Critical Reasoning (CR) | Re: In humans, ingested protein is broken down into amino acids,

Good question, domleon. It's a basic principle that shows up in many CR questions. The principle is that "If X, then Y" does not mean the same thing as "If Y, then X". This seems obvious when expressed this way, and it is -- but the testmakers have ways of writing "If X, then Y" statements that are hard to recognize.

In this case, the evidence definitely proves that (as long as protein is ingested first), ingestion of sugar will increase the brain's serotonin level.