MCAT questions!

[qweewq11]

Can someone explain why the answer to this question is B? I couldn't decide between B and D. Isn't converting fatty acids to glucose gluconeogenesis? And I thought all the glycogen stores were depleted after 1 day, so the body woulds start burning fat?

Ater one day of starvation, which of the following is most likely to occur?

A. Epinephrine stimulates glycogen synthesis in muscle to protect the energy supply of muscle
B. Glucagon stimulates glycogen phosphorylase activity and gluconeogenesis in liver
C. Glycogen stores are converted into acetyl-CoA and, subsequently, fatty acids
D. Fatty acids are converted into glucose
E. Badger

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[Mr. Z]

Its not D because fatty acids don't get converted to glucose. Fatty acids, when metabolized, will enter into the krebs cycle via beta oxidation.

 

[souljah1]

Usually after 24 hours, glycogen stores are essentially depleted. However, after one day of fasting glucagon would be elevated and glycogenolysis would be activated via signal transduction pathways. Whatever glycogen that is still in the liver, muscle, or kidney..would be getting broken down. Gluconeogenesis would be elevated b/c glucagon stimulates the regulated enzymes of the gluconeogenesis pathway. So B is very much correct.

D is completely wrong. Fatty acids do not contribute Carbon gluconeogenesis. Fatty acids are broken down into Acetyl CoA, which enters the TCA cycle. Glucogenic amino acids contribute carbon for gluconeogenesis. Fat does not get made into Carbohydrate, but the reverse can happen (though does not really happen all that much in humans).

 

[limit]

Originally posted by souljah1
Fat does not get made into Carbohydrate, but the reverse can happen (though does not really happen all that much in humans).

Yes it does!!

 

[souljah1]

Originally posted by limit


Yes it does!!

You might think so, but it really doesn't. Look up Hellerstein et al..They've done brilliant work following high carbohydrate diets. High carbohydrate diets will lead to a decrease in mobilization of fatty acids in adipose as well as a decrease in oxidation of fatty acids eaten in diet. The effects are massive preservation of adipose occurring simulataneously with a decreased use of them. Our body will choose carbohydrates to oxidize in time of plenty. Yes, some will be made into fat..but it is very little. Tracer studies have shown this. Other animals are much more efficient at storing excess CHO as Fat, but humans are not.

 

[Sonya]

I'd guess B, but i ' m not sure.

D is ou t, fatty acids straigt enter kreb cycle.

C. I doubt it. Why would you be making fatty acids when you need USABLE energy? not likely.


A may be true. But, wouldn't the body be more likely to be protecting glucose levels over the muscle glycogen supply?

not sure, though.

 

[limit]

With the MCAT tomorrow morning, I'll have to take your word on it since I have no time to hunt for sources. Efficiency in humans vs. animals is one particular aspect of this research that I have never considered, simply because it is of no use to me. I don't recall the Hellerstein work, but does it by any chance have anything to do with an experiment where they had told a group of obese individuals to eat as much bread as they could in the morning and then continue their regular routine for the rest of the day? Regardless, as far as my homework goes, if your muscle and liver glycogen stores are full, there is but no choice than to store incoming carbs as palmitic acid (saturated fat). The high carbohydrate diet, so heavily promoted, demands excessive insulin production in order to be metabolized. Excessive insulin production causes insulin resistance. Insulin resistance causes nutrients in the blood to bypass muscle cells, only to be readily stored in fat cells, which are not insulin resistant.

Originally posted by souljah1


You might think so, but it really doesn't. Look up Hellerstein et al..They've done brilliant work following high carbohydrate diets. High carbohydrate diets will lead to a decrease in mobilization of fatty acids in adipose as well as a decrease in oxidation of fatty acids eaten in diet. The effects are massive preservation of adipose occurring simulataneously with a decreased use of them. Our body will choose carbohydrates to oxidize in time of plenty. Yes, some will be made into fat..but it is very little. Tracer studies have shown this. Other animals are much more efficient at storing excess CHO as Fat, but humans are not.

 

[Neuronix]

Originally posted by qweewq11
Can someone explain why the answer to this question is B? I couldn't decide between B and D. Isn't converting fatty acids to glucose gluconeogenesis? And I thought all the glycogen stores were depleted after 1 day, so the body woulds start burning fat?

Ater one day of starvation, which of the following is most likely to occur?

A. Epinephrine stimulates glycogen synthesis in muscle to protect the energy supply of muscle
B. Glucagon stimulates glycogen phosphorylase activity and gluconeogenesis in liver
C. Glycogen stores are converted into acetyl-CoA and, subsequently, fatty acids
D. Fatty acids are converted into glucose
E. Badger

You do make a good point that glycogen would likely be depleted in the liver after 1 day. I learned that glycogen acts a short-term store and is generally depleted in something like 4 - 6 hours. However, the rest of the answers are just plain wrong. Here's a breakdown:

A - Glycogen is not synthesized in muscle. It is synthesized in the liver, and that is only when the body is full of food, not deprived of it.
B - This is correct. Gluconeogenesis = "The generation of new glucose". This does occur in the liver and is a function of the steroid glucagon (pneumonic - "Gluc(ose) all gone"). The main pathway for this has to do with the conversion of glycogen to glucose.
C - This again occurs when the body is full of food, not deprived of it.
D - This requires pretty good physiology/biochemistry knowledge. Fatty acids cannot be converted to glucose. Rather, they are degraded into Acetyl-CoA which enters the citric acid (Kreb's) cycle.
E - I prefer beaver myself.

 

[souljah1]

Originally posted by limit
Regardless, as far as my homework goes, if your muscle and liver glycogen stores are full, there is but no choice than to store incoming carbs as palmitic acid (saturated fat). The high carbohydrate diet, so heavily promoted, demands excessive insulin production in order to be metabolized. Excessive insulin production causes insulin resistance. Insulin resistance causes nutrients in the blood to bypass muscle cells, only to be readily stored in fat cells, which are not insulin resistant.

Maybe if someone where to eat a high carbohydrate diet around 4000+ calories/day...otherwise almost all fat that is consumed would be stored, lipolysis would decrease dramatically, and almost all energy would be derived from carboydrate oxidation. Please, trust me on this one. As far as the insulin resistance thing goes. High carbohydrate diets do not lead to insulin resistance. Insulin is not necessarily excessive. Usually it is secreted at the proper concentration to lead to enough GLUT4 receptors to absorb glucose. High carb diets do not necessarily lead to insulin resistance. Obesity, on the other hand, usually is associated with insulin resistance, but that has more to do with the paracrine/hormonal effects of adipose tissue and the inflammation promotatory compounds it secretes such as TNFa and IL6. High carb diets, when not eaten in excessive caloric intake, will not even lead to hypertriglyceridemia..and there is no research that shows that a high carb diet (without positive energy balance) leads to insulin resistance.

Anyway...best of luck tomorrow on the MCAT.

With certainty, the answer is B.