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# How does the brain know when carbohydrates have been or should be cons

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How does the brain know when carbohydrates have been or should be cons  [#permalink]

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Updated on: 22 Sep 2019, 01:32
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New Project RC Butler 2019 - Practice 2 RC Passages Everyday
Passage # 171, Date : 27-APR-2019
This post is a part of New Project RC Butler 2019. Click here for Details

How does the brain know when carbohydrates have been or should be consumed? The answer to this question is not known, but one element in the explanation seems to be the neurotransmitter serotonin, one of a class of chemical mediators that may be released from a presynaptic neuron and that cause the transmission of a nerve impulse across a synapse to an adjacent postsynaptic neuron. In general, it has been found that drugs that selectively facilitate serotonin-mediated neurotransmission tend to cause weight loss, whereas drugs that block serotonin-mediated transmission often have the opposite effect: they often induce carbohydrate craving and consequent weight gain.

Serotonin is a derivative of tryptophan, an amino acid that is normally present at low levels in the bloodstream. The rate of conversion is affected by the proportion of carbohydrates in an individual’s diet: carbohydrates stimulate the secretion of insulin, which facilitates the uptake of most amino acids into peripheral tissues, such as muscles. Blood tryptophan levels, however, are unaffected by insulin, so the proportion of tryptophan in the blood relative to the other amino acids increases when carbohydrates are consumed. Since tryptophan competes with other amino acids for transport across the blood-brain barrier into the brain, insulin secretion indirectly speeds tryptophan’s entry into the central nervous system where, in a special cluster of neurons, it is converted into serotonin.

The level of serotonin in the brain in turn affects the amount of carbohydrate an individual chooses to eat. Rats that are allowed to choose among synthetic foods containing different proportions of carbohydrate and protein will normally alternate between foods containing mostly protein and those containing mostly carbohydrate. However, if rats are given drugs that enhance the effect of serotonin, the rats’ carbohydrate intake is reduced. On the other hand, when rats are given drugs that interrupt serotonin-mediated neurotransmission, their brains fail to respond when carbohydrates are eaten, so the desire for them persists.

In human beings a serotoninlike drug, d-fenfluramine (which release serotonin into brain synapses and then prolong its action by blocking its reabsorption into the presynaptic neuron), selectively suppresses carbohydrate snacking (and its associated weight gain) in people who crave carbohydrates. In contrast, drugs that block serotonin-mediated transmission or that interact with neurotransmitters other than serotonin have the opposite effect: they often induce carbohydrate craving and subsequent weight gain. People who crave carbohydrates report feeling refreshed and invigorated after eating a carbohydrate-rich meal (which would be expected to increase brain serotonin levels), in contrast, those who do not crave carbohydrates become sleepy following a high-carbohydrate meal. These findings suggest that serotonin has other effects that may be useful indicators of serotonin levels in human beings.
1. Which one of the following best states the main idea of the passage

(A) The body’s need for carbohydrates varies with the level of serotonin in the blood.
(B) The body’s use of carbohydrates can be regulated by the administration of serotoninlike drugs.
(C) The role of serotonin in regulating the consumption of carbohydrates is similar in rats and in humans.
(D) The body’s desire for carbohydrates can be influenced by serotonin or serotoninlike drugs.
(E) Tryptophan initiates a chain of events that regulates the body’s use of carbohydrates.

2. The term “rate” (Highlighted) refers to the rate at which

(A) serotonin is produced from tryptophan
(B) carbohydrates are taken into the body
(C) carbohydrates stimulate the secretion of insulin
(D) insulin facilitates the uptake of amino acids into peripheral tissues
(E) tryptophan enters the bloodstream

3. It can be inferred that a person is likely to crave carbohydrates when

(A) the amount of insulin produced is too high
(B) the amount of serotonin in the brain is too low
(C) more tryptophan than usual crosses the blood-brain barrier
(D) neurotransmission by neurotransmitters other than serotonin is interrupted
(E) amino acids other than tryptophan are taken up by peripheral tissues

4. The information in the passage indicates that if human beings were given a drug that inhibits the action of serotonin, which one of the following might be expected to occur?

