and it peaks by 12 hours. And it basically it is depleted by 24 hours of fasting. So glycogen degradation.
This is the glycogen degradation here mediated by glycogen phosphorylase and that is stimulated by glucagon hormone. So glycogen degradation starts two to three hours of the meals peaks by twelve hours and depletes completely by 24 hours.

an individual who is fasting will deplete his or her energy stores in about-0
an individual who is fasting will deplete his or her energy stores in about-0

It can vary from person to person depending on their carbohydrate previous carbohydrate intake. Some people their glucose and glycogen can last only for 12 hours. And some other it can last for 24 hours.
In a person. Who is taking well balanced diet. Who is 70 kg in those people.
So glycogen can lost for 24 hours. So. This glycogen.
Which is breaking down into glucose 1 phosphate further. It is converted into glucose and that will maintain the blood glucose level under fasting condition. So basically person will have a fasting blood glucose level.
So now what else is going on here so as the glucagon is increased here so as the glucagon is increased here glucagon not only breaks up glycogen stores. It is also going to break a lipid store present in our body. And where do you have a lipid store.
Where is the lipid stored in our body. It is stored beneath our skin. That is adipose tissue adipose tissue has got white adipose tissue has got triacylglycerol and this triacylglycerol can be broken down to give energy for ourselves so glucagon.
It is going to go and activate hormone sensitive lipase glucagon is binding to glucagon receptors on the adipose tissue and its going to activate hormone sensitive lipase and that hormone sensitive lipase. It is going to release free fatty acids and glycerol into the blood and free fatty acids. Will be taken up by peripheral tissues.
Almost all tissues in our body. Except red blood cells. And the brain so they can use free fatty acids for their energy.
So they will conduct beta oxidation process skeletal muscles cardiocytes all other tissues including the liver. Its going to conduct the beat oxidation breakdown fatty acids and releases acetyl coa and acetyl coa get into tca cycle to give energy. Okay so in the influence of glucagon.
So your free fatty acids floods into the blood. So im going to write that now so two to three hours after meals so free fatty acid. Continuously.
Theyre going to rise as the fasting goes on and as the person gets into starvation free fatty acid rises. So these free fatty acids will be used by almost all tissues in our body. Except red cells.
And the brain. So let me gradual means there will be continuous rise in the fatty acids in our body. Okay so now the glycerol that is coming from triacylglycerol as the adipose tissue is broken down into free fatty acid.
And the glycerol. So glycerol is taken up by the liver free fatty acids. Predominantly taken up by the liver and the other tissues glycerol mainly it will be taken up by the liver.
So what liver does with the glycerol glycerol. It will be converted into glucose in the liver liver has glycerol kinase enzyme. Glycerol.
3. Phosphate dehydrogenase enzyme and ultimately glycerol is going into glucose formation by a process called as gluconeogenesis. So not only glycerol coming from triacylglycerol gets into gluconeogenesis gluconeogenesis can also be done with using lactate lactate is coming from red blood cells.
Because red blood cells. They conduct anaerobic glycolysis so in anaerobic glycolysis you get lactate lactate gets into the blood. It will be taken up by the liver and heart so in the liver.
What happens. The lactate is converted back into glucose by gluconeogenesis now some of the amino acids can also be converted into glucose like out of 20 amino acids 18 of them can be converted to glucose so lets see what is the timeline for gluconeogenesis gluconeogenesis. It starts two to three hours after meals.
But it will be there at basal level. So this is the gluconeogenesis line here it will be there at the base level. Because you are still glycogen degradation is going on so at 12 hours your glycogen degradation is at peak and it is depleted by 24 hours and the glycogen degradation is depleted.
So the gluconeogenesis that is going on at a very basal level here it will start to rise or 12 hours by 12 hours. It is steaming up and by 24 hours.

an individual who is fasting will deplete his or her energy stores in about-1
an individual who is fasting will deplete his or her energy stores in about-1

