drduck
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A 26 yr old womwn is stranded for more than a week when her hiking trip is interrupted by an avanchle. bcos she expexted to be gone foronly few hours, she didnt not bring any food.
which of the following substances can be converted to glucose to supply the needs of the brain during this period...
acetoacetate
acetone
amino acids
beta hydroxybutraye
fatty acids
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| ssrpk
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amino acids
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| pat05
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why is it amino acids?
at first i was thinking about ketone... but then acetoacetate, b hydroxyb, and acetone would all imply that. ketone bodies are produced after days w/o eating..?
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| SILVER
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y not fatty acids?
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| doc179
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I was also thinking the same way, but I guess this is why it can not be fatty acids : fatty acids undergo beta oxidation to produce acetyl coA which is converted to ketone body which is used as energy by thr brain. fatty acids are not a source of glucose, they are not used for gluconeogenesis. The qn asks which is the surce of glucose , so I guess amino acids is the answer. Lets wait for dr duck.
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| Luckyall
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after 1 week---> brain switches to KETONES as preferential energy fuel.
Ketones bodies--> decr. Alanine release from muscle& decr demands for producing gluc.--> in turn helping Protein conservation( decrease glucose production is essential for conserving protein muscle)----> interruption alanine cycle. As long as the ketones bodies conc is inc-----> proteolysis is restricted& conservation of muscle occurs .
After 1 week starvation---> brain uses KETONES bodies & Muscle&liver ---> fatty acids !!!
Among the 3 ketones , i d go for Acetoacetate, since is is the first formed, then 2nd i think is betahydroxybutyrate, and as far as Acetone---> is just a minor side product and is not even used as fuel in tisues.
My answer is primarly acetoacetate and 2 nd beta hydroxybutyrate
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| Luckyall
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Ohh, Qs ask" which of the following substances can be converted to GLUCOSE to supply the needs of the brain during this periods" and after 1 week brain preferentially gets ketones and it decreses gluc production from AminoAcids....
Muscle&liver use F A , brain preferentially gets Ketones and proteolysis is already restricted
So, A A surely is the answer
Edited by Luckyall on 06/06/07 - 11:08 AM
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| SILVER
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good point doc179
i realized fatty acid can't be the right answer also because the question is asking about the brain. and fatty acids can't cross the BBB therefore they can never be a source of energy for the brain.
revised some biochem. and read that some organs do depend on fatty acids for energy during prolonged fasting, but not the brain. in a prolonged fasting situation the brain gains the capacity to use ketones for energy.
so i guess the ans. should be acetoacetate
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| doc179
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silver good point, forgot to mention tht FA can not enter the BBB. But then they can not be converted to glucose and the ketone bodies also can not be converted to glucose and the qn asks which can be converted to glucose. remember the graph which shows the source of energy to the brain and the days of starvation on the x axis, there after a week of starvation brain mostly uses KB but protein breakdown is also still positive. if the qn asked which is source of energy I would have chosen KB but it says which is converted to glucose and amongst the choices only amino acid can be converted to glucose. I might be wrong, but this is what I feel right now.
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| SILVER
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well ur right doc179, the ques. is asking specifically what can be converted into glucose for the brain to use. emphasis is on the word glucose and not "energy".......
but there's another aspect to all of this--isn't the limited amount of glucose that is in the body by now in this hiker being redirected to the brain anyway?? and eventually i think the brain even spares that and shifts entirely to KB so that the glucose can be saved up and given entirely to RBCs.
i think the ques. would make more sense if it was talking about sources of 'energy' and not glucose.
Edited by silver on 06/06/07 - 12:44 PM
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| whournameiz
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i think it is specifically refering to glucose, and none of the option can be converted to glucose besides amino acids
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| new_n_lost
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In Starvation State the Brain Uses Moslty Ketones For energy purposes Ur basic substrate for Ketones bodies are Fatty Acids. The Brain only absorbs Ketones and starts using them in the CAC(citric acid cycle). All the Choices will result in Glucose Production but in Starvation the basic substrate for the production of Acetyl CoA are Fatty Acids. Brain itself doesnt use AA or FFA or makes the Ketones for energy purposes; for that matter it heavily relies on ketones which r coming from FFA form the liver cos if the amino acids r the substrate then we will have increased muscle protein catabolism for the availability of AA for the production of glucose.