(A) Subjects would probably show a preference for carbohydrate-rich snacks rather than protein-rich snacks.
(B) Subjects would probably become sleepy after eating a carbohydrate-rich meal.
(C) Subjects would be more likely to lose weight than before they took the drug.
(D) Subjects’ blood tryptophan levels would probably increase.
(E) Subjects’ desire for both carbohydrates and proteins would increase.

5. The primary purpose of the second paragraph in the passage is to

(A) provide an overview of current research concerning the effect of serotonin on carbohydrate consumption
(B) contrast the role of tryptophan in the body with that of serotonin
(C) discuss the role of serotonin in the transmission of neural impulses
(D) explain how the brain knows that carbohydrates should be consumed
(E) establish a connection between carbohydrate intake and the production of serotonin

6. It can be inferred that after a person has taken d-fenflurarmine, he or she will probably be

(A) inclined to gain weight
(B) sleepy much of the time
(C) unlikely to crave carbohydrates
(D) unable to sleep as much as usual
(E) likely to secrete more insulin than usual

7. The author’s primary purpose is to

(A) defend a point of view
(B) correct a misconception
(C) assess conflicting evidence
(D) suggest new directions for investigation
(E) provide information that helps explain a phenomenon

• Source: LSAT Official PrepTest 12 (October 1994)
• Difficulty Level: 650

Originally posted by nitya34 on 05 Mar 2009, 10:41.
Last edited by SajjadAhmad on 22 Sep 2019, 01:32, edited 8 times in total.
Updated - Complete topic (607).
Director
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Re: How does the brain know when carbohydrates have been or should be cons  [#permalink]

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05 Mar 2009, 16:12
1
Got D,A,B,A,E,C,E

Found the first Q awfully tough. Ended up taking 4 min for that single Q and took 18 on a whole. Jeez! I should learn to let go..

Desire vs need??

role of serotonin levels vs serotonin like drugs.

Chose A flipped to D,flipped back to A and then back to D

That Q is nasty. The final line suggests that the answer probably is A

The first para suggests that the answer probably is D.

OE for the first one please?

Chose A for Q 24 very uncomfortably as I needed to bring the results from Rats experiment.

Thanks
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Re: How does the brain know when carbohydrates have been or should be cons  [#permalink]

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01 May 2019, 04:27
1
From 3rd to 5th questions, I chose wrong all of them: D, C, and B. Could anybody kindly tell me how you find the right answers for these three questions? Many thanks!
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Re: How does the brain know when carbohydrates have been or should be cons  [#permalink]

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01 May 2019, 06:27
1
keysersozebrook wrote:
From 3rd to 5th questions, I chose wrong all of them: D, C, and B. Could anybody kindly tell me how you find the right answers for these three questions? Many thanks!

3. It can be inferred that a person is likely to crave carbohydrates when

(A) the amount of insulin produced is too high
(B) the amount of serotonin in the brain is too low
(C) more tryptophan than usual crosses the blood-brain barrier
(D) neurotransmission by neurotransmitters other than serotonin is interrupted
(E) amino acids other than tryptophan are taken up by peripheral tissues

Question Type: Inference.
When is a person likely to crave carbohydrates?

We have few instances in the passage, eg. Para 1: which say the drugs that block Serotonin cause Carbohydrates craving. So Carbohydrate craving occurs when amount of serotonin is low. And also we were told serotonin levels in brain affect the amount of carbohydrate we eat. More Serotonin in brain, lesser the craving and vice-versa. With this understanding I will try to eliminate other options and try to reach the right answer.

A) Insulin produced is too High.

In Para 2, the passage explains the indirect relationship between Insulin and Tryptophan. If insulin produced is too high, most amino acids are taken up by peripheral muscles. As a consequence, the amount of tryptophan relative to other amino acids in the blood stream would be higher than it was before. Further, this increases the amount of Tryptophan crossing the blood-brain barrier and it's subsequent conversion into serotonin. So. more serotonin in brain, less carbohydrate craving.
INCORRECT.

more amino acids other than tryptophan taken up----->relative rise in tryptophan levels------>more tryptophan crossing blood brain barrier-----> more tryptophan to serotonin conversion----> more serotonin levels in brain-----> LESS CRAVING

Understanding this will help me eliminate 3 options A,C,E.

B) the amount of serotonin in the brain is too low.

It matches my understanding of the passage. Hold and look at other options.