It is at peak gluconeogenesis is at peak by 24 hours by the time your glycogen degradation is already down it is totally depleted. So now the glucose maintenance. This is the glucose here so the maintenance of blood glucose level.
It will be done by gluconeogenesis process. Now as the gluconeogenesis process reaches peak here by 24 hours your gluconeogenesis is at peak by that time. You know that the glycogen degradation degradation is already down here or total depleted.
Now the gluconeogenesis further it is going to continue for at the peak level for another one week basically for one week. It will be on peak as you can see this is one week period here soon gluconeogenesis is at peak and after that it is going to fall down gluconeogenesis is going to drop by second week. And then it just maintains at a constant level.
Okay. So now what all the sources. There you can see as the gluconeogenesis is falling down.
Even the blood glucose levels also falls down and just maintaining a constant pace. Now what all the sources for what all the precursors for gluconeogenesis under this condition. So initially from 2 to 3 hours.
And then 12 hours up to 24 hours in this particular periods. How the glucose is made it is made from by converting glycerol into glucose glycerol. You know it is coming from triacylglycerol.
It is made from converting lactate into glucose in the liver. And also some of the amino acids present in the hepatocytes and also amino acids coming from the muscle not really because of breakdown of skeletal muscles of breakdown of skeletal muscle. Doesnt go on until more than 24 hours to 48 hours.
It takes 24 hours to 48 hours for our skeletal muscle proteins to undergo proteolysis. So the amino acids that are coming from skeletal muscle. Actually they are all come they are all coming from the transamination process.
What happens during transamination let me explain they transamination. So whenever person is in fasting condition. See.
There is a glucagon is elevated releasing free. Fatty acids. Free.
Fatty acids. Gets into the skeletal muscles. Skeletal.
Muscles will conduct. The beta oxidation process and they will get acetyl coa from beta oxidation acetyl coa level rises as the acetyl coa level rises acetyl coa will have a negative effect on pyruvate dehydrogenase complex so that means the glucose. Which is converted to pyruvate.
Which is further is not converted to acetyl coa. Because excess acetyl coa from beta oxidation of fatty acids in skeletal muscle will have a negative effect on pyruvate dehydrogenase complex. There by pyruvate accumulates and this accumulated pyruvate.
It will undergo transamination reaction. So with the glutamate amino acid glutamate. So in this transformation.
What happens pyruvate is converted to alanine and glutamate is converted to alpha keto glutarate. You can watch my video on transamination. The link for that is appearing right now on the right upper corner now with pyruvate.
Undergoing transamination. It is converted to a alanine and the alanine is coming into the blood and that will be taken up by the liver and that will participate in gluconeogenesis process. That is what happens within 24 hours of this gluconeogenesis period.
So the major sources or the precursors for gluconeogenesis within that 24 hours. I would say it is glycerol lactate and the amino acid coming from the muscle specially from transamination reaction. Which is coming from pyruvate okay.
After 24 hours. What happens so after 24 hours. 24 to 48 hours period.
So it is the proteolysis that can go on in the skeletal muscle especially because by that time even the cortisol will be elevated. Because the person is in starvation.

an individual who is fasting will deplete his or her energy stores in about-2
an individual who is fasting will deplete his or her energy stores in about-2

Its a stressful situation. So the cortisol will build up in the blood. So that cortisol can lead to proteolysis in the skeletal muscle and skeletal muscle proteins will be broken down to release more amino acids.
So that amino acids will go to the liver and in the liver. Some of these amino acids go into gluconeogenesis process to maintain blood glucose level and also some amino acids in the liver can be going into protein synthesis to make essential proteins that are needed for anabolic processes in the liver. That is why the skeletal muscle protein acts as a source for amino acids in the liver.
But so far that happen. The starvation has to be 24 48 hours and then continues there until like 7 days after that you see there is a drop in these gluconeogenesis. Because by that time 50 to 60 of the skeletal muscle is already proteolysed.
So growth hormone comes in and it will decrease. The proteolysis and thereby gluconeogenesis this drops and continues and the constant phase and in this gluconeogenic precursors are glycerol and lactate again. Because glycerol is coming from continuous degradation of triacylglycerol into free fatty acids and glycerol and the lactate is coming from anaerobic glycolysis.
So this is what happens in relation to gluconeogenesis process. Now see a the free. Fatty acids.
Are coming into the liver. So. Liver.
Is. Conducting beta oxidation beta. Oxidation will give you acetyl.
Coa and acetyl coa. Will saturate. Tca cycle.
Once tca cycle is saturated. So excess acetyl coa. They will be diverted into ketone body formation ketone bodies for formation note like 12 hours after fasting.
They basically are at the baseline. You wont really see them much in the blood. But after 24 hours so they start to appear in the blood.
So by the three days as you can see there is a significant appearance of ketone bodies. And they are going to rise as the starvation like fasting and going into starvation as the starvation occurs ketone body rises. This is the ketones here so ketones will rise they will appear significantly twenty four hours after starvation and they are continuously rise as long as free fatty acids are available.
Then it now you have most of our cells. When when a person is in fasting condition. They are all like adopted like theyre using free fatty acids as the ketone bodies are appearing in the blood.
So our cells will adopt themselves to use ketone also specially the brain brain takes two to three days to adopt for ketone. Bodies. And start using ketone bodies as the brain start using ketone bodies by third day like around 30 percent of the energy needs of brain will come from ketone bodies that means.
It is going to save this glucose for red blood cells. So because it knows there as the days goes you know gluconeogenesis is going to fall down blood glucose level goes to go down that means by that time your brain is already adapting itself to use ketone bodies by one week. Brain is almost using ketone bodies as its energy source.
It is sparing glucose for red blood cell in the same way the skeletal muscle and other tissues like heart. They all will use ketone bodies. Eventually.
They will revert back to use of fatty acids. Because ketone bodies are predominantly used by the brain. After one to two weeks of starvation it so this is how the metabolic adaptations will go on in our bodies.
For that there will be a lot of hormonal changes going on so insulin dropping glucagon increasing and later on cortisol coming in and then growth hormone coming in so all these hormones. They will alter the metabolic pathways and make sure that our cells will get all the energy that they need so in one or the other form. It initially it will be coming from a glycogen.
Giving glucose after that gluconeogenesis giving glucose and then the free fatty acids. Giving energy and later body getting into keto adaptation process. So this is all about metabolic changes that are going on during fasting condition and starvation.
I hope this video has helped you in understanding this concept. If you have any question. So kindly let me know about that in the comment section below.
I will answer to them as soon as possible so thanks for watching and for regular updates. You can consider subscribing to this channel and also click on the bell button there so that you get the notification as in when i upload the video thanks again and have a good day .

an individual who is fasting will deplete his or her energy stores in about-3
an individual who is fasting will deplete his or her energy stores in about-3

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