In humans, ingested protein is broken down into amino acids, all of wh

OFFICIAL EXPLANATION

The importance of this problem is not just in answering it correctly, but also in answering it quickly. The first step is to recognize the argument structure: Premise: In humans, ingested protein is broken down into amino acids, all of which must compete to enter the brain. Premise: Subsequent ingestion of sugars leads to the production of insulin, a hormone that breaks down the sugars and also rids the bloodstream of residual amino acids, except for tryptophan. Premise: Tryptophan then slips into the brain uncontested and is transformed into the chemical serotonin, increasing the brain’s serotonin level. Conclusion: Sugars can play a major role in mood elevation, helping one to feel relaxed and anxiety-free. At this point in your preparation, you should constantly be on the lookout for new elements that appear in the conclusion. This problem contains the new conclusion element of “a major role in mood elevation, helping one to feel relaxed and anxiety-free.” Because this element immediately follows the assertion that the brain’s serotonin level has been increased, you should attack the answer choices by looking for an answer that fits the Supporter relationship that an increase serotonin leads to an elevated mood.

Only answer choices (A) and (D) contain these two elements, and you should examine them first as you seek to accelerate through this problem:

Answer choice (A): Although the author assumes that raising the level of serotonin is sufficient to elevate mood, this answer claims that it is necessary. Hence, this answer is incorrect.

Answer choice (D): This is the correct answer. The author states that after the action of the sugars, more serotonin enters the brain. The author then concludes that this leads to a mood elevation. Thus, the author assumes that serotonin has an effect on the mood level.

Answer choice (B): The argument refers to what happens when sugars are ingested. No assumption is made about what occurs when foods rich in sugars are not ingested.

Answer choice (C): Although the argument states that tryptophan is transformed into serotonin, no assumption is made that this is the only way serotonin is produced.

Answer choice (E): The author does not assume the statement in this answer. We know from the first sentence of the stimulus that ingested protein is broken down into amino acids which compete to enter the brain. This competition could result in mood elevation even without the ingestion of sugars since some amino acids will enter the brain (some could be tryptophan, for example). Thus, since the author’s argument contains a scenario that would allow for the opposite of this answer choice to occur, this answer is not an assumption of the argument.