C) more tryptophan than usual crosses the blood-brain barrier

So again it's the opposite. If more tryptophan crosses the blood brain barrier, more serotonin is likely to be produced in brain and thus lesser carbohydrate craving.

D) neurotransmission by neurotransmitters other than serotonin is interrupted

This is a slightly difficult to eliminate. Para 4 says,
"In contrast, drugs that block serotonin-mediated transmission or that interact with neurotransmitters other than serotonin have the opposite effect: they often induce carbohydrate craving"

It says, drugs that interact with neurotransmitters other than serotonin induce carbohydrate craving. Interact doesn't mean interrupt. Those drugs by interacting may actually enhance other neurotransmitters action or may interrupt their's. The option says "interrupt".
So. INCORRECT.

E) amino acids other than tryptophan are taken up by peripheral tissues

Again the same explanation as for option A. Amino acids taken up----->relative rise in tryptophan levels------>more Tryptophan to Serotonin conversion----> more serotonin levels in brain-----> LESS CRAVING

So option B.
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Re: How does the brain know when carbohydrates have been or should be cons  [#permalink]

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01 May 2019, 07:14
1
1
keysersozebrook wrote:
From 3rd to 5th questions, I chose wrong all of them: D, C, and B. Could anybody kindly tell me how you find the right answers for these three questions? Many thanks!

4. The information in the passage indicates that if human beings were given a drug that inhibits the action of serotonin, which one of the following might be expected to occur?

(A) Subjects would probably show a preference for carbohydrate-rich snacks rather than protein-rich snacks.
(B) Subjects would probably become sleepy after eating a carbohydrate-rich meal.
(C) Subjects would be more likely to lose weight than before they took the drug.
(D) Subjects’ blood tryptophan levels would probably increase.
(E) Subjects’ desire for both carbohydrates and proteins would increase.

Question type: Indicate---> Inference

For finding the answer for this I refer to para 1, last line and para 4.

The passages states that drugs that inhibit the action of serotonin have the opposite action to that of serotonin like drugs.
So carbohydrate craving actually increases. Further in Para 4 it is mentioned that serotonin facilitators actually reduce carbohydrate craving by prolonging serotonin's action. Subjects feel sleepy after given a carbohydrate meal and the opposite is true for people who take drugs that block serotonin. With this understanding, I shall try to eliminate options and try to reach the correct answer.

A) Subjects would probably show a preference for carbohydrate-rich snacks rather than protein-rich snacks.

Seems correct. But, we might get a doubt whether is there any craving for protein snacks at all, which is not explicitly stated in the passage. If it is present, is it more or less or equal? I'll try to eliminate others or will mark other option, if found to be more convincing than this is.

B) Subjects would probably become sleepy after eating a carbohydrate-rich meal.

Opposite. INCORRECT
They become refreshed and invigorated.

C) Subjects would be more likely to lose weight than before they took the drug.

Again opposite. INCORRECT.
They would more likely to gain weight. To increase or to reach the required levels of serotonin in brain, people excessively crave for carbohydrate diet and probably take more carbohydrates and gain weight.

D) Subjects’ blood tryptophan levels would probably increase.

No information given. We don't know what increases the blood tryptophan levels with the information given in the passage. We were only given Tryptophan----->Serotonin but no information whether low serotonin levels lead to more tryptophan.
INCORRECT

(The passage explicitly states that, when the carbohydrate level in the blood stream increases, insulin is produced and amino acids other than tryptophan are taken up by peripheral tissues. This in turn indirectly increases the "relative tryptophan levels".
Its actual levels or its levels prior to intake of carbohydrates are the same. Its just that others are taken up and it seems to be the one that exists.)

E) Subjects’ desire for both carbohydrates and proteins would increase.

Desire for carbohydrates increases, but protein's; not mentioned. So I would rule out E too.

INCORRECT

Left with A. I would refer to the experiment given in the third passage.
"On the other hand, when rats are given drugs that interrupt serotonin-mediated neurotransmission, their brains fail to respond when carbohydrates are eaten, so the desire for them persists." I will try to draw an analogy so that I would be even more confident in marking this. The question stem says "might", so no harm.

"Normally" rats alternate between protein and carbohydrate. But when given drugs that inhibit serotonin action, their brains fail to respond. Desire persists, so we can infer that given more carbohydrate they will continue to eat until the serotonin level in the brain reaches to that level, which says to brain, it's enough. So, it's clear they don't care if we feed them more protein snacks.

