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Daily RC Article 41

How carbohydrate is consumed

Paragraph 1

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.

Paragraph 2

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.

Paragraph 3

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.

Paragraph 4

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.

Topic and Scope:

The brain and what it knows about carbohydrate consumption; specifically, the link between serotonin levels in the brain and the body’s (the brain’s) desire for carbohydrates. Serotonin is definitely the “star” of this particular passage.

Purpose and Main Idea:

Author’s purpose is to describe the link between serotonin and carbohydrate desire; the author’s specific main idea is that, in general, the more serotonin that is present in the brain, the less likely the body is to crave carbohydrates, and viceversa. It’s an inverse relationship, one that you need to be clear on. (But that’s about all you need to be clear on!)

Paragraph Structure:

Paragraph 1 sets out the basic issue at hand — the link between serotonin levels and the body’s desire for carbohydrates, suggesting that there is an inverse relationship between the two.

Paragraph 2, once you cut through all of the scientific factology, says that the production of serotonin itself is linked to the amount of carbohydrates that the body is receiving. (As we’ll see, the role of tryptophan in the passage — as opposed to its role in the bloodstream — is mainly to generate wrong answer choices.)

Paragraph 3 reverts to the original issue of how serotonin levels affect the body’s craving for carbohydrates. Rats, we learn, are affected in much the same way as humans: increased serotonin levels make them desire carbohydrates less.

Paragraph 4 brings us full circle to Paragraph 1: Drugs that enhance serotonin levels decrease the human body’s desire for carbohydrates, while those that depress serotonin levels increase it. Yadda yadda yadda; same-o same-o. If you ever doubted or were unaware of the redundancy that can often drive a Reading Comp. passage, especially a science passage, we hope that this one was an eye-opener.

The Big Picture:

  • Like most science passages, this one contains a lot of details, but the passage’s main idea — the inverse relationship between serotonin levels and the body’s desire for carbohydrates — is evident from the outset, is straightforward, and never really develops further. Chances are you saved this passage till last; hope you got a pleasant surprise from how manageable it was.
  • Underneath all of the scientific jargon is a straightforward, easy-to-understand idea, so don’t let yourself be thrown by the complex prose. And be sure that you understand the main idea, because many of the questions in science passages will reward you for doing nothing more than grasping it.

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