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.
21. 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.
22. The term “rate” (line 17) 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
23. It can be inferred that a person is likely to CRAVE
(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
24. 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.
25. 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
26. 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
27. 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