In Starvation the Liver rate of Gluconeogenesis decreases cos as the supply of AA decreases cos they will catabolize more of muscle proteins therefore after 3-5 days of starvation the reliance is on FFA and Ketones to maintain Blood Glucose.
So i my Opinion it shud be FFA. U cant Chose Choices 1 and 4 cos they both r Ketones and Choice 2 is not heavily metabolised therefore u have fruity breath of Ketoacidosis. Choice 3 is the last resort of the Body after Very long period of Starvation such as > 10 days thus leaving the Choice 5.
PLZ FOR THE LOVE OF GOD POST COMPLETE QUESTION OR ATLEAST POST THE WORDS WHICH TRULY MAKE SENSE COS POSTING INCOMPLETE QS RESULTS IN HALF OF THE PPL WHACKING THEIR BRAINS OVER A SIMPLE CONCEPT AND THEN ENDING UP IN DOUBTING THEMSELVES
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| new_n_lost
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JUST TO MAKE THE POINT MORE CLEAR
PLZ FOR THE LOVE OF GOD POST COMPLETE QUESTION OR ATLEAST POST THE WORDS WHICH TRULY MAKE SENSE COS POSTING INCOMPLETE QS RESULTS IN HALF OF THE PPL WHACKING THEIR BRAINS OVER A SIMPLE CONCEPT AND THEN ENDING UP IN DOUBTING THEMSELVES
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| SILVER
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ok, after thinking about it more and reading nnl's expln, which also clears up some concepts.
i'm sticking to my original ans. of fatty acids.
in the initial 12 hrs. the brain switches from glucose derived from liver glycogenolysis to glucose derived from gluconeogenesis. then after a week's time, the brain again switches, this time from glucose derived from gluconeogenesis to ketones derived from fatty acids.
Glycolysis and subsequent glucose uptake in the brain decreases, and this switch spares the protein thereby ultimately allowing the brain to indirectly metabolize fatty acids as ketone bodies. This does not mean the FA enter the brain, that is not possible at all, rather the FA are metabolized in the body to ketone bodies which are then supplied to the brain....AND this is what the ques. is really asking---->which of the following can be supplied to the brain for the glucose--->ketone bodies derived from FA.
important to note is also where we are on the timeline of starvation--->just over a week, so right now the body can't be going after the amino acids. when amino acids start being utilized that's the last resort and indication that the starvation is beyond 10/15 days.
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| ssrpk
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well USMLE guys are smart enough not to unnecassarily play with words, so we shouldn't assume what they mean rather we should try to concentrtate on what the question is asking.....
its true that after starving for a prolonged period brain switches to ketones but still maybe even less than 10% glucose is still being utilized, and even after one week of starvation the only place glucose can come from is still gonna be amino acids...and that is exactlyt what the question has asked in clear words, which of the following can be converted into glucose....not a very fair question probably!
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| doc179
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exactly that was what I was or trying to say. Thanx ssrpk.
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| SILVER
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y would the body turn to amino acids this early on in starvation??? body only starts consuming the amino acids for glucose when all other stores are depleted and that happens much later on in starvation; it's only been just over ONE week
it would make more sense to metabolize the FA for the supply of glucose for the brain.
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| doc179
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silver pls go thru that picture I told you about in biochem. You would see there that first 12hrs its glycogenolysis, next 12hrs to one week its protein breakdown( gluconeogenesis) , after one week its mostly ketone bodies and little protein.
Edited by doc179 on 06/06/07 - 06:07 PM
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| new_n_lost
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Ok Big Time Mistake on my part Didnt Understand the Question
AcetylCoA doesnt get into the Gluconeogenesis pathway i.e
FFA --> AcetylCoA --> TCA but the Acetyl CoA is not forming Pyruvate so therefore AcetylCoA is not a precursor for Glucose.
And AA r substrate of Glucose.
Man I so need to go thru Biochem.
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| SILVER
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alright, all cleared up....i misunderstood the part about the malate shuttle in the TCA
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