A.
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Re: How does the brain know when carbohydrates have been or should be cons  [#permalink]

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01 May 2019, 08:41
Hi,
I have taken 12mins 39secs to solve this passage with 100% accuracy. Am I taking too long? If yes, how much time I should devote per reading passage.
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Re: How does the brain know when carbohydrates have been or should be cons  [#permalink]

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03 May 2019, 12:50
arorni wrote:
Hi,
I have taken 12mins 39secs to solve this passage with 100% accuracy. Am I taking too long? If yes, how much time I should devote per reading passage.

Given the length of the passage, number of questions and accuracy with which you solved I'd say you are doing fine. But the point here is that GMAT doesn't give you passages with 7 questions. So If you solved the first three questions (including passage comprehension) within 7 minutes, great or else practice practice and practice!!!
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Re: How does the brain know when carbohydrates have been or should be cons  [#permalink]

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25 Nov 2019, 18:57
4. The information in the passage indicates that if human beings were given a drug that inhibits the action of serotonin, which one of the following might be expected to occur?

I didn't know the meaning of word "inhibit" so I got tripped by this question. Not a difficult RC but tricky at places.
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Re: How does the brain know when carbohydrates have been or should be cons  [#permalink]

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26 Nov 2019, 07:36
can anyone suggest how much time one should take to complete this whole passage? i got all right but took perhaps more time .
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How does the brain know when carbohydrates have been or should be cons  [#permalink]

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05 Dec 2019, 02:07
1
Hi everyone,
Got 6/7 correct in 14:30 minutes.

------------------------------------------------------------------------------------------------

P1

Paragraph 1 starts with a question about how the brain knows when to eat carbos. According to the passage there is no bulletproof answer but an explanation might be related to the neurotransmitter serotonin and its interaction with a presynaptic neuron and a postsynaptic neuron. Serotonin's function in such relation is to give a nerve impulse from the presynaptic neuron to the postsynaptic neuron. The more enhanced such impulse is, the less our brain thinks about carbos. On the other hand when such impulse is blocked, our brain craves carbos more.

Brief summary: how serotonin might be responsible for the desire (or not) to eat carbos.

P2

Paragraph 2 describes the process through which serotonin is produced. Serotonin is produced from an amino acid called T. T is in the blood stream and needs to arrive in the brain before the conversion is possible. The latter is affected by the quantity of carbos present in one's diet. The more the carbos the higher the secretion of insulin and consequently the levels of other amino acids (except T). There is some competition between the amino acids to get to the brain and so when more carbos are introduced, T speeds up the process to get to the brain.

Brief summary: How serotonin is produced

P3

The author states that the levels of serotonin in the brain affect one's desire to eat carbos. To support such idea an experiment with rats is presented. Rats normally would eat equally protein and carbos foods but when given a serotonin drug they would avoid the carbos.

Brief summary: serotonin affects one's desire to eat carbos

P4

Paragraph 4 just rehighlight the effects of serotonin, or its blockage, on individual's desire to eat carbos. When someone is given a serotonin drug, their desire of carbos is reduced; on the opposite scenario, when someone takes a drug that blocks the production of serotonin, they crave carbos even more. We are also given that when someone eats carbos, they feel refreshed (if serotonin levels are low) and when someone eats carbos (and their serotonin levels are high), they almost fall asleep.

Brief summary: effects of serotonin post eating carbos

Main point

The main point is to describe serotonin and the effect of it on one's desire to eat carbos

------------------------------------------------------------------------------------------------

1. Which one of the following best states the main idea of the passage

Pre-thinking

Main point question

Refer to main point and summaries above.

(A) The body’s need for carbohydrates varies with the level of serotonin in the blood.
I believe this answer choice to be inconsistent because the serotonin levels affect the desire and not the need for carbos

(B) The body’s use of carbohydrates can be regulated by the administration of serotoninlike drugs.
Not the main point but an application

(C) The role of serotonin in regulating the consumption of carbohydrates is similar in rats and in humans.
not broad enough and such similarity is never confirmed

(D) The body’s desire for carbohydrates can be influenced by serotonin or serotoninlike drugs.
In line with pre-thinking

(E) Tryptophan initiates a chain of events that regulates the body’s use of carbohydrates.
partial scope

------------------------------------------------------------------------------------------------

2. The term “rate” (Highlighted) refers to the rate at which

Pre-thinking

Detail question

From P2: "Serotonin is a derivative of tryptophan,"
the rate hence refers to the rate of conversion from T to serotonin

(A) serotonin is produced from tryptophan
In line with pre-thinking

(B) carbohydrates are taken into the body
Not in line with pre-thinking

(C) carbohydrates stimulate the secretion of insulin
Not in line with pre-thinking

(D) insulin facilitates the uptake of amino acids into peripheral tissues
Not in line with pre-thinking

(E) tryptophan enters the bloodstream
Not in line with pre-thinking

------------------------------------------------------------------------------------------------

3. It can be inferred that a person is likely to crave carbohydrates when

Pre-thinking

Inference question

From P4: "In contrast, drugs that block serotonin-mediated transmission or that interact with neurotransmitters other than serotonin have the opposite effect: they often induce carbohydrate craving and subsequent weight gain."

We can infer that when the production of serotonin is low or blocked, the desire for carbos is higher

(A) the amount of insulin produced is too high
Not in line with pre-thinking

(B) the amount of serotonin in the brain is too low
in line with pre-thinking

(C) more tryptophan than usual crosses the blood-brain barrier
Opposite

(D) neurotransmission by neurotransmitters other than serotonin is interrupted
Not in line with pre-thinking

(E) amino acids other than tryptophan are taken up by peripheral tissues
Not in line with pre-thinking

------------------------------------------------------------------------------------------------

4. The information in the passage indicates that if human beings were given a drug that inhibits the action of serotonin, which one of the following might be expected to occur?

Pre-thinking

Detail question

From P4: "People who crave carbohydrates report feeling refreshed and invigorated after eating a carbohydrate-rich meal (which would be expected to increase brain serotonin levels),"

(A) Subjects would probably show a preference for carbohydrate-rich snacks rather than protein-rich snacks.
in line with pre-thinking

(B) Subjects would probably become sleepy after eating a carbohydrate-rich meal.
Not in line with pre-thinking

(C) Subjects would be more likely to lose weight than before they took the drug.
Not in line with pre-thinking

(D) Subjects’ blood tryptophan levels would probably increase.
Not in line with pre-thinking. "Blood tryptophan levels, however, are unaffected by insulin, so the proportion of tryptophan in the blood relative to the other amino acids increases when carbohydrates are consumed."

(E) Subjects’ desire for both carbohydrates and proteins would increase.
Not in line with pre-thinking

------------------------------------------------------------------------------------------------

5. The primary purpose of the second paragraph in the passage is to

Pre-thinking

Purpose/function question

Paragraph 2 establishes a relation between T and the production of serotonin

(A) provide an overview of current research concerning the effect of serotonin on carbohydrate consumption
Not in line with pre-thinking

(B) contrast the role of tryptophan in the body with that of serotonin
Not in line with pre-thinking

(C) discuss the role of serotonin in the transmission of neural impulses
Not in line with pre-thinking

(D) explain how the brain knows that carbohydrates should be consumed
Not in line with pre-thinking

(E) establish a connection between carbohydrate intake and the production of serotonin
in line with pre-thinking

------------------------------------------------------------------------------------------------

6. It can be inferred that after a person has taken d-fenflurarmine, he or she will probably be

Pre-thinking

Inference question

[b]According to P4 such drugs enhances the production of serotonin and hence diminishes the cravings for carbos and consequently leads to weight loss

[/b]

(A) inclined to gain weight
opposite

(B) sleepy much of the time
extreme

(C) unlikely to crave carbohydrates
in line with pre-thinking

(D) unable to sleep as much as usual
Not in line with pre-thinking

(E) likely to secrete more insulin than usual
Not in line with pre-thinking

------------------------------------------------------------------------------------------------

7. The author’s primary purpose is to

Pre-thinking

Main point question

Refer to main point and summaries above

(A) defend a point of view
Not in line with pre-thinking

(B) correct a misconception
Not in line with pre-thinking

(C) assess conflicting evidence
Not in line with pre-thinking

(D) suggest new directions for investigation
Not in line with pre-thinking

(E) provide information that helps explain a phenomenon
in line with pre-thinking

------------------------------------------------------------------------------------------